A project log for The 74LS181 ALU

Taming the 74LS181 ALU chip

agp.cooperagp.cooper 08/12/2016 at 03:560 Comments

A SSI ALU for my bit-slice CPU

There is not much on the Internet about DIY ALUs if you don't like the 74LS181.

For my Weird CPU I used an adder (74LS83A), a comparator (74LS85) and the NAND function (74LS00). I should have included the XOR and SHR functions as they were expensive to implement in software.

Here is the schematic from my Bit-Slice SSI CPU (it is on hold at the moment):

So you should recognise the NAND gate equilavent of an XOR gate (U1-4 and U6-9) that make up an adder (with U5). The remaining gates (U13-16) make up a comparator. I have also picked off the NAND and XOR outputs.

Other than SHR it is all you really need for a DIY ALU. The rest is efficiently implemented in software.