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Computing (FOLDOC) dictionary
RCA 1802
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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 programcounter, 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 onSapphire, giving radiation and static resistance, ideal for
space operation.