Intel 8008 - Design

Design

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 subsequent Intel 8080 and 8085 CPUs were assembly-language (not binary) compatible with the 8008 and also heavily based on the same basic internal design. 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 large 32-bit based instruction set of a modern x86 processor (although the encodings are different in all three).

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 8008 family is also referred to as the MCS-8.