RCA 1802

RCA 1802
CDP1802

   <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.

   More history (http://cosmacelf.com).

   (2002-04-09)