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rca 1802
(foldoc) | RCA 1802
(Or "COSMAC") An extremely simple microprocessor
architecture, fabricated by
RCA, Harris and others. These were the first CMOS
microprocessors and were produced
as the two-chip CDP1801R and CDP1801U, and later combined into the
single-chip CDP1802.
The CDP1802 ran 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 Galileo space probes as it was also
fabricated in Silicon on Sapphire, giving radiation
and static resistance ideal for space operation.
More history (http://cosmacelf.com).
(2002-04-09)
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