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
The RCA 1805
was an enhanced version.
The 1802 was used in the COSMAC
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).