RCA 1802


(Or "COSMAC") An extremely simple microprocessor architecture, fabricated by RCA, Harris and others. These were the first CMOS microprocessors and were produced as the two-chip CDP1801R and CDP1801U, and later combined into the single-chip CDP1802.

The CDP1802 ran 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 Galileo space probes as it was also fabricated in Silicon on Sapphire, giving radiation and static resistance ideal for space operation.

More history.

Last updated: 2002-04-09

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