An Intel 8008 Microprocessor
|Produced||From mid 1972 to 1983|
|Max. CPU clock rate||0.2 MHz to 0.8 MHz|
|Min. feature size||10μm|
|Application||Dumb terminals, general calculators, bottling machines, data/character manipulation|
The Intel 8008 ("eight-thousand-eight" or "eighty-oh-eight") was an early byte-oriented microprocessor designed and manufactured by Intel and introduced in April 1972. It was an 8-bit CPU with an external 14-bit address bus that could address 16KB of memory. Originally known as the 1201, the chip was commissioned by Computer Terminal Corporation (CTC) to implement an instruction set of their design for their Datapoint 2200 programmable terminal. As the chip was delayed and did not meet CTC's performance goals, the 2200 ended up using CTC's own TTL based CPU instead. An agreement permitted Intel to market the chip to other customers after Seiko expressed an interest in using it for a calculator.
CTC formed in San Antonio in 1968 under the direction of Austin O. "Gus" Roche and Phil Ray, both NASA engineers. Roche, in particular, was primarily interested in producing a desktop computer. However, given the immaturity of the market, the company's business plan mentioned only a Teletype Model 33 ASR replacement, which shipped as the Datapoint 3300. The case, designed by John "Jack" Frassanito, was deliberately designed to fit in the same space as an IBM Selectric typewriter, and used a video screen shaped to be the same aspect ratio as an IBM punched card. Although commercially successful, the 3300 had ongoing heat problems due to the amount of circuitry packed into such a small space.
In order to address the heating and other issues, a re-design started that featured the CPU part of the internal circuitry re-implemented on a single chip. Looking for a company able to produce their chip design, Roche turned to Intel, then primarily a vendor of memory chips. Roche met with Bob Noyce, who expressed concern with the concept; Frassanito recalls that "Noyce said it was an intriguing idea, and that Intel could do it, but it would be a dumb move. He said that if you have a computer chip, you can only sell one chip per computer, while with memory, you can sell hundreds of chips per computer." Another major concern was that Intel's existing customer base purchased their memory chips for use with their own processor designs; if Intel introduced their own processor, they might be seen as a competitor, and their customers might look elsewhere for memory. Nevertheless, Noyce agreed to a $50,000 development contract in early 1970. Texas Instruments (TI) was also brought in as a second supplier.
TI was able to make samples of the 1201 based on Intel drawings, but these proved to be buggy and were rejected. Intel's own versions were delayed. CTC decided to re-implement the new version of the terminal using discrete TTL instead of waiting for a single chip CPU. The new system was released as the Datapoint 2200 in the spring 1970, with their first sale to General Mills on May 25, 1970. CTC paused development of the 1201 after the 2200 was released, as it was no longer needed. Six months later, Seiko approached Intel expressing an interest in using the 1201 in a scientific calculator, likely after seeing the success of the simpler Intel 4004 used by Busicom in their business calculators. A small re-design followed, under the leadership of Federico Faggin, the designer of the 4004, now project leader of the 1201, expanding from a 16-pin to 18-pin design, and the new 1201 was delivered to CTC in late 1971.
By that point CTC had once again moved on, this time to the Datapoint 2200 II, which was faster and included a hard drive. The 1201 was no longer powerful enough for the new model. CTC voted to end their involvement with the 1201, leaving the design's intellectual property to Intel instead of paying the $50,000 contract. Intel renamed it the 8008, and put it in their catalog in April 1972 priced at $120. Intel's initial worries about their existing customer base leaving them proved unfounded, and the 8008 went on to be a commercially successful design. This was followed by the Intel 8080, and then the hugely successful Intel x86 family.
One of the first teams to build a complete system around the 8008 was Bill Pentz' team at California State University, Sacramento. The Sac State 8008 was possibly the first true microcomputer, with a disk operating system built with IBM Basic assembly language in PROM all driving a color display, hard drive, keyboard, modem, audio/paper tape reader and printer. The project started in the spring of 1972 and with key help from Tektronix the system was fully functional a year later. Bill assisted Intel with the MCS-8 kit and provided key input to the Intel 8080 instruction set which helped make it useful for the industry and hobbyists.
In the UK, a team at S. E. Laboratories Engineering (EMI) led by Tom Spink in 1972 built a microcomputer based on a pre-release sample of the 8008. Joe Hardman extended the chip with an external stack. This, among other things, gave it power-fail save and recovery. Joe also developed a direct screen printer. The operating system was written using a meta-assembler developed by L. Crawford and J. Parnell for a Digital Equipment Corporation PDP-11. The operating system was burnt into a PROM. It was interrupt-driven, queued, and based on a fixed page size for programs and data. An operational prototype was prepared for management, who decided not to continue with the project.
|Intel 8008 registers|
Implemented in 10 μm silicon-gate enhancement load PMOS; initial versions of the 8008 could work at clock frequencies up to 0.5 MHz, this was later increased in the 8008-1 to a specified maximum of 0.8 MHz. Instructions took between 5 and 11 T-states where each T-state was 2 clock cycles. Register-register loads and ALU operations took 5T (20 μs at 0.5 MHz), register-memory 8T (32 μs), while calls and jumps (when taken) took 11 T-states (44 μs). The 8008 was a little slower in terms of instructions per second (36,000 to 80,000 at 0.8 MHz) than the 4-bit Intel 4004 and Intel 4040, but the fact that the 8008 processed data eight bits at a time and could access significantly more RAM still gave it a significant speed advantage in most applications. The 8008 had 3,500 transistors.
The chip (limited by its 18-pin DIP packaging) had a single 8-bit bus and required a significant amount of external support logic. For example, the 14-bit address, which could access "16 K x 8 bits of memory", needed to be latched by some of this logic into an external Memory Address Register (MAR). The 8008 could access 8 input ports and 24 output ports.
For controller and CRT terminal use, this was an acceptable design, but it was rather cumbersome to use for most other tasks, at least compared to the next generations of microprocessors. A few early computer designs were based on it, but most would use the later and greatly improved Intel 8080 instead.
The subsequent Intel 8080 and 8085 CPUs were heavily based on the same basic internal design of the 8008; even though they were neither assembly-language nor binary compatible with it, they expanded upon the same basic register set and memory bus architecture. The original x86 processor, the 8086, was in turn a non-strict extension of the 8080, therefore loosely resembling the original Datapoint 2200 design as well. This means that each 8008 instruction has an equivalent not only in the larger instruction set of the 8080 (and 8085, Z80, etc.) but also, in most cases, in the larger 32-bit based instruction set of modern x86 processors (although the opcode encodings are different in all three).
- Eight 8-bit "scratchpad" registers: The main accumulator (A) and seven other registers (B, C, D, E, H, and L).
- 14-bit Program counter (PC).
- Seven-level push-down address call stack. Eight registers are actually used, with the top-most register being the PC.
- Four condition code status flags: Carry (C), even Parity (P), Zero (Z), and Sign (S).
- Indirect memory access using the H and L registers (HL) as a 14-bit data pointer (the upper two bits are ignored).
The following 8008 assembler source code is for a subroutine named
MEMCPY that copies a block of data bytes of a given size from one location to another.
; MEMCPY -- ; Copy a block of memory from one location to another. ; ; Entry parameters ; SRC: 14-bit address of source data block ; DST: 14-bit address of target data block ; CNT: 14-bit count of bytes to copy ORG 1700Q ;Data at 001700q 001700 000 SRC DFB 0 ;SRC, low byte 001701 000 DFB 0 ; high byte 001702 000 DST DFB 0 ;DST, low byte 001703 000 DFB 0 ; high byte 001704 000 CNT DFB 0 ;CNT, low byte 001705 000 DFB 0 ; high byte ORG 2000Q ;Code at 002000q 002000 066 304 MEMCPY LLI CNT+0 ;HL = addr(CNT) 002002 056 003 LHI CNT+1 002004 327 LCM ;BC = CNT 002005 060 INL 002006 317 LBM 002007 302 LOOP LAC ;If BC = 0, 002010 261 ORB 002011 053 RTZ ;Return 002012 066 300 GETSRC LLI SRC+0 ;HL = addr(SRC) 002014 056 003 LHI SRC+1 002016 347 LEM ;DE = SRC 002017 060 INL 002020 337 LDM 002021 364 LLE 002022 302 LAC ;HL = HL+BC 002023 206 ADL 002024 360 LLA 002025 301 LAB 002026 215 ACH 002027 350 LHA 002030 307 LAM ;Load A from (HL) 002031 066 302 GETDST LLI DST+0 ;HL = addr(DST) 002033 056 003 LHI DST+1 002035 347 LEM ;DE = DST 002036 060 INL 002037 337 LDM 002040 364 LLE 002041 353 LHD 002042 330 LDA ;D = A 002043 302 LAC ;HL = HL+BC 002044 206 ADL 002045 360 LLA 002046 301 LAB 002047 215 ACH 002050 350 LHA 002051 373 LMD ;Store D to (HL) 002052 302 DECCNT LAC ;BC = BC-1 002053 024 001 SUI 1 002054 320 LCA 002055 301 LAB 002056 034 000 SBI 0 002057 310 LBA 002060 104 007 004 JMP LOOP ;Repeat the loop 002063 END
In the code above, all values are given in octal. Locations
CNT are 16-bit parameters for the subroutine named
MEMCPY. In actuality, only 14 bits of the values are used, since the CPU has only a 14-bit addressable memory space. The values are stored in little-endian format, although this is an arbitrary choice, since the CPU is incapable of reading or writing more than a single byte into memory at a time. Since there is no instruction to load a register directly from a given memory address, the HL register pair must first be loaded with the address, and the target register can then be loaded from the M operand, which is an indirect load from the memory location in the HL register pair. The BC register pair is loaded with the
CNT parameter value, and decremented at the end of the loop until it becomes zero. Note that most of the instructions used occupy a single 8-bit opcode.
- CTC (Instruction set and architecture): Victor Poor and Harry Pyle.
- Intel (Implementation in silicon):
- Ted Hoff and Stan Mazor and Larry Potter (IBM Chief Scientist of IBM) proposed a single-chip implementation of the CTC architecture, using RAM register memory rather than shift register memory, and also added a few instructions and interrupt facility. The 8008 (originally called 1201) chip design started before the 4004 development. Hoff and Mazor, however, could not and did not develop a "silicon design" because they were neither chip designers nor process developers and furthermore the necessary silicon-gate-based design methodology and circuits, under development by Federico Faggin for the 4004, were not yet available.
- Federico Faggin, having finished the design of the 4004, became leader of the project from January 1971 until its successful completion in April 1972, after it had been suspended – for lack of progress – for about seven months.
- Hal Feeney project engineer did the detailed logic design, circuit design, and physical layout under Faggin's supervision, employing the same design methodology that Faggin had originally developed for the Intel 4004 microprocessor, and utilizing the basic circuits he had developed for the 4004. Note his combined HF logo on the chip about half way between the D5 and D6 bonding pads.
- MCS-8 User Manual with 8008 data sheet (1972)
- Computer World Forgotten PC history: The true origins of the personal computer
- The Intel 8008 support page unofficial
- The DigiBarn Computer Museum's page on Bill Pentz' Sacramento State machine, a full microcomputer built around the 8008
- CPU History – The CPU Museum – Life Cycle of the CPU
- Lamont Wood, "Forgotten PC history: The true origins of the personal computer", Computerworld, August 8, 2008
- Brunel University, 1974. Master of Technology dissertation, L. R. Crawford
- "MCS-8 Micro Computer Set Users Manual" (PDF). Intel Corporation. 1972. Retrieved December 4, 2010.
- "Intel 8008 Opcodes". Retrieved December 4, 2010.
- "Intel 8008 (i8008) microprocessor family". CPU World. 2003–2010. Retrieved December 4, 2010.
- [dead link]
- Faggin, Federico; Hoff, Marcian E.; Mazor, Stanley; Shima, Masatoshi (December 1996), "The History of the 4004", IEEE Micro (Los Alamitos: IEEE Computer Society) 16 (6): 10–19, ISSN 0272-1732