Reverse engineering the barrel shifter circuit on the Intel 386 processor die

The Intel 386 processor (1985) was a large step from the 286 processor, moving x86 to a 32-bit architecture. The 386 also dramatically improved the performance of shift and rotate operations by adding a "barrel shifter", a circuit that can shift by multiple bits in one step. The die photo below shows the 386's barrel shifter, highlighted in the lower left and taking up a substantial part of the die.

The 386 die with the main functional blocks labeled. Click this image (or any other) for a larger version.)

Shifting is a useful operation for computers, moving a binary value left or right by one or more bits. Shift instructions can be used for multiplying or dividing by powers of two, and as part of more general multiplication or division. Shifting is also useful for extracting bit fields, aligning bitmap graphics, and many other tasks.1

Barrel shifters require a significant amount of circuitry. A common approach is to use a crossbar, a matrix of switches that can connect any input to any output. By closing switches along a desired diagonal, the input bits are shifted. The diagram below illustrates a 4-bit crossbar barrel shifter with inputs X (vertical) and outputs Y (horizontal). At each point in the grid, a switch (triangle) connects a vertical input line to a horizontal output line. Energizing the blue control line, for instance, passes the value through unchanged (X0 to Y0 and so forth). Energizing the green control line rotates the value by one bit position (X0 to Y1 and so forth, with X3 wrapping around to X0). Similarly, the circuit can shift by 2 or 3 bits. The shift control lines select the amount of shift. These lines run diagonally, which will be important later.

A four-bit crossbar switch with inputs X and outputs Y. Image by Cmglee, CC BY-SA 3.0.

The main problem with a crossbar barrel shifter is that it takes a lot of hardware. The 386's barrel shifter has a 64-bit input and a 32-bit output,2 so the approach above would require 2048 switches (64×32). For this reason, the 386 uses a hybrid approach, as shown below. It has a 32×8 crossbar that can shift by 0 to 28 bits, but only in multiples of 4, making the circuit much smaller. The output from the crossbar goes to a second circuit that can shift by 0, 1, 2, or 3 bits. The combined circuitry supports an arbitrary shift, but requires less hardware than a complete crossbar. The inputs to the barrel shifter are two 32-bit values from the processor's register file, stored in latches for use by the shifter.

Block diagram of the barrel shifter circuit.

The figure below shows how the shifter circuitry appears on the die; this image shows the two metal layers on the die's surface. The inputs from the register file are at the bottom, for bits 31 through 0. Above that, the input latches hold the two 32-bit inputs for the shifter. In the middle is the heart of the shift circuit, the crossbar matrix. This takes the two 32-bit inputs and produces a 32-bit output. The matrix is controlled by sloping polysilicon lines, driven by control circuitry on the right. The matrix output goes to the circuit that applies a shift of 0 to 3 positions. Finally, the outputs exit at the top, where they go to other parts of the CPU. The shifter performs right shifts, but as will be explained below, the same circuit is used for the left shift instructions.

The barrel shifter circuitry as it appears on the die. I have cut out repetitive circuitry from the middle because the complete image is too wide to display clearly.

The barrel shifter crossbar matrix

In this section, I'll describe the matrix part of the barrel shifter circuit. The shift matrix takes 32-bit values a and b. Value b is shifted to the right, with bits from a filling in at the left, producing a 32-bit output. (As will be explained below, the output is actually 37 bits due to some complications, but ignore that for now.) The shift count is a multiple of 4 from 0 to 28.

The diagram below illustrates the structure of the shift matrix. The two 32-bit inputs are provided at the bottom, interleaved, and run vertically. The 32 output lines run horizontally. The 8 control lines run diagonally, activating the switches (black dots) to connect inputs and outputs. (For simplicity, only 3 control lines are shown.) For a shift of 0, control line 0 (red) is selected and the output is b₃₁b₃₀...b₁b₀. (You can verify this by matching up inputs to outputs through the dots along the red line.)

Diagram of the shift matrix, showing three of the shift control lines.

For a shift right of 4, the cyan control line is activated. It can be seen that the output in this case is a₃a₂a₁a₀b₃₁b₃₀...b₅b₄, shifting b to the right 4 bits and filling in four bits from a as desired. For a shift of 28, the purple control line is activated, producing the output a₂₇...a₀b₃₁...b₂₈. Note that the control lines are spaced four bits apart, which is why the matrix only shifts by a multiple of 4. Another important feature is that below the red diagonal, the b inputs are connected to the output, while above the diagonal, the a inputs are connected to the output. (In other words, the black dots are shifted to the right above the diagonal.) This implements the 64-bit support, taking bits from a or b as appropriate.

Looking at the implementation on the die, the vertical wires use the lower metal layer (metal 1) while the horizontal wires use the upper metal layer (metal 2), so the wires don't intersect. NMOS transistors are used as the switches to connect inputs and outputs.4 The transistors are controlled by diagonal wires constructed of polysilicon that form the transistor gates. When a particular polysilicon wire is energized, it turns on the transistors along a diagonal line, connecting those inputs and outputs.

The image below shows the left side of the matrix.5 The polysilicon control lines are the green horizontal lines stepping down to the right. These control the transistors, which appear as columns of blue-gray squares next to the polysilicon lines. The metal layers have been removed; the position of the lower metal 1 layer is visible in the vertical bluish lines.

The left side of the matrix as it appears on the die.

The diagram below shows four of these transistors in the shifter matrix. There are four circuitry layers involved. The underlying silicon is pinkish gray; the active regions are the squares with darker borders. Next is the polysilicon (green), which forms the control lines and the transistor gates. The lower metal layer (metal 1) forms the blue vertical lines that connect to the transistors.3 The upper metal layer (metal 2) forms the horizontal bit output lines. Finally, the small black dots are the vias that connect metal 1 and metal 2. (The well taps are silicon regions connected to ground to prevent latch-up.)

Four transistors in the shifter matrix. The polysilicon and metal lines have been drawn in.

To see how this works, suppose the upper polysilicon line is activated, turning on the top two transistors. The two vertical bit-in lines (blue) will be connected through the transistors to the top two bit out lines (purple), by way of the short light blue metal segments and the via (black dot). However, if the lower polysilicon line is activated, the bottom two transistors will be turned on. This will connect the bit-in lines to the fifth and sixth bit-out lines, four lines down from the previous ones. Thus, successive polysilicon lines shift the connections down by four lines at a time, so the shifts change in steps of 4 bit positions.

As mentioned earlier, to support the 64-bit input, the transistors below the diagonal are connected to b input while the transistors above the diagonal are connected to the a input. The photo below shows the physical implementation: the four upper transistors are shifted to the right by one wire width, so they connect to vertical a wires, while the four lower transistors are connected to b wires. (The metal wires were removed for this photo to show the transistors.)

This photo of the underlying silicon shows eight transistors. The top four transistors are shifted one position to the right. the irregular lines are remnants of other layers that I couldn't completely remove from the die.

In the matrix, the output signals run horizontally. In order for signals to exit the shifter from the top of the matrix, each horizontal output wire is connected to a vertical output wire. Meanwhile, other processor signals (such as the register write data) must also pass vertically through the shifter region. The result is a complicated layout, packing everything together as tightly as possible.

The precharge/keepers

At the left and the right of the barrel shifter, repeated blocks of circuitry are visible. These blocks contain precharge and keeper circuits to hold the value on one of the lines. During the first clock phase, each horizontal bit line is precharged to +5 volts. Next, the matrix is activated and horizontal lines may be pulled low. If the line is not pulled low, the inverter and PMOS transistor will continuously pull the line high. The inverter and transistor can be viewed as a bus keeper, essentially a weak latch to hold the line in the 1 state. The keeper uses relatively weak transistors, so the line can be pulled low when the barrel shifter is activated. The purpose of the keeper is to ensure that the line doesn't drift into a state between 0 and 1. This is a bad situation with CMOS circuitry, since the pull-up and pull-down transistors could both turn on, yielding a short circuit.

The precharge/keeper circuit

The motivation behind this design is that implementing the matrix with "real" CMOS would require twice as many transistors. By implementing the matrix with NMOS transistors only, the size is reduced. In a standard NMOS implementation, pull-up transistors would continuously pull the lines high, but this results in fairly high power consumption. Instead, the precharge circuit pulls the line high at the start. But this results in dynamic logic, dependent on the capacitance of the circuit to hold the charge. To avoid the charge leaking away, the keeper circuit keeps the line high until it is pulled low. Thus, this circuit minimizes the area of the matrix as well as minimizing power consumption.

There are 37 keepers in total for the 37 output lines from the matrix.6 (The extra 5 lines will be explained below.) The photo below shows one block of three keepers; the metal has been removed to show the silicon transistors and some of the polysilicon (green).

One block of keeper circuitry, to the right of the shift matrix. This block has 12 transistors, supporting three bits.

The register latches

At the bottom of the shift circuit, two latches hold the two 32-bit input values. The 386 has multi-ported registers, so it can access two registers and write a third register at the same time. This allows the shift circuit to load both values at the same time. I believe that a value can also come from the 386's constant ROM, which is useful for providing 0, 1, or all-ones to the shifter.

The schematic below shows the register latches for one bit of the shifter. Starting at the bottom are the two inputs from the register file (one appears to be inverted for no good reason). Each input is stored in a latch, using the standard 386 latch circuit.7 The latched input is gated by the clock and then goes through multiplexers allowing either value to be used as either input to the shifter. (The shifter takes two 32-bit inputs and this multiplexer allows the inputs to be swapped to the other sides of the shifter.) A second latch stage holds the values for the output; this latch is cleared during the first clock phase and holds the desired value during the second clock phase.

Circuit for one bit of the register latch.

The die photo below shows the register latch circuit, contrasting the metal layers (left) with the silicon layer (right). The dark spots in the metal image are vias between the metal layers or connections to the underlying silicon or polysilicon. The metal layer is very dense with vertical wiring in the lower metal 1 layer and horizontal wiring in the upper metal 2 layer. The density of the chip seems to be constrained by the metal wiring more than the density of the transistors.

One of the register latch circuits.

The 0-3 shifter

The shift matrix can only shift in steps of 4 bits. To support other shifts, a circuit at the top of the shifter provides a shift of 0 to 3 bits. In conjunction, these circuits permit a shift by an arbitrary amount.8 The schematic below shows the circuit. A bit enters at the bottom. The first shift stage passes the bit through, or sends it one bit position to the right. The second stage passes the bit through, or sends it two bit positions to the right. Thus, depending on the control lines, each bit can be shifted by 0 to 3 positions to the right. At the top, a transistor pulls the circuit low to initialize it; the NOR gate at the bottom does the same. A keeper transistor holds the circuit low until a data bit pulls it high.

One bit of the 0-3 shifter circuit.

The diagram below shows the silicon implementation corresponding to two copies of the schematic above. The shifters are implemented in pairs to slightly optimize the layout. In particular, the two NOR gates are mirrored so the power connection can be shared. This is a small optimization, but it illustrates that the 386 designers put a lot of work into making the layout dense.

Two bits of the 0-3 shifter circuit as it appears on the die.

Complications

As is usually the case with x86, there are a few complications. One complication is that the shift matrix has 37 outputs, rather than the expected 32. There are two reasons behind this. First, the upper shifter will shift right by up to 3 positions, so it needs 3 extra bits. Thus, the matrix needs to output bits 0 through 34 so three bits can be discarded. Second, shift instructions usually produce a carry bit from the last bit shifted out of the word. To support this, the shift matrix provides an extra bit at both ends for use as the carry. The result is that the matrix produces 37 outputs, which can be viewed as bits -1 through 35.

Another complication is that the x86 instruction set supports shifts on bytes and 16-bit words as well as 32-bit words. If you put two 8-bit bytes into the shifter, there will be 24 unused bits in between, posing a problem for the shifter. The solution is that some of the diagonal control lines in the matrix are split on byte and word boundaries, allowing an 8- or 16-bit value to be shifted independently. For example, you can perform a 4-bit right shift on the right-hand byte, and a 28-bit right shift on the left-hand byte. This brings the two bytes together in the result, yielding the desired 4-bit right shift. As a result, there are 18 diagonal control lines in the shifter (if I counted correctly), rather than the expected 8 control lines. This makes the circuitry to drive the control lines more complicated, as it must generate different signals depending on the size of the operand.

The control circuitry

The control circuitry at the right of the shifter drives the diagonal polysilicon lines in the matrix, selecting the desired shift. It also generates control signals for the 0-3 shifter, selecting a shift-by-1 or shift-by-2 as necessary. This circuitry operates under the control of the microcode, which tells it when to shift. It gets the shift amount from the instruction or the CL register and generates the appropriate control signals.

The distribution of control signals is more complex than you might expect. If possible, the polysilicon diagonals are connected on the right of the matrix to the control circuitry, providing a direct connection. However, many of the diagonals do not extend all the way to the right, either because they start on the left or because they are segmented for 8- or 16-bit values. Some of these signals are transmitted through polysilicon lines that run underneath the matrix. Others are transmitted through horizontal metal lines that run through the register latches. (These latches don't use many horizontal lines, so there is available space to route other signals.) These signals then ascend through the matrix at various points to connect with the polysilicon lines. This shows that the routing of this circuitry is carefully optimized to make it as compact as possible. Moreover, these "extra" lines disrupt the layout; the matrix is almost a regular pattern, but it has small irregularities throughout.

Implementing x86 shifts and rotates with the barrel shifter

The x86 has a variety of shift and rotate instructions.9 It is interesting to consider how they are implemented using the barrel shifter, since it is not always obvious. In this section, I'll discuss the instructions supported by the 386.

One important principle is that even though the circuitry shifts to the right, by changing the inputs this can achieve a shift to the left. To make this concrete, consider two input words a and b, with the shifter extracting the portion in red below. (I'll use 8-bit examples instead of 32-bit here and below to keep the size manageable.) The circuit shifts b to the right five bits, inserting bits from a at the left. Alternatively, the result can be viewed as shifting a to the left three bits, inserting bits from b at the right. Thus, the same result can be viewed as a right shift of b or a left shift of a. This holds in general, with a 32-bit right shift by N bits equivalent to a left shift by 32-N bits, depending on which word10 you focus on.

a₇a₆a₅a₄a₃a₂a₁a₀b₇b₆b₅b₄b₃b₂b₁b₀

Double shifts

The double-shift instructions (Shift Left Double (SHLD) and Shift Right Double (SHRD)) were new in the 386, shifting two 32-bit values to produce a 32-bit result. The last bit shifted out goes into the carry flag (CF). These instructions map directly onto the behavior of the barrel shifter, so I'll start with them.

Actions of the double shift instructions.

The examples below show how the shifter implements the SHLD and SHRD instructions; the shifter output is highlighted in red. (These examples use an 8-bit source (s) and destination (d) to keep them manageable.) In either case, 3 bits of the source are shifted into the destination; shifting left or right is just a matter of whether the destination is on the left or right.

SHLD 3: ddddddddssssssss

SHRD 3: ssssssssdddddddd

Shifts

The basic shift instructions are probably the simplest. Shift Arithmetic Left (SAL) and Shift Logical Left (SHL) are synonyms, shifting the destination to the left and filling with zeroes. This can be accomplished by performing a shift with the word on the left and zeroes on the right. Shift Logical Right (SHR) is the opposite, shifting to the right and filling with zeros. This can be accomplished by putting the word on the right and zeroes on the left. Shift Arithmetic Right (SAR) is a bit different. It fills with the sign bit, the top bit. The purpose of this is to shift a signed number while preserving its sign. It can be implemented by putting all zeroes or all ones on the left, depending on the sign bit. Thus, the shift instructions map nicely onto the barrel shifter.

Actions of the shift instructions.

The 8-bit examples below show how the shifter accomplishes the SHL, SHR, and SAR instructions. The destination value d is loaded into one half of the shifter. For SAR, the value's sign bit s is loaded into the other half of the shifter, while the other instructions load 0 into the other half of the shifter. The red box shows the output from the shifter, selected from the input.

SHL 3: dddddddd00000000

SHR 3: 00000000dddddddd

SAR 3: ssssssssdddddddd

Rotates

Unlike the shift instructions, the rotate instructions preserve all the bits. As bits shift off one end, they fill in the other end, so the bit sequence rotates. A rotate left or right is implemented by putting the same word on the left and right.

Actions of the rotate instructions.

The shifter implements rotates as shown below, using the destination value as both shifter inputs. A left shift by N bits is implemented by shifting right by 32-N bits.

ROL 3: d₇d₆d₅d₄d₃d₂d₁d₀d₇d₆d₅d₄d₃d₂d₁d₀

ROR 3: d₇d₆d₅d₄d₃d₂d₁d₀d₇d₆d₅d₄d₃d₂d₁d₀

Rotates through carry

The rotate through carry instructions perform 33-bit rotates, rotating the value through the carry bit. You might wonder how the barrel shifter can perform a 33-bit rotate, and the answer is that it can't. Instead, the instruction takes multiple steps. If you look at the instruction timings, the other shifts and rotates take three clock cycles. Rotating through the carry, however, takes nine clock cycles, performing multiple steps under the control of the microcode.

Actions of the rotate through carry instructions.

Without looking at the microcode, I can only speculate how it takes place. One sequence would be to get the top bits by putting zeroes in the right 32 bits and shifting. Next, get the bottom bits by putting the carry bit in the left 32 bits and shifting one bit more. (That is, set the left 32-bit input to either the constant 0 or 1, depending on the carry.) Finally, the result can be generated by ORing the two shift values together. The example below shows how an RCL 3 could be implemented. In the second step, the carry value C is loaded into the left side of the shifter, so it can get into the result. Note that bit d₅ ends up in the carry bit, rather than the result. The RCR instruction would be similar, but adjusting the shift parameters accordingly.

First shift: d₇d₆d₅d₄d₃d₂d₁d₀00000000

Second shift: 0000000Cd₇d₆d₅d₄d₃d₂d₁d₀

Result from OR: d₄d₃d₂d₁d₀Cd₇d₆

Conclusions

The shifter circuit illustrates how the rapidly increasing transistor counts in the 1980s allowed new features. Programming languages make it easy to shift numbers with an expression such as a>>5. But it takes a lot of hardware in the processor to perform these shifts efficiently. The additional hardware of the 386's barrel shifter dramaticallly improved shift performance for shifts and rotates compared to earlier x86 processors. I estimate that the barrel shifter requires about 2000 transistors, about half the number of the entire 6502 processor (1975). But by 1985, putting 2000 transistors into a feature was practical. (In total, the 386 contains 285,000 transistors, a trivial number now, but a large number for the time.)

I plan to write more about the 386, so follow me on Twitter @kenshirriff or RSS for updates. I'm also on Mastodon occasionally as @[email protected].

Notes and references

1. The earliest reference for a barrel shifter is often given as "A barrel switch design", Computer Design, 1972, but the idea of a barrel shifter goes back to 1964 at least. (The "barrel switch" name presumably comes from a physical barrel switch, a cylindrical multi-position switch such as a car ignition.) The CDC 6600 supercomputer (1964) had a 6-stage shifter able to shift up to 63 positions in one cycle (details); it was called a "parallel shifting network" rather than a "barrel shifter". A Burroughs patent filed in 1965 describes a barrel switch "capable of performing logical switching operations in a single time involving any amount of binary information," so the technology is older.

   Early microprocessors shifted by one bit position at a time. Although the Intel 8086 provided instructions to shift by multiple bits at a time, this was implemented internally by a microcode loop, so the more bits you shifted, the longer the instruction took, four clock cycles per bit. Shifting on the 286 was faster, taking one additional cycle for each bit position shifted. The first ARM processor (ARM1, 1985) included a 32-bit barrel shifter. It was considerably simpler than the 386's design, following ARM's RISC philosophy. ↩

2. The 386 Hardware Reference Manual states that the 386 contains a 64-bit barrel shifter. I find this description a bit inaccurate, since the output is only 32 bits, so the barrel shifter is much simpler than a full 64-bit barrel shifter. ↩

3. The 386 has two layers of metal. The vertical lines are in the lower layer of metal (metal 1) while the horizontal lines are in the upper layer of metal (metal 2). Transistors can only connect to lower metal, so the connection between the horizontal line and the transistor uses a short piece of lower metal to bridge the layers. ↩

4. Each row of the matrix can be considered a multiplexer with 8 inputs, implemented by 8 pass transistors. One of the eight transistors is activated, passing that input to the output. ↩

5. The image below shows the full shift matrix. Click the image for a much larger view.

   

   The matrix with the metal layer removed.

    ↩

6. The keepers are arranged with 6 blocks of three on the left and 6 blocks of 3 on the right, plus an additional one at the bottom right. ↩

7. The standard latch in the 386 consists of two cross-coupled inverters forming a static circuit to hold a bit. The input goes through a transmission gate (back-to-back NMOS and PMOS transistors) to the inverters. One inverter is weak, so it can be overpowered by the input. The 8086, in contrast, uses dynamic latches that depend on the gate capacitance to hold a bit. ↩

8. Some shifters take the idea of combining shift circuits to the extreme. If you combine a shift-by-one circuit, a shift-by-two circuit, a shift-by-four circuit, and so forth, you end up with a logarithmic shifter: selecting the appropriate stages provide an arbitrary shift. (This design was used in the CDC 6600.) This design has the advantage of reducing the amount of circuitry since it uses log₂(N) layers rather than N layers. However, the logarithmic approach has performance disadvantages since the signals need to go through more circuitry. This paper describes various design alternatives for barrel shifters. ↩

9. The basic rotate left and right instructions date back to the Datapoint 2200, the ancestor of the 8086 and x86. The rotate left through carry and rotate right through carry instructions in x86 were added in the Intel 8008 processor and the 8080 was the same. The MOS 6502 had a different set of rotates and shifts: arithmetic shift left, rotate left, logical shift right, and rotate right; the rotate instructions rotated through the carry. The Z-80 had a more extensive set: rotates left and right, either through the carry or not, shift left, shift right logical, shift right arithmetic, and 4-bit digit rotates left and right through two bytes. The 8086's set of rotates and shifts was similar to the Z-80, except it didn't have the digit rotates. The 8086 also supported shifting and rotating by multiple positions. This illustrates that there isn't a "natural" set of shift and rotate instructions. Instead, different processors supported different instructions, with complexity generally increasing over time. ↩

10. The x86 uses "word" to refer to a 16-bit value and "double word" or "dword" to refer to a 32-bit value. I'm going to ignore the word/dword distinction. ↩

Inside the Intel 386 processor die: the clock circuit

Processors are driven by a clock, which controls the timing of each step inside the chip. In this blog post, I'll examine the clock-generation circuitry inside the Intel 386 processor. Earlier processors such as the 8086 (1978) were simpler, using two clock phases internally. The Intel 386 processor (1985) was a pivotal development for Intel as it moved x86 to CMOS (as well as being the first 32-bit x86 processor). The 386's CMOS circuitry required four clock signals. An external crystal oscillator provided the 386 with a single clock signal and the 386's internal circuitry generated four carefully-timed internal clock signals from the external clock.

The die photo below shows the Intel 386 processor with the clock generation circuitry and clock pad highlighted in red. The heart of a processor is the datapath, the components that hold and process data. In the 386, these components are in the lower left: the ALU (Arithmetic/Logic Unit), a barrel shifter to shift data, and the registers. These components form regular rectangular blocks, 32 bits wide. In the lower right is the microcode ROM, which breaks down machine instructions into micro-instructions, the low-level steps of the instruction. Other parts of the chip prefetch and decode instructions, and handle memory paging and segmentation. All these parts of the chip run under the control of the clock signals.

The 386 with the main functional blocks labeled. Click this image (or any other) for a larger version.

A brief discussion of clock phases

Many processors use a two-phase clock to control the timing of the internal processing steps. The idea is that the two clock phases alternate: first phase 1 is high, and then phase 2 is high, as shown below. During each clock phase, logic circuitry processes data. A circuit called a "transparent latch" is used to hold data between steps.2 The concept of a latch is that when a latch's clock input is high, the input passes through the latch. But when the latch's clock input is low, the latch remembers its previous value. With two clock phases, alternating latches are active one at a time, so data passes through the circuit step by step, under the control of the clock.

The two-phase clock signal used by the Intel 8080 processor. The 8080 uses asymmetrical clock signals, with phase 2 longer than phase 1. From the 8080 datasheet.

The diagram below shows an abstracted model of the processor circuitry. The combinational logic (i.e. the gate logic) is divided into two blocks, with latches between each block. During clock phase 1, the first block of latches passes its input through to the output. Thus, values pass through the first logic block, the first block of latches, and the second logic block, and then wait.

Action during clock phase 1.

During clock phase 2 (below), the first block of latches stops passing data through and holds the previous values. Meanwhile, the second block of latches passes its data through. Thus, the first logic block receives new values and performs logic operations on them. When the clock switches to phase 1, processing continues as in the first diagram. The point of this is that processing takes place under the control of the clock, with values passed step-by-step between the two logic blocks.1

Action during clock phase 2.

This circuitry puts some requirements on the clock timing. First, the clock phases must not overlap. If both clocks are active at the same time, data will flow out of control around the loop, messing up the results.3 Moreover, because the two clock phases probably don't arrive at the exact same time (due to differences in the wiring paths), a "dead zone" is needed between the two phases, an interval where both clocks are low, to ensure that the clocks don't overlap even if there are timing skews. Finally, the clock frequency must be slow enough that the logic has time to compute its result before the clock switches.

Many processors such as the 8080, 6502, and 8086 used this type of two-phase clocking. Early processors such as the 8008 (1972) and 8080 (1974) required complicated external circuitry to produce two asymmetrical clock phases.4 For the 8080, Intel produced a special clock generator chip (the 8224) that produced the two clock signals according to the required timing. The Motorola 6800 (1974) required two non-overlapping (but at least symmetrical) clocks, produced by the MC6875 clock generator chip. The MOS 6502 processor (1975) simplified clock generation by producing the two phases internally (details) from a single clock input. This approach was used by most later processors.

An important factor is that the Intel 386 processor was implemented with CMOS circuitry, rather than the NMOS transistors of many earlier processors. A CMOS chip uses both NMOS transistors (which turn on when the gate is high) and PMOS transistors (which turn on when the gate is low).7 Thus, the 386 requires an active-high clock signal and an active-low clock signal for each phase,5 four clock signals in total.6 In the rest of this article, I'll explain how the 386 generates these four clock signals.

The clock circuitry

The block diagram below shows the components of the clock generation circuitry. Starting at the bottom, the input clock signal (CLK2, at twice the desired frequency) is divided by two to generate two drive signals with opposite phases. These signals go to the large driver circuits in the middle, which generate the two main clock signals (phase 1 and phase 2). Each driver sends an "inhibit" signal to the other when active, ensuring that the phases don't overlap. Each driver also sends signals to a smaller driver that generates the inverted clock signal. The "enable" signal shapes the output to prevent overlap. The four clock output signals are then distributed to all parts of the processor.

Block diagram of the clock circuitry. The layout of the blocks matches their approximate physical arrangement.

The diagram below shows a closeup of the clock circuitry on the die. The external clock signal enters the die at the clock pad in the lower right. The signal is clamped by protection diodes and a resistor before passing to the divide-by-two logic, which generates the two clock phases. The four driver blocks generate the high-current clock pulses that are transmitted to the rest of the chip by the four output lines at the left.

Details of the clock circuitry. This image shows the two metal layers. At the right, bond wires are connected to the pads on the die.

Input protection

The 386 has a pin "CLK2" that receives the external clock signal. It is called CLK2 because this signal has twice the frequency of the 386's clock. The chip package connects the CLK2 pin through a tiny bond wire (visible above) to the CLK2 pad on the silicon die. The CLK2 input has two protection diodes, created from MOSFETs, as shown in the schematic below. If the input goes below ground or above +5 volts, the corresponding diode will turn on and clamp the excess voltage, protecting the chip. The schematic below shows how the diodes are constructed from an NMOS transistor and a PMOS transistor. The schematic corresponds to the physical layout of the circuit, so power is at the bottom and the ground is at the top.

The input protection circuit. The left shows the physical circuit built from an NMOS transistor and a PMOS transistor, while the right shows the equivalent diode circuit.

The diagram below shows the implementation of these protection diodes (i.e. transistors) on the die. Each transistor is much larger than the typical transistors inside the 386, because these transistors must be able to handle high currents. Physically, each transistor consists of 12 smaller (but still relatively large) transistors in parallel, creating the stripes visible in the image. Each transistor block is surrounded by two guard rings, which I will explain in the next section.

This diagram shows the circuitry next to the clock pad.

Latch-up and the guard rings

The phenomenon of "latch-up" is the hobgoblin of CMOS circuitry, able to destroy a chip. Regions of the silicon die are doped with impurities to form N-type and P-type silicon. The problem is that the N- and P-doped regions in a CMOS chip can act as parasitic NPN and PNP transistors. In some circumstances, these transistors can turn on, shorting power and ground. Inconveniently, the transistors latch into this state until the power is removed or the chip burns up. The diagram below shows how the substrate, well, and source/drain regions can combine to act as unwanted transistors.8

This diagram illustrates how the parasitic NPN and PNP transistors are formed in a CMOS chip. Note that the 386's construction is opposite from this diagram, with an N substrate and P well. Image by Deepon, CC BY-SA 3.0.

Normally, P-doped substrate or wells are connected to ground and the N-doped substrate or wells are connected to +5 volts. As a result, the regions act as reverse-biased diodes and no current flows through the substrate. However, a voltage fluctuation or large current can disturb the reverse biasing and the resulting current flow will turn on these parasitic transistors. Unfortunately, these parasitic transistors drive each other in a feedback loop, so once they get started, they will conduct more and more strongly and won't stop until the chip is powered down. The risk of latch-up is highest with circuits connected to the unpredictable voltages of the outside world, or high-current circuits that can cause power fluctuations. The clock circuitry has both of these risks.

One way of protecting against latch-up is to put a guard ring around a potentially risky circuit. This guard ring will conduct away the undesired substrate current before it can cause latch-up. In the case of the 386, two concentric guard rings are used for additional protection.9 In the earlier die photo, these guard rings can be seen surrounding the transistors. Guard rings will also play a part in the circuitry discussed below.

Polysilicon resistor

After the protection diodes, the clock signal passes through a polysilicon resistor, followed by another protection diode. Polysilicon is a special form of silicon that is used for wiring and also forms the transistor gates. The polysilicon layer sits on top of the base silicon; polysilicon has a moderate amount of resistance, considerably more than metal, so it can be used as a resistor.

The image below shows the polysilicon resistor along with a protection diode. This circuit provides additional protection against transients in the clock signal.10 This circuit is surrounded by two concentric guard rings for more latch-up protection.

The polysilicon resistor and associated diode.

The divide-by-two logic

The input clock to the 386 runs at twice the frequency of the internal clock. The circuit below divides the input clock by 2, producing complemented outputs. This circuit consists of two set-reset latch stages, one driven by the input clock inverted and the second driven by the input clock, so the circuit will update once per input clock cycle. Since there are three inversions in the loop, the output will be inverted for each update, so it will cycle at half the rate of the input clock. The reset input is asymmetrical: when it is low, it will force the output low and the complemented output high. Presumably, this ensures that the processor starts with the correct clock phase when exiting the reset state.

The divide-by-two circuit.

I have numbered the gates above to match their physical locations below. In this image, I have etched the chip down to the silicon so you can see the active silicon regions. Each logic gate consists of PMOS transistors in the upper half and NMOS transistors in the lower half. The thin stripes are the transistor gates; the two-input NAND gates have two PMOS transistors and two NMOS transistors, while the three-input NAND gates have three of each transistor. The AND-NOR gates need to drive other circuits, so they use paralleled transistors and are much larger. Each AND-NOR gate contains 12 PMOS transistors, four for each input, but uses only 9 NMOS transistors. Finally, the inverter (7) inverts the input clock signal for this circuit. The transistors in each gate are sized to maximize performance and minimize power consumption. The two outputs from the divider then go through large inverters (not shown) that feed the driver circuits.11

The silicon for the divide-by-two circuit as it appears on the die.

The drivers

Because the clock signals must be transmitted to all parts of the die, large transistors are required to generate the high-current pulses. These large transistors, in turn, are driven by medium-sized transistors. Additional driver circuitry ensures that the clock signals do not overlap. There are four driver circuits in total. The two larger, lower driver circuits generate the positive clock pulses. These drivers control the two smaller, upper driver circuits that generate the inverted clock pulses.

First, I'll discuss the larger, positive driver circuit. The core of the driver consists of the large PMOS transistor (1) to pull the output high, and the large NMOS transistor (1) to pull the output low. Each transistor is driven by two inverters (2/3 and 6/7 respectively). The circuit also produces two signals to shape the outputs from the other drivers. When the clock output is high, the "inhibit" signal goes to the other lower driver and inhibits that driver from pulling its output high.12 This prevents overlap in the output between the two drivers. When the clock output is low, an "enable" output goes to the inverted driver (discussed below) to enable its output. The transistor sizes and propagation delays in this circuit are carefully designed to shape the internal clock pulses as needed.

Schematic of the lower driver.

The diagram below shows how this driver is implemented on the die. The left image shows the two metal layers. The right image shows the transistors on the underlying silicon. The upper section holds PMOS transistors, while the lower section holds NMOS transistors. Because PMOS transistors have poorer performance than NMOS transistors, they need to be larger, so the PMOS section is larger. The transistors are numbered, corresponding to the schematic above. Each transistor is physically constructed from multiple transistors in parallel. The two guard rings are visible in the silicon, surrounding and separating the PMOS and NMOS regions.

One of the lower drivers. The left image shows metal while the right image shows silicon.

The 386 has two layers of metal wiring. In this circuit, the top metal layer (M2) provides +5 for the PMOS transistors, ground for the NMOS transistors, and receives the output, all through large rectangular regions. The lower metal layer (M1) provides the physical source and drain connections to the transistors as well as the wiring between the transistors. The pattern of the lower metal layer is visible in the left photo. The dark circles are connections between the lower metal layer and the transistors or the upper metal layer. The connections to the two guard rings are visible around the edges.

Next, I'll discuss the two upper drivers that provided the inverted clock signals. These drivers are smaller, presumably because less circuitry needs the inverted clocks. Each upper driver is controlled by enable and drive from the corresponding lower driver. As before, two large transistors pull the output high or low, and are driven by inverters. The enable input must be high for inverter 4 to go low Curiously, the enable input is wired to the output of inverter 4. Presumably, this provides a bit of shaping to the signal.

Schematic of the upper driver.

The layout (below) is roughly similar to the previous driver, but smaller. The driver transistors (1) are arranged vertically rather than horizontally, so the metal 2 rectangle to get the output is on the left side rather than in the middle. The transistor wiring is visible in the lower (metal 1) layer, running vertically through the circuit. As before, two guard rings surround the PMOS and NMOS regions.

One of the upper drivers. The left image shows metal while the right image shows silicon.

Distribution

Once the four clock signals have been generated, they are distributed to all parts of the chip. The 386 has two metal layers. The top metal layer (M2) is thicker, so it has lower resistance and is used for clock (and power) distribution where possible. The clock signal will use the lower M1 metal layer when necessary to cross other M2 signals, as well as for branch lines off the main clock lines.

The diagram below shows part of the clock distribution network; the four parallel clock lines are visible similarly throughout the chip. The clock signal arrives at the upper right and travels to the datapath circuitry on the left. As you can see, the four clock lines are much wider than the thin signal lines; this width reduces the resistance of the wiring, which reduces the RC (resistive-capacitive) delay of the signals. The outlined squares at each branch are the vias, connections between the two metal layers. At the right, the incoming clock signals are in layer M1 and zig-zag to cross under other signals in M2. The clock distribution scheme in the 386 is much simpler than in modern processors.

Part of the wiring for clock distribution. This image spans about 1/5 of the chip's width.

Clocks in modern processors

The 386's internal clock speed was simply the external clock divided by 2. However, modern processors allow the clock speed to be adjusted to optimize performance or to overclock the chip. This is implemented by an on-chip PLL (Phase-Locked Loop) that generates the internal clock from a fixed external clock, multiplying the clock speed by a selectable multiplier. Intel introduced a PLL to the 80486 processor, but the multipler was fixed until the Pentium.

The Intel 386's clock can go up to 40 megahertz. Although this was fast for the time, modern processors are over two orders of magnitude faster, so keeping the clock synchronized in a modern processor requires complex techniques.13 With fast clocks, even the speed of light becomes a constraint; at 6 GHz, light can travel just 5 centimeters during a clock cycle.

The problem is to ensure that the clock arrives at all circuits at the same time, minimizing "clock skew". Modern processors can reduce the clock skew to a few picoseconds. The clock is typically distributed by a "clock tree", where the clock is split into branches with each branch buffered and the same length, so the delays nearly match. One approach is an "H-tree", which distributes the clock through an H-shaped path. Each leg of the H branches into a smaller H recursively, forming a space-filling fractal, as shown below.

Clock distribution in a PowerPC chip. The recursive H pattern is only approximate since other layout factors constrain the clock tree. From ISSCC 2000.

Delay circuitry can actively compensate for differences in path time. A Delay-Locked Loop (DLL) circuit adds variable delays to counteract variations along different clock paths. The Itanium used a clock distribution hierarchy with global, regional, and local distribution of the clock. The main clock was distributed to eight regions that each deskewed the clock (in 8.5 ps steps) and drove a regional clock grid, keeping the clock skew under 28 ps. The Pentium 4's complex distribution tree and skew compensation circuitry got clock skew below ±8 ps.

Conclusions

The 386's clock circuitry turned out to be more complicated than I expected, with a lot of subtlety and complications. However, examining the circuit illustrates several features of CMOS design, from latch circuits and high-current drivers to guard rings and multi-phase clocks. Hopefully you have found this interesting.

I plan to write more about the 386, so follow me on Twitter @kenshirriff or RSS for updates. I'm also on Mastodon occasionally as @[email protected].

Thanks to William Jones for discussing a couple of errors.

Notes and references

1. You might wonder why processors use transparent latches and two clock phases instead of using edge-triggered flip-flops and a single clock phase. First, edge-triggered flip-flops take at least twice as many transistors as latches. (An edge-triggered flip flop is often built from two latch stages.) Second, the two-phase approach allows processing to happen twice per clock cycle, rather than once per clock cycle. This may allow a faster implementation with more pipelining. ↩

2. The transparent latch was implemented by a single pass transistor in processors such as the MOS 6502. When the transistor was on, the input signal passed through. But when the transistor was off, the former value was held by the transistor's gate capacitance. Eventually the charge on the gate would leak away (like DRAM), so a minimum clock speed was required for reliable operation. ↩

3. To see why having multiple stages active at once is bad, here's a simplified example. Consider a circuit that increments the accumulator register. In the first clock phase, the accumulator's value might go through the adder circuit. In the second clock phase, the new value can be stored in the accumulator. If both clock phases are high at the same time, the circuit will form a loop and the accumulator will get incremented multiple times, yielding the wrong result. Moreover, different parts of the adder probably have different delays, so the result is likely to be complete garbage. ↩

4. To generate the clocks for the Intel 8008 processor, the suggested circuit used four analog (one-shot) delays to generate the clock phases. The 8008 and 8080 required asymmetrical clocks because the two blocks of logic took different amounts of time to process their inputs. The asymemtrical clock minimized wasted time, improving performance. (More discussion here.) ↩

5. You might think that the 386 could use two clock signals: one latch could use phase 1 for NMOS and phase 2 for PMOS, while the next stage is the other way around. Unfortunately, that won't work because the two phases aren't exactly complements. During the "dead time" when phase 1 and phase 2 are both low, the PMOS transistors for both stages will turn on, causing problems. ↩

6. Even though the 80386 has four clock signals internally, there are really just two clock phases. This is different from four-phase logic, a type of logic that was used in the late 1960s in some MOS processor chips. Four-phase logic was said to provide 10 times the density, 10 times the speed, and 1/10 the power consumption of standard MOS logic techniques. Designer Lee Boysel was a strong proponent of four-phase logic, forming the company Four Phase Systems and building a processor from a small number of MOS chips. Improvements in MOS circuitry in the 1970s (in particular depletion-mode logic) made four-phase logic obsolete. ↩

7. The clocking scheme in the 386 is closely tied to the latch circuit used in the processor, shown below. This is a transparent latch: when enable is high and the complemented enable is low, the input is passed through to the output (inverted). When enable is low and the complemented enable is high, the latch remembers the previous value. The important factor is that the enable and complemented enable inputs must switch in lockstep. (In comparison, earlier chips such as the 8086 used a dynamic latch built from one transistor that used a single enable input.)

   

   The basic latch circuit used in the 386.

   The circuit on the right shows the implementation of the 386 latch. The two transistors on the left form a transmission gate: when both transistors are on, the input is passed through, but when both transistors are off, the input is blocked. Data storage is implemented through the two inverters connected in a loop. The bottom inverter is "weak", generating a small output current. Because of this, its output will be overpowered by the input, replacing the value stored in the latch. This latch uses 6 transistors in total.

   The 386 uses several variants of the latch circuit, for instance with set or reset inputs, or multiplexers to select multiple data inputs. ↩

8. The parasitic transistors responsible for latch-up can also be viewed as an SCR (silicon-controlled rectifier) or thyristor. An SCR is a four-layer (PNPN) silicon device that is switched on by its gate and remains on until power is removed. SCRs were popular in the 1970s for high-current applications, but have been replaced by transistors in many cases. ↩

9. The 386 uses two guard rings to prevent latch-up. NMOS transistors are surrounded by an inner N+ guard ring connected to ground and an outer P+ guard ring connected to +5. The guard rings are reversed for PMOS transistors. This page has a diagram showing how the guard rings prevent latch-up. ↩

10. The polysilicon resistor appears to be unique to the clock input. My hypothesis is that the CLK2 signal runs at a much higher frequency than other inputs (since it is twice the clock frequency), which raises the risk of ringing or other transients. If these transients go below ground, they could cause latch-up, motivating additional protection on the clock input. ↩

11. To keep the main article focused, I'll describe the inverters in this footnote. The circuitry below is between the divider logic and the polysilicon resistor, and consists of six inverters of various sizes. The large inverters 1 and 2 buffer the output from the divider to send to the drivers. Inverter 3 is a small inverter that drives larger inverter 4. I think this clock signal goes to the bus interface logic, perhaps to ensure that communication with the outside world is synchronized with the external clock, rather than the internal clock, which is shaped and perhaps slightly delayed. The output of small inverter 5 appears to be unused. My hypothesis is that this is a "dummy" inverter to match inverter 3 and ensure that both clock phases have identical circuitry. Otherwise, the load from inverter 3 might make that phase switch slightly slower.

    

    The inverters that buffer the divider's output.

    The final block of logic is shown below. This logic appears to take the chip reset signal from the reset pin and synchronize it with the clock. The first three latches use the CLK2 input as the clock, while the last two latches use the internal clock. Using the external reset signal directly would risk metastability because the reset signal could change asynchronously with respect to the rest of the system. The latches ensure that the timing of the reset signal matches the rest of the system, minimizing the risk of metastability. The NAND gate generates a reset pulse that resets the divide-by-two counter to ensure that it starts in a predictable state.

    

    The reset synchronizer. (Click for a larger image.)

     ↩

12. The gate (2) that receives the inhibit signal is a bit strange, a cross between an inverter and a NAND gate. The gate goes low if the clk' input is high, but goes high only if both inputs are low. In other words, it acts like an inverter but the inhibit signal blocks the transition to the high output. Instead, the output will "float" with its previous low value. This will keep the driver's output low, ensuring that it doesn't overlap with the other driver's high output.

    The upper driver has a similar gate (4), except the extra input (enable) is on the NMOS side so the polarity is reversed. That is, the enable input must be high in order for the inverter to go low. ↩

13. An interesting 2004 presentation is Clocking for High Performance Processors. A 2005 Intel presentation also discusses clock distribution. ↩

Reverse engineering the Intel 386 processor's register cell

The groundbreaking Intel 386 processor (1985) was the first 32-bit processor in the x86 line. It has numerous internal registers: general-purpose registers, index registers, segment selectors, and more specialized registers. In this blog post, I look at the silicon die of the 386 and explain how some of these registers are implemented at the transistor level. The registers that I examined are implemented as static RAM, with each bit stored in a common 8-transistor circuit, known as "8T". Studying this circuit shows the interesting layout techniques that Intel used to squeeze two storage cells together to minimize the space they require.

The diagram below shows the internal structure of the 386. I have marked the relevant registers with three red boxes. Two sets of registers are in the segment descriptor cache, presumably holding cache entries, and one set is at the bottom of the data path. Some of the registers at the bottom are 32 bits wide, while others are half as wide and hold 16 bits. (More registers with different circuits, but I won't discuss them in this post.)

The 386 with the main functional blocks labeled. Click this image (or any other) for a larger version. I created this image using a die photo from Antoine Bercovici.

The 6T and 8T static RAM cells

First, I'll explain how a 6T or 8T static cell holds a bit. The basic idea behind a static RAM cell is to connect two inverters into a loop. This circuit will be stable, with one inverter on and one inverter off, and each inverter supporting the other. Depending on which inverter is on, the circuit stores a 0 or a 1.

Two inverters in a loop can store a 0 or a 1.

To write a new value into the circuit, two signals are fed in, forcing the inverters to the desired new values. One inverter receives the new bit value, while the other inverter receives the complemented bit value. This may seem like a brute-force way to update the bit, but it works. The trick is that the inverters in the cell are small and weak, while the input signals are higher current, able to overpower the inverters.1 The write data lines (called bitlines) are connected to the inverters by pass transistors.2 When the pass transistors are on, the signals on the write lines can pass through to the inverters. But when the pass transistors are off, the inverters are isolated from the write lines. Thus, the write control signal enables writing a new value to the inverters. (This signal is called a wordline since it controls access to a word of storage.) Since each inverter consists of two transistors7, the circuit below consists of six transistors, forming the 6T storage cell.

Adding pass transistor so the cell can be written.

The 6T cell uses the same bitlines for reading and writing. Adding two transistors creates the 8T circuit, which has the advantage that you can read one register and write to another register at the same time. (I.e. the register file is two-ported.) In the 8T cell below, two additional transistors (G and H) are used for reading. Transistor G buffers the cell's value; it turns on if the inverter output is high, pulling the read output bitline low.3 Transistor H is a pass transistor that blocks this signal until a read is performed on this register; it is controlled by a read wordline.

Schematic of a storage cell. Each transistor is labeled with a letter.

To form registers (or memory), a grid is constructed from these cells. Each row corresponds to a register, while each column corresponds to a bit position. The horizontal lines are the wordlines, selecting which word to access, while the vertical lines are the bitlines, passing bits in or out of the registers. For a write, the vertical bitlines provide the 32 bits (along with their complements). For a read, the vertical bitlines receive the 32 bits from the register. A wordline is activated to read or write the selected register.

Static memory cells (8T) organized into a grid.

Silicon circuits in the 386

Before showing the layout of the circuit on the die, I should give a bit of background on the technology used to construct the 386. The 386 was built with CMOS technology, with NMOS and PMOS transistors working together, an advance over the earlier x86 chips that were built with NMOS transistors. Intel called this CMOS technology CHMOS-III (complementary high-performance metal-oxide-silicon), with 1.5 µm features. While Intel's earlier chips had a single metal layer, CHMOS-III provided two metal layers, making signal routing much easier.

Because CMOS uses both NMOS and PMOS transistors, fabrication is more complicated. In an MOS integrated circuit, a transistor is formed where a polysilicon wire crosses active silicon, creating the transistor's gate. A PMOS transistor is constructed directly on the silicon substrate (which is N-doped). However, an NMOS transistor is the opposite, requiring a P-doped substrate. This is created by forming a P well, a region of P-doped silicon that holds NMOS transistors. Each P well must be connected to ground; this is accomplished by connecting ground to specially-doped regions of the P well, called "well taps"`.

The diagram below shows a cross-section through two transistors, showing the layers of the chip. There are four important layers: silicon (which has some regions doped to form active silicon), polysilicon for wiring and transistors, and the two metal layers. At the bottom is the silicon, with P or N doping; note the P-well for the NMOS transistor on the left. Next is the polysilicon layer. At the top are the two layers of metal, named M1 and M2. Conceptually, the chip is constructed from flat layers, but the layers have a three-dimensional structure influenced by the layers below. The layers are separated by silicon dioxide ("ox") or silicon oxynitride4; the oxynitride under M2 caused me considerable difficulty.

A cross-section of circuitry formed with the CHMOS-III process. From A double layer metal CHMOS III technology.

The image below shows how circuitry appears on the die;5 I removed the metal layers to show the silicon and polysilicon that form transistors. (As will be described below, this image shows two static cells, holding two bits.) The pinkish and dark regions are active silicon, doped to take part in the circuits, while the "background" silicon can be ignored. The green lines are polysilicon lines on top of the silicon. Transistors are the most important feature here: a transistor gate is formed when polysilicon crosses active silicon, with the source and drain on either side. The upper part of the image has PMOS transistors, while the lower part of the image has the P well that holds NMOS transistors. (The well itself is not visible.) In total, the image shows four PMOS transistors and 12 NMOS transistors. At the bottom, the well taps connect the P well to ground. Although the metal has been removed, the contacts between the lower metal layer (M1) and the silicon or polysilicon are visible as faint circles.

A (heavily edited) closeup of the die.

Register layout in the 386

Next, I'll explain the layout of these cells in the 386. To increase the circuit density, two cells are put side-by-side, with a mirrored layout. In this way, each row holds two interleaved registers.6 The schematic below shows the arrangement of the paired cells, matching the die image above. Transistors A and B form the first inverter,7 while transistors C and D form the second inverter. Pass transistors E and F allow the bitlines to write the cell. For reading, transistor G amplifies the signal while pass transistor H connects the selected bit to the output.

Schematic of two static cells in the 386. The schematic approximately matches the physical layout.

The left and right sides are approximately mirror images, with separate read and write control lines for each half. Because the control lines for the left and right sides are in different positions, the two sides have some layout differences, in particular, the bulging loop on the right. Mirroring the cells increases the density since the bitlines can be shared by the cells.

The diagram below shows the various components on the die, labeled to match the schematic above. I've drawn the lower M1 metal wiring in blue, but omitted the M2 wiring (horizontal control lines, power, and ground). "Read crossover" indicates the connection from the read output on the left to the bitline on the right. Black circles indicate vias between M1 and M2, green circles indicate contacts between silicon and M1, and reddish circles indicate contacts between polysilicon and M1.

The layout of two static cells. The M1 metal layer is drawn in blue; the horizontal M2 lines are not shown.

One more complication is that alternating registers (i.e. rows) are reflected vertically, as shown below. This allows one horizontal power line to feed two rows, and similarly for a horizontal ground line. This cuts the number of power/ground lines in half, making the layout more efficient.

Multiple storage cells.

Having two layers of metal makes the circuitry considerably more difficult to reverse engineer. The photo below (left) shows one of the static RAM cells as it appears under the microscope. Although the structure of the metal layers is visible in the photograph, there is a lot of ambiguity. It is difficult to distinguish the two layers of metal. Moreover, the metal completely hides the polysilicon layer, not to mention the underlying silicon. The large black circles are vias between the two metal layers. The smaller faint circles are contacts between a metal layer and the underlying silicon or polysilicon.

One cell as it appears on the die, with a diagram of the upper (M2) and lower (M1) metal layers.

With some effort, I determined the metal layers, which I show on the right: M2 (upper) and M1 (lower). By comparing the left and right images, you can see how the structure of the metal layers is somewhat visible. I use black circles to indicate vias between the layers, green circles indicate contacts between M1 and silicon, and pink circles indicate contacts between M1 and polysilicon. Note that both metal layers are packed as tightly as possible. The layout of this circuit was highly optimized to minimize the area. It is interesting to note that decreasing the size of the transistors wouldn't help with this circuit, since the size is limited by the metal density. This illustrates that a fabrication process must balance the size of the metal features, polysilicon features, and silicon features since over-optimizing one won't help the overall chip density.

The photo below shows the bottom of the register file. The "notch" makes the registers at the very bottom half-width: 4 half-width rows corresponding to eight 16-bit registers. Since there are six 16-bit segment registers in the 386, I suspect these are the segment registers and two mystery registers.

The bottom of the register file.

I haven't been able to determine which registers in the 386 correspond to the other registers on the die. In the segment descriptor circuitry, there are two rows of register cells with ten more rows below, corresponding to 24 32-bit registers. These are presumably segment descriptors. At the bottom of the datapath, there are 10 32-bit registers with the T8 circuit. The 386's programmer-visible registers consist of eight general-purpose 32-bit registers (EAX, etc.). The 386 has various control registers, test registers, and segmentation registers8 that are not well known. The 8086 has a few registers for internal use that aren't visible to the programmer, so the 386 presumably has even more invisible registers. At this point, I can't narrow down the functionality.

Conclusions

It's interesting to examine how registers are implemented in a real processor. There are plenty of descriptions of the 8T static cell circuit, but it turns out that the physical implementation is more complicated than the theoretical description. Intel put a lot of effort into optimizing this circuit, resulting in a dense block of circuitry. By mirroring cells horizontally and vertically, the density could be increased further.

Reverse engineering one small circuit of the 386 turned out to be pretty tricky, so I don't plan to do a complete reverse engineering. The main difficulty is the two layers of metal are hard to untangle. Moreover, I lost most of the polysilicon when removing the metal. Finally, it is hard to draw diagrams with four layers without the diagram turning into a mess, but hopefully the diagrams made sense.

I plan to write more about the 386, so follow me on Twitter @kenshirriff or RSS for updates. I'm also on Mastodon occasionally as @[email protected].

Notes and references

1. Typically the write driver circuit generates a strong low on one of the bitlines, flipping the corresponding inverter to a high output. As soon as one inverter flips, it will force the other inverter into the right state. To support this, the pullup transistors in the inverters are weaker than normal. ↩

2. The pass transistor passes its signal through or blocks it. In CMOS, this is usually implemented with a transmission gate with an NMOS and a PMOS transistor in parallel. The cell uses only the NMOS transistor, which makes it worse at passing a high signal, but substantially reduces the size, a reasonable tradeoff for a storage cell. ↩

3. The bitline is typically precharged to a high level for a read, and then the cell pulls the line low for a 0. This is more compact than including circuitry in each cell to pull the line high. ↩

4. One problem is that the 386 uses a layer of insulating silicon oxynitride as well as the usual silicon dioxide. I was able to remove the oxynitride with boiling phosphoric acid, but this removed most of the polysilicon as well. I'm still experimenting with the timing; 20 minutes of boiling was too long. ↩

5. The image is an edited composite of multiple cells since the polysilicon was highly damaged when removing the metal layers. Unfortunately, I haven't found a process for the 386 to remove one layer of metal at a time. As a result, reverse-engineering the 386 is much more difficult than earlier processors such as the 8086; I have to look for faint traces of polysilicon and puzzle over what connections the circuit requires. ↩

6. You might wonder why they put two cells side-by-side instead of simply cramming the cells together more tightly. The reason for putting two cells in each row is presumably to match the size of each bit with the rest of the circuitry in the datapath. If the register circuitry is half the width of the ALU circuitry, a bunch of space will be wasted by the wiring to line up each register bit with the corresponding ALU bit. ↩

7. A CMOS inverter is constructed from an NMOS transistor (which pulls the output low on a 1 input) and a PMOS transistor (which pulls the output high on a 0 input), as shown below.

   

   A CMOS inverter.

    ↩↩

8. The 386 has multiple registers that are documented but not well known. Chapter 4 of the 386 Programmers Reference Manual discusses various registers that are only relevant to operating systems programmers. These include the Global Descriptor Table Register (GDTR), Local Descriptor Table Register (LDTR), Interrupt Descriptor Table Register (IDTR), and Task Register (TR). There are four Control Registers CR0-CR3; CR0 controls coprocessor usage, paging, and a few other things. The six Debug Registers for hardware breakpoints are named DR0-DR3, DR6, and DR7 (which suggests undocumented DR4 and DR5 registers). The two Test Registers for TLB testing are named TR6 and TR7 (which suggests undocumented TR0-TR5 registers). I expect that these registers are located near the relevant functional units, rather than part of the processing datapath. ↩