RCA 1802

Meaning of RCA 1802 in English

< 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 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 .


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