APOLLO181  has been  conceived  and assembled in Italy in 2012 by Gianluca G. (author of the homemade Z80/AM95 microcomputer) using early 1970s TTL technology. Designed and tested with the aid of a hardware simulator, APOLLO181 is running today at 2.5 MHz on a 12x12 inches single perfboard.

APOLLO181 uses 8-bit instruction word and 8-bit address bus which can access 256 Byte of user program memory. The reason we classify it a 4-bit processor is that the internal registers and the arithmetic logic unit perform computation on 4-bit (or nibble) intermediate results: advantages of a shorter word are simpler circuits and higher speeds.

The instruction set consists of sixteen basic commands which perform  input and output interfacing, conditional jumps and operations like addition, subtraction, increment, decrement, shift operand, magnitude comparison, Exclusive-OR, AND, NAND, OR, NOR on 4-bit data words.

APOLLO181 is a multi-chip board and its peculiarity is that each TTL component here employed  has been described in the Bugbook® I & II (LOGIC & MEMORY EXPERIMENTS USING TTL INTEGRATED CIRCUITS, written by Dr. Peter R. Rony © 1974, 1st edition), as a Gianluca G.'s personal tribute to these books. By happy coincidence we are also approaching the 40 years Bugbook® publication anniversary.

This project obviously aims to be more educational and recreational than being a practical useful processor: the major limitations compared with normal CPUs are the maximum program length of only 256 instructions, the lack of subroutine calls and the absence of memory manipulation instructions. The RAM contains only the user program but we can use up to sixteen internal registers to store 4-bit temporary results. The processor is anyway capable of driving sixteen independent input and output ports, so the theoretical areas of application of APOLLO181 could be the same as microcontrollers: small domestic appliances, white and brown goods, security systems, toys, office equipment and industrial control applications.