<processor> An extremely simple microprocessor fabricated in CMOS,
running at 6.4 MHz at 10V (very fast for 1974). It could be suspended with the
clock stopped. It was an 8-bit processor, with 16-bit addressing. Simplicity was
the primary design goal, and in that sense it was one of the first RISC chips.
It had sixteen 16-bit registers, which could be accessed as thirty-two 8-bit
registers, and an accumulator D used for arithmetic and memory access - memory
to D, then D to registers and vice versa, using one 16-bit register as an
address. This led to one person describing the 1802 as having 32 bytes of RAM
and 65535 I/O ports. A 4-bit control register P selected any one general
register as the program counter, while control registers X and N selected
registers for I/O Index and the operand for the current instruction. All
instructions were 8 bits - a 4-bit op code (total of 16 operations) and 4-bit
operand register stored in N. There was no real conditional branching, no
subroutine support and no actual stack but these could be implemented by clever
use of registers, e.g. changing P to another register allowed jump to a
subroutine. Similarly, on an interrupt P and X were saved, then R1 and R2 were
selected for P and X until an RTI restored them.
The RCA 1805 was an enhanced version.
The 1802 was used in the COSMAC (VIP?) microcomputer kit, some video games from
RCA and Radio Shack, and the ETI-660 computer. It was chosen for the Voyager,
Viking and Galileo space probes as it was also fabricated in Silicon on
Sapphire, giving radiation and static resistance, ideal for space operation.
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