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74181 Chip History
CPU Specification
Instruction Set
CPU Architecture
Programming PROM
First Program Example
Binary Clock Algorithm
Shift-and-Add Multiplication
Prime Numbers Benchmark
PWM LED Dimmer
Step Motor Controller
Sound Generator: Part 1
Sound Generator: Part 2
APOLLO181 Schematic
EPROM Data Storage
My Previous Z80 Project


APOLLO181 is a homemade didactic 4-bit CPU

made exclusively of TTL logics and bipolar memories


All employed chips are described in the Bugbook® I and II,

in particular the 74181 Arithmetic and Logic Unit. 



After having intensively explored the Z80 CPU, I decided to design and make a processor from scratch by myself. The Bugbooks® volumes written by Dr. Peter R. Rony in 1974, Virginia, (which were my first study books) inspired and encouraged me to realize this.


Peculiarity of APOLLO181 is the effective programmability: like real processors it can be instructed to perform high speed operations at different times and conditions, while exchanging data with the outside world.


I’m very delighted that Dr. Peter Rony, to whom I have recently communicated this project, has defined my APOLLO181 “a labour of love”.


Gianluca G., May 2012, Italy 




APOLLO181 was designed and built in 2012 by Gianluca G. (an Italian electronic engineer, author of the homemade Z80/AM95 8-bit NANO COMPUTER) adopting early 1970s TTL technology and using original vintage components, with retro style layout. One of the goals was in fact to obtain as much historical accuracy as was possible.


The machine was built using extremely accurate soldering with a miniature iron and sockets for over sixty TTL integrated circuits on a single square foot perfboard. Designed and tested with the aid of a hardware simulator, APOLLO181 originally run at 2.5 MHz, by using Low Power Schottky 74LS family chips.

Then, after being tested, nearly all integrated circuits were changed into pure Schottky 74S 
 technology: a previous, but faster TTL logic family, introduced by Texas Instruments in 1971, which allowed APOLLO181 to run up to 3 MHz.


The project is based on the famous SN74181 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.


You can find author’s research about history of  the 74181 chip in commercial minicomputers on the following pages.


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 operations can be anyway performed by propagating carry or borrow flag, permitting powerful 8-, 16-, 24-, and even 32-bit computations (at reduced speed).


The symple set of instructions consists of sixteen basic commands, which perform  input and output interfacing, conditional jumps and arthmetic and logic operations like addition, subtraction, increment, decrement, operand shifting, 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 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.


A famous benchmark ("BM9", a Tom Fox algorithm to find out prime numbers, written in BASIC in 1980) has been implemented on APOLLO181 to test the performance: the nice result obtained  proved its capability.


APOLLO181 is a multi-chip board and its peculiarity is that each TTL component here employed  was 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, in 2014, 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 256 Byte of 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 4-bit independent input and latched output ports and includes also a digital to analog converter, thus 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.


In the following pages you will find algorithm examples showing computation of the four basic arithmetic operations, a light dimmer, a stepper-motor control, a chronograph and a basic sound generator.



Copyright (c) 2012 by Gianluca G. Italy 

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APOLLO181 processor technology belongs to the 3rd Computer Generation

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The information presented here is just that: INFORMATION. Use it at your own risk and for only non commercial purpose. The information here presented is believed to be technically correct and everything presented on this site is done so in good faith. Anyhow you (the reader) are responsible for anything that you might do as a result of reading this article. You assume complete and total responsibility for your actions! Author is not responsible for any misuse or damage coming from the reading and using this information.

Text and images from original typewritten Bugbooks I and II in 1974 are permission courtesy of Dr. Peter R. Rony, the original author and sole copyright owner of the Bugbooks I, II, IIA, III, V, and VI.

The background image on the header of each page of the site is "Sunset over western South America" photographed on 12 April 2011 by an Expedition 27 crew member on the International Space Station. (Image credit: NASA). On it I have merged titles and a my photo of TIL302 displays.

Texas Instruments data are Texas Instruments Copyright and reported by Courtesy of Texas Instruments.


TERM OF USE: With clear exception for texts and images which are not author's property, Gianluca G. freely authorizes you the downloading, printing and reproducing of APOLLO181 data, texts and images ONLY for non-commercial usage and ONLY if you give a clear reference to its source and project namewithout any right to resell or redistribute them or to compile or create derivative works.

Any rights not expressly granted herein are reserved.


Copyright (c) 2012 by Gianluca G. Italy