RCA 1802

An extremely simple {microprocessor} fabricated in {CMOS},

which allowed it to run at 6.4 MHz (at 10V, but very fast for

1974) or suspended with the clock stopped. It was an 8 bit

processor, with 16-bit addressing but the major features were

its extreme simplicity, and the flexibility of its large

{register set}. Simplicity was the primary design goal, and

in that sense it was one of the first {RISC} chips. It had

sixteen 16-bit {register}s, 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 clever use of the register set allowed

these to be implemented - for example, 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.

Apart from the {COSMAC} (VIP?) {microcomputer} kit, the 1802

saw action in some video games from {RCA} and {Radio Shack}.

It was used in the {DREAM 6800} and {ETI-660} computers and is

also the heart of the Voyager, Viking and Galileo probes. One

reason for this is that the 1802 was also fabricated on

sapphire ({Silicon on Sapphire}), which leads to radiation and

static resistance, ideal for space operation.

(1994-11-16)