RCA 1802 (CDP1802) microprocessor family

RCA 1802 (CDP1802) is an 8-bit microprocessor, and it was one of the first, if not the first, CMOS processors. The 1802 had low power consumption, wide range of operating voltages and military operating temperature range. The processor had very interesting architecture. Any one of the 16 registers of the 1802 microprocessor could be set as a data pointer register or program counter. It was possible to jump to any memory location by placing a new address into any free register, and set this register as a program counter. This trick was usually used when there was a need to call a subroutine as the microprocessor didn't have a dedicated "call subroutine" instruction. Another interesting feature was that the RCA 1802 didn't have stack instructions. Stack operations could be simulated using data loading/storing instructions.

RCA also produced 1804 (CDP1804) microcontroller based on the RCA 1802 architecture. The 1804 had 64 bytes of memory, 2 KB ROM, on-chip oscillator and 8-bit timer. Instruction set of the 1804 was enhanced with 32 additional instructions, including subroutine call and return from the subroutine instructions. RCA 1805 (CDP1805) and 1806 (CDP1806) were similar to the 1804, but they didn't have on-chip ROM.