A National Semiconductor version of the Intel 4004 (manufactured under
license) and a Unicom 141P calculator that used it.
A National Semiconductor version of the Intel 4004 (manufactured under
license) and a Unicom 141P calculator that used it.
The first complete Micro-Central Processing Unit (Micro-CPU or
microprocessor) on a single chip was the Intel 4004, released in 1971. This CPU
had a maximum clock speed of 740 kHz. It had a single four-bit bus that
multiplexed addresses, data, and instructions. External support chips
demultiplexed this to separate data, instruction, and address buses (Harvard
architecture). It operated on 4-bit data words, 8-bit instructions, and 12-bit
addresses and could access 4096 memory addresses. The 4004 came in a 16-pin
Dual-Inline Package (DIP) and was mainly used in calculators.
Intel 8008
An Intel 8008 chip and a Micral N personal computer
The next-generation microprocessor from Intel was the 8008. It was released
in April 1972 for $120. The main improvement was an 8-bit bus. It had a maximum
clock speed of 500 kHz but operated on 8 bits simultaneously, so it outperformed
the 4004. It used 14-bit addresses and could access 16,384 bytes (16k) of
memory. The 8008 was used in the first commercial, personal computer, the
Micral, released in 1973.
Intel 8080
An Advanced Micro Devices (AMD) version of the 8080 (manufactured under
license).
A MITS Altair 8800
(left) and an IMSAI 8080
The 8080 was a significant improvement over the 8008. I had separate address
and data buses, thus not requiring external demultiplexing of these buses. It
also used the same memory space for instructions and data (Von Neumann
architecture), simplifying memory access. It had a 16-bit address bus and could
access 65,536 bytes (64k) of memory. The 8080 was an 8-bit processor with three
16-bit registers and could operate on 16-bit words.
Although the 8080 could use memory-mapped I/O (since that is implemented
outside the processor), it usually used port-mapped I/O[1] and had dedicated I/O
instructions. It was also capable of implementing a stack that was outside the
regular memory space.
The 8080 was used in the famous MITS Altair 8800 microcomputer. Even though
the Altair was predated by the similar Micral N, the Altair started the personal
computer revolution.
The 8080 changed the way computers were designed and manufactured. For the
first time, manufacturers chose a microprocessor and designed a computer around
it rather than designing a computer and CPU together.
Zilog Z80
Some of the team that designed the 8080 left Intel and started Zilog. The Z80
was designed as a direct competitor and an improvement to the 8080. It
eventually became more popular than its inspiration.
Osbourne 1, Timex Sinclair 1000, and TRS-80 Model III computers that
used the Z80
The Altair and IMSAI were not capable of desktop computing. They were
programmed by switch panels, and output was made through LEDs. Some hobbyists
attached teletypes or video terminals to their Altairs and IMSAIs. After this
improvement, Bill Gates and Paul Allen developed a version of BASIC, a simple
programming language that could be loaded via a teletype and paper tape. This
allowed programmers to type instructions on a keyboard and have results printed
on paper or a video screen. Additional memory and small (5-1/4") floppy drives
were eventually added. After that, Gary Kildall developed a Disk-based operating
system called Control Program for Microcomputers (CP/M). With CP/M and a video
terminal, it was fairly easy to load and run user applications like primitive
word processors. Manufacturers began to sell computers with built-in floppy
drives and video terminalss; thus, desktop computing had become available to the
masses. For most manufacturers, the Z80 was the CPU of choice for these
machines.
The Z80 is still in widespread use as of 2023. It is popular for embedded
systems[2]
where the sophisticated microprocessors of today's desktop computers are
overqualified.
