Intel 8008

The Intel 8008 ("eight-thousand-eight" or "eighty-oh-eight") is an early 8-bit microprocessor capable of addressing 16 KB of memory, introduced in April 1972. The 8008 architecture was designed by Computer Terminal Corporation (CTC) and was implemented and manufactured by Intel. While the 8008 was originally designed for use in CTC's Datapoint 2200 programmable terminal, an agreement between CTC and Intel permitted Intel to market the chip to other customers after Seiko expressed an interest in using it for a calculator.
In order to address several issues with the Datapoint 3300, including excessive heat radiation, Computer Terminal Corporation (CTC) designed the architecture of the 3300's planned successor with a CPU as part of the internal circuitry re-implemented on a single chip. Looking for a company able to produce their chip design, CTC co-founder Austin O. "Gus" Roche turned to Intel, then primarily a vendor of memory chips. Roche met with Bob Noyce, who expressed concern with the concept; John Frassanito recalls that:
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 US$50,000 development contract in early 1970 (equivalent to $405,000 in 2024). Texas Instruments (TI) was also brought in as a second supplier.
In December 1969, Intel engineer Stan Mazor and a representative of CTC met to discuss options for the logic chipset to power a new CTC business terminal. Mazor, who had been working with Ted Hoff on the development of the Intel 4004, proposed that a one-chip programmable microprocessor might be less cumbersome and ultimately more cost effective than building a custom logic chipset. CTC agreed and development work began on the chip, which at the time was known as the 1201.
TI was able to make samples of the 1201 based on Intel drawings, calling it the TMX 1795. 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 serial discrete TTL instead of waiting for a single-chip CPU. The new system was released as the Datapoint 2200 in the spring of 1970, with their first sale to General Mills on 25 May 1970. CTC paused development of the 1201 after the 2200 was released, as it was no longer needed. Later in early 1971, Seiko approached Intel, expressing an interest in using the 1201 in a scientific calculator, likely after seeing the success of the simpler 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 parallel-architecture Datapoint 2200 II, which was faster than the 1201. 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 US$120 (equivalent to $902 in 2024). This renaming tried to ride off the success of the 4004 chip, by presenting the 8008 as simply a 4 to 8 port, but the 8008 is not based on the 4004. The 8008 went on to be a commercially successful design. This was followed by the popular 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's 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.
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.
The 8008 was the CPU for the very first commercial non-calculator personal computers (excluding the Datapoint 2200 itself): the US SCELBI kit and the pre-built French Micral N and Canadian MCM/70. It was also the controlling microprocessor for the first several models in Hewlett-Packard's 2640 family of computer terminals.
In 1973, Intel offered an instruction set simulator for the 8008 named INTERP/8. It was written in FORTRAN IV by Gary Kildall while he worked as a consultant for Intel.
The 8008 was implemented in 10 μm silicon-gate enhancement-mode PMOS logic. Initial versions 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 take between 3 and 11 T-states, where each T-state is 2 clock cycles. Register–register loads and ALU operations take 5T (20 μs at 0.5 MHz), register–memory 8T (32 μs), while calls and jumps (when taken) take 11 T-states (44 μs). The 8008 is 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 since the 8008 processes data 8 bits at a time and can access significantly more RAM, in most applications it has a significant speed advantage over these processors. The 8008 has 3,500 transistors.
The chip, limited by its 18-pin DIP, has a single 8-bit bus working triple duty to transfer 8 data bits, 14 address bits, and two status bits. The small package requires about 30 TTL support chips to interface to memory. For example, the 14-bit address, which can access "16 K × 8 bits of memory", needs to be latched by some of this logic into an external memory address register (MAR). The 8008 can access 8 input ports and 24 output ports.
For controller and CRT terminal use, this is an acceptable design, but it is 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 40-pin NMOS Intel 8080 expanded upon the 8008 registers and instruction set and implements a more efficient external bus interface (using the 22 additional pins). Despite a close architectural relationship, the 8080 was not made binary compatible with the 8008, so an 8008 program would not run on an 8080. However, as two different assembly syntaxes were used by Intel at the time, the 8080 could be used in an 8008 assembly-language backward-compatible fashion.
The Intel 8085 is an electrically modernized version of the 8080 that uses depletion-mode transistors and also added two new instructions.
The Intel 8086, the original x86 processor, is a non-strict extension of the 8080, so it loosely resembles the original Datapoint 2200 design as well. Almost every Datapoint 2200 and 8008 instruction has an equivalent not only in the instruction set of the 8080, 8085, and Z80, but also in the instruction set of modern x86 processors (although the instruction encodings are different).
The 8008 architecture includes the following features:
Instructions are all one to three bytes long, consisting of an initial opcode byte, followed by up to two bytes of operands which can be an immediate operand or a program address. Instructions operate on 8-bits only; there are no 16-bit operations. There is only one mechanism to address data memory: indirect addressing pointed to by a concatenation of the H and L registers, referenced as M. The 8008 does, however, support 14-bit program addresses. It has automatic CAL and RET instructions for multi-level subroutine calls and returns which can be conditionally executed, like jumps. Eight one-byte call instructions (RST) for subroutines exist at the fixed addresses 00h, 08h, 10h, ..., 38h. These are intended to be supplied by external hardware in order to invoke interrupt service routines, but can employed as fast calls. Direct copying may be made between any two registers or a register and memory. Eight math/logic functions are supported between the accumulator (A) and any register, memory, or an immediate value. Results are always deposited in A. Increments and decrements are supported for most registers but, curiously, not A. Register A does, however, support four different rotate instructions. All instructions are executed in 3 to 11 states. Each state requires two clocks.
The following 8008 assembly source code is for a subroutine named MEMCPY that copies a block of data bytes of a given size from one location to another. Intel's 8008 assembler supported only + and - operators. This example borrows the 8080's assembler AND and SHR (shift right) operators to select the low and high bytes of a 14-bit address for placement into the 8 bit registers. A contemporaneous 8008 programmer was expected to calculate the numbers and type them in for the assembler.
In the code above, all values are given in octal. Locations SRC, DST, and 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.
The following 8008 assembly source code is for a simplified subroutine named MEMCPY2 that copies a block of data bytes from one location to another. By reducing the byte counter to 8 bits, there is enough room to load all the subroutine parameters into the 8008's register file.
Interrupts on the 8008 are only partially implemented. After the INT line is asserted, the 8008 acknowledges the interrupt by outputting a state code of S0,S1,S2 = 011 at T1I time. At the subsequent instruction fetch cycle, an instruction is "jammed" (Intel's word) by external hardware on the bus. Typically this is a one-byte RST instruction.
At this point, there is a problem. The 8008 has no provision to save its architectural state. The 8008 can only write to memory via an address in the HL register pair. When interrupted, there is no mechanism to save HL so there is no way to save the other registers and flags via HL. Because of this, some sort of external memory device such as a hardware stack or a pair of read/write registers must be attached to the 8008 via the I/O ports to help save the state of the 8008.