The first 8-bit microprocessor, Intel 8008 (i8008) was released 5 months
after Intel 4004. The 8008 was available
in two speed grades - 500 KHz and 800 KHz. Because it took the CPU from 5 to
8 cycles to execute each instruction, the effective rate of
instruction execution was:
- From 45,000 to 100,000 instructions per second for Intel 8008
- From 72,000 to 160,000 instruction per second for Intel 8088-1
These numbers assume that the CPU uses fast memory and doesn't
require wait states to access the memory. Although the effective
speed in instructions per second of the 8008 microprocessor sometimes
is lower than the effective speed of the 4004 CPU, overall
performance of the i8008 was greater due to faster effective speed of
some instructions, 8-bit architecture and more efficient instruction set.
The 8008 had other advantages over the 4004:
- The processor supported of 16 KB of memory (ROM and RAM combined).
- The size of internal CPU stack was 7 levels in contrast to 3
level-stack for the i4004.
- The Intel 8008 could handle interrupts.
One of the drawbacks of the Intel 8008 was the absence of direct
memory addressing. To access data in memory the memory address had to
be stored in H and L registers, and only then the processor could
indirectly access the memory. This limitation was removed in
Intel 8080.
Intel 8008 microprocessor was used in Mark-8 computer, which is
considered to be the first personal computer.
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