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.