APOLLO181  has been completely designed and built - in Italy, 2012 -  by Gianluca G. (author of the homemade Z80/AM95 microcomputer) using early 1970s TTL technology. He built the machine using painstakingly accurate soldering with a miniature iron, sockets for over 60 integrated circuits, and a single square foot perfboard. Designed and tested with the aid of a hardware simulator, APOLLO181 runs at 3 MHz.

The project is based on the famous 74181 Integrated Circuit that is a TTL Arithmetic and Logic Unit frequently used in the 1970s during the third generation of computers. This chip is the hub of datapath activity with its output the primary feedback path to the registers: as early minicomputers, APOLLO181 uses the content of a memory location as one operand of the 74181 and an accumulator as the second operand. The result is stored back in the accumulator.

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 computations on 4-bit (or nibble) intermediate results: advantages of a shorter word are simpler circuits and higher speed. Multi-nibble instructions and operations, however, permit powerful 8-, 16-, 24-, and even 32-bit computations (at reduced speed).

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.

The move time for register-to-register transfers is 2,7 microseconds, which requires two basic instructions and eight clock cycles. Just as a performance indicator, APOLLO181 processor is able to perform a 12-bit integer multiplication (giving a 24-bit long result) in less than 1,2 milliseconds.

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'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 16 independent input and latched output 4-bit 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.