RCA 1802
CDP1802

    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>.

   (2002-04-09)