50 Years Later: The revolutionary 8008 microprocessor


Intel’s groundbreaking 8008 microprocessor was first produced over 50 years in the past. This was Intel’s first 8-bit microprocessor and the ancestor of the x86 processor household that you could be be utilizing proper now. I could not discover good die images of the 8008, so I opened one up and took some detailed images. These new die images are on this article, together with a dialogue of the 8008’s inner design.

The photograph beneath exhibits the tiny silicon die contained in the 8008 bundle (click on for greater decision photograph). You can barely see the wires and transistors that make up the chip. The squares across the outdoors are the 18 pads which are linked to the exterior pins by tiny bond wires.

You can see the textual content “8008” on the fitting fringe of the chip and “© Intel 1971” on the decrease edge. The initials HF seem on the highest proper for Hal Feeney, who did the chip’s logic design and bodily format. Other key designers of the 8008 have been Ted Hoff, Stan Mazor, and Federico Faggin.

Inside the chip

The diagram beneath highlights a few of the main purposeful blocks of the chip. On the left is the 8-bit Arithmetic/Logic Unit (ALU), which performs the precise information computations.3

The ALU makes use of two momentary registers to carry its enter values. These registers take up important space on the chip, not as a result of they’re complicated, however as a result of they want massive transistors to drive indicators by way of the ALU circuitry.

Below the registers is the carry look forward circuitry. For addition and subtraction, this circuit computes all eight carry values in parallel to enhance efficiency.2 Since the low-order carry will depend on simply the low-order bits, whereas the higher-order carries depend upon a number of bits, the circuit block has a triangular form.

The triangular format of the ALU is uncommon. Most processors stack the circuitry for every bit into an everyday rectangle (a bit-slice format). The 8008, nevertheless, has eight blocks (one for every bit) organized haphazardly to suit across the house left by the triangular carry generator. The ALU helps eight easy operations.3

In the middle of the chip is the instruction register and the instruction decoding logic that determines the which means of every 8-bit machine instruction. Decoding is finished with a Programmable Logic Array (PLA), an association of gates that matches bit patterns and generates the suitable management indicators for the remainder of the chip. On the fitting are the storage blocks. The 8008’s seven registers are within the higher proper. In the decrease proper is the tackle stack, which consists of eight 14-bit tackle phrases. Unlike most processors, the 8008’s name stack is saved on the chip as a substitute of in reminiscence. The program counter is only one of those addresses, making subroutine calls and returns quite simple. The 8008 makes use of dynamic reminiscence for this storage

The bodily construction of the chip could be very near the block diagram within the 8008 User’s Manual (beneath), with blocks positioned on the chip in almost the identical positions as within the block diagram.

The construction of the chip

What does the die photograph present? For our functions, the chip will be considered three layers. The diagram beneath exhibits a closeup of the chip, mentioning these layers. The topmost layer is the metallic wiring. It is probably the most seen function, and appears metallic (not surprisingly). In the element beneath, these wires are largely horizontal. The polysilicon layer is beneath the metallic and seems orange underneath the microscope.

The basis of the chip is the silicon wafer, which seems purplish-gray within the photograph. Pure silicon is successfully an insulator. Regions of it are “doped” with impurities to create semiconducting silicon. Being on the underside, the silicon layer is tough to differentiate, however you may see black strains alongside the border between doped silicon and undoped silicon. A couple of vertical silicon “wires” are seen within the photograph.4

Transistors are the important thing part of the chip, and a transistor is shaped the place a polysilicon wire crosses doped silicon. In the photograph, the polysilicon seems as a brighter orange the place it types a transistor.

Why an 18 pin chip?

One inconvenient function of the 8008 is it solely has 18 pins, which makes the chip slower and far more tough to make use of. The 8008 makes use of 14 tackle bits and eight information bits so with 18 pins there aren’t sufficient pins for every sign. Instead, the chip has 8 information pins which are reused in three cycles to transmit the low tackle bits, excessive tackle bits, and information bits. A pc utilizing the 8008 requires many help chips to work together with this inconvenient bus structure.5

There was no good purpose to power the chip into 18 pins. Packages with 40 or 48 pins have been widespread with different producers, however 16 pins was “a faith at Intel”.6 Only with nice reluctance did they transfer to 18 pins. By the time the 8080 processor got here out just a few years later, Intel had come to phrases with 40-pin chips. The 8080 was far more fashionable, partially as a result of it had an easier bus design permitted by the 40-pin bundle.

Power and information paths within the chip

The information bus supplies information movement by way of the chip. The diagram beneath exhibits the 8-bit information bus of the 8008 with rainbow colours for the 8 information strains. The information bus connects to the 8 information pins alongside the skin of the higher half of the chip. The bus runs between the ALU on the left, the instruction register (higher heart), and the registers and stack on the fitting. The bus is break up on the left with half alongside both sides of the ALU.

The pink and blue strains present energy routing. Power routing is an under-appreciated side of microprocessors. Power is routed within the metallic layer resulting from its low resistance. But since there is just one metallic layer in early microprocessors, energy distribution have to be fastidiously deliberate so the paths do not cross.7 The diagram above exhibits Vcc strains in blue and Vdd strains in pink. Power is equipped by way of the Vcc pin on the left and the Vdd pin on the fitting, then branches out into skinny, interlocking wires that offer all components of the chip.

The register file

To present what the chip appears like intimately, I’ve zoomed in on the 8008’s register file within the photograph beneath. The register file consists of an 8 by 7 grid of dynamic RAM (DRAM) storage cells, every utilizing three transistors to carry one bit.8 (You can see the transistors because the small rectangles the place the orange polysilicon takes on a barely extra vivid coloration.) Each row is without doubt one of the 8008’s seven 8-bit registers (A, B, C, D, E, H, L). On the left, you may see seven pairs of horizontal wires: the learn choose and write choose strains for every register. At the highest, you may see eight vertical wires to learn or write the contents of every bit, together with 5 thicker wires to provide Vcc. Using DRAM for registers (relatively than the extra widespread static latches) is an fascinating selection. Since Intel was main a reminiscence firm on the time, I anticipate they selected DRAM resulting from their experience within the space.

How PMOS works

The 8008 makes use of PMOS transistors. To simplify barely, you may consider a PMOS transistor as a change between two silicon wires, managed by a gate enter (of polysilicon). The change closes when its gate enter is low and it could possibly pull its output excessive. If you are acquainted with the NMOS transistors utilized in microprocessors just like the 6502, PMOS could also be a bit complicated as a result of the whole lot is backwards.

A easy PMOS NAND gate will be constructed as proven beneath. When each inputs are excessive, the transistors are off and the resistor pulls the output low. When any enter is low, the transistor will conduct, connecting the output to +5. Thus, the circuit implements a NAND gate. For compatibility with 5-volt TTL circuits, the PMOS gate (and thus the 8008) is powered with uncommon voltages: -9V and +5V.

For technical causes, the resistor is definitely carried out with a transistor. The diagram beneath exhibits how the transistor is wired to behave as a pull-down resistor. The element on the fitting exhibits how this circuit seems on the chip. The -9V metallic wire is on the prime, the transistor is within the center, and the output is the silicon wire on the backside.

History of the 8008

The 8008’s difficult story begins with the Datapoint 2200, a preferred laptop launched in 1970 as a programmable terminal. (Some individuals take into account the Datapoint 2200 to be the primary private laptop.) Rather than utilizing a microprocessor, the Datapoint 2200 contained a board-sized CPU construct from particular person TTL chips. (This was the usual method to construct a CPU within the minicomputer period.) Datapoint and Intel determined that it might be attainable to switch this board with a single MOS chip, and Intel began the 8008 challenge to construct this chip. A bit later, Texas Instruments additionally agreed to construct a single-chip processor for Datapoint. Both chips have been designed to be appropriate with the Datapoint 2200’s 8-bit instruction set and structure.

Around March 1971, Texas Instruments accomplished their processor chip, calling it the TMC 1795. After delaying the challenge, Intel completed the 8008 chip later, across the finish of 1971. For quite a lot of causes, Datapoint rejected each microprocessors and constructed a quicker CPU based mostly on newer TTL chips together with the 74181 ALU chip.

TI tried unsuccessfully to market the TMC 1795 processor to firms similar to Ford, however ended up abandoning the processor, specializing in highly-profitable calculator chips as a substitute. Intel, however, marketed the 8008 as a general-purpose microprocessor, which finally led to the x86 structure you are in all probability utilizing proper now. Although TI was first with the 8-bit processor, it was Intel who made their chip a hit, creating the microprocessor business.

The diagram above summarizes the “household tree” of the 8008 and a few associated processors.10 The Datapoint 2200’s structure was used within the TMC 1795, the Intel 8008, and the subsequent model Datapoint 220011. Thus, 4 fully totally different processors have been constructed utilizing the Datapoint 2200’s instruction set and structure. The Intel 8080 processor was a much-improved model of the 8008. It considerably prolonged the 8008’s instruction set and reordered the machine code directions for effectivity. The 8080 was utilized in groundbreaking early microcomputers such because the Altair and the Imsai. After engaged on the 4004 and 8080, designers Federico Faggin and Masatoshi Shima left Intel to construct the Zilog Z-80 microprocessor, which improved on the 8080 and have become extremely popular.

The leap to the 16-bit 8086 processor was a lot much less evolutionary. Most 8080 meeting code might be transformed to run on the 8086, however not trivially, because the instruction set and structure have been radically modified. Nonetheless, some traits of the Datapoint 2200 nonetheless exist in right now’s x86 processors. For occasion, the Datapoint 2200 had a serial processor, processing bytes one bit at a time. Since the bottom bit must be processed first, the Datapoint 2200 was little-endian. For compatibility, the 8008 was little-endian, and that is nonetheless the case in Intel’s processors. Another function of the Datapoint 2200 was the parity flag, since parity calculation was vital for a terminal’s communication. The parity flag has continued to the x86 structure.

The 8008 is architecturally unrelated to Intel’s 4-bit 4004 processor12. The 8008 is just not an 8-bit model of the 4-bit 4004 in any manner. The related names are purely a advertising and marketing invention; throughout its design section the 8008 had the unexciting title “1201”.

If you need extra early microprocessor historical past, I wrote an in depth article for the IEEE Spectrum. I additionally wrote a publish about TI’s TMC 1795.

How the 8008 suits into the historical past of semiconductor expertise

The 4004 and 8008 each used silicon-gate enhancement-mode PMOS, a semiconductor expertise that was solely used briefly. This places the chips at an fascinating level in chip fabrication expertise.

The 8008 (and trendy processors) makes use of MOS transistors. These transistors had an extended path to acceptance, being slower and fewer dependable than the bipolar transistors utilized in most computer systems of the Sixties. By the late Sixties, MOS built-in circuits have been turning into extra widespread; the usual expertise was PMOS transistors with metallic gates. The gates of the transistor consisted of metallic, which was additionally used to attach elements of the chip. Chips basically had two layers of performance: the silicon itself, and the metallic wiring on prime. This expertise was utilized in many Texas Instruments calculator chips, in addition to the TMC 1795 chip (the chip that had the identical instruction set because the 8008).

A key innovation that made the 8008 sensible was the self-aligned gate – a transistor utilizing a gate of polysilicon relatively than metallic. Although this expertise was invented by Fairchild and Bell Labs, it was Intel that pushed the expertise forward. Polysilicon gate transistors had significantly better efficiency than metallic gate (for complicated semiconductor causes). In addition, including a polysilicon layer made routing of indicators within the chip a lot simpler, making the chips denser. The diagram beneath exhibits the advantage of self-aligned gates: the metal-gate TMC 1795 is greater than the 4004 and 8008 chips mixed.

Shortly afterwards, semiconductor expertise improved once more with using NMOS transistors as a substitute of PMOS transistors. Although PMOS transistors have been simpler to fabricate initially, NMOS transistors are quicker, so as soon as NMOS might be fabricated reliably, they have been a transparent win.

NMOS led to extra highly effective chips such because the Intel 8080 and the Motorola 6800 (each 1974). Another expertise enchancment of this time was ion-implantation to vary the traits of transistors. This allowed the creation of “depletion-mode” transistors to be used as pull-up resistors. These transistors improved chip efficiency and diminished energy consumption. They additionally allowed the creation of chips that ran on customary five-volt provides.13

The mixture of NMOS transistors and depletion-mode pull-ups was used for many of the microprocessors of the late Nineteen Seventies and early Eighties, such because the 6502 (1975), Z-80 (1976), 68000 (1979), and Intel chips from the 8085 (1976) to the 80286 (1982).

In the mid Eighties, CMOS took over, utilizing NMOS and PMOS transistors collectively to dramatically scale back energy consumption, with chips such because the 80386 (1986), 68020 (1984) and ARM1 (1985). Now virtually all chips are CMOS.14

As you may see, the Nineteen Seventies have been a time of huge adjustments in semiconductor chip expertise. The 4004 and 8008 have been created when the technological functionality intersected with the fitting market.

How to take die images

In this part, I clarify how I received the images of the 8008 die. The first step is to open the chip bundle to show the die. Most chips are available epoxy packages, which will be dissolved with harmful acids.

Since I’d relatively keep away from boiling nitric acid, I took an easier strategy. The 8008 can be obtainable in a ceramic bundle (above), which I received on eBay. Tapping the chip alongside the seam with a chisel pops the 2 ceramic layers aside. The photograph beneath exhibits the decrease half of the ceramic bundle, with the die uncovered. Most of the metallic pins have been eliminated, however their positions within the bundle are seen. To the fitting of the die is a small sq.; this connects floor (Vcc) to the substrate. A few the tiny bond wires are nonetheless seen, linked to the die.

Once the die is uncovered, a microscope can be utilized to take images. A normal microscope shines the sunshine from beneath, which does not work properly for die images. Instead, I used a metallurgical microscope, which shines the sunshine from above to light up the chip.

I took 48 images by way of the microscope after which used the Hugin stitching software program to mix them into one high-resolution picture (particulars). Finally, I adjusted the picture distinction to make the chip’s buildings extra seen. The unique picture (which is roughly what you see by way of the microscope) is beneath for comparability.

Conclusion

I took detailed die images of the 8008 that reveal the circuitry it used. While the 8008 wasn’t the primary microprocessor and even the primary 8-bit microprocessor, it was actually revolutionary, triggering the microprocessor revolution and resulting in the x86 structure that dominated private computer systems for many years to return. In future posts, I plan to clarify the 8008’s circuits intimately to supply a glimpse into the roots of right now’s computer systems.

I announce my newest weblog posts on Twitter, so comply with me at kenshirriff. Or you need to use the RSS feed.

Notes and references





Source hyperlink

Leave a Reply

Your email address will not be published. Required fields are marked *