# Hackaday bitcoin calculator

1970s: it provided fast 4-bit arithmetic and logic functions, and could be combined to handle larger words, making it hackaday bitcoin calculator key part of many CPUs. But if you look at the chip more closely, there are a few mysteries. Schematic of the 74LS181 ALU chip, from the datasheet. The internal structure of the chip is surprisingly complex and difficult to understand at first.

The 74181 chip is important because of its key role in minicomputer history. Before the microprocessor era, minicomputers built their processors from boards of individual chips. Early minicomputers built ALUs out of a large number of simple gates. The datasheet for the 74181 ALU chip shows a strange variety of operations. So how is the 74181 implemented and why does it include such strange operations? Is there any reason behind the 74181’s operations, or did they just randomly throw things in? And why are the logic functions and arithmetic functions in any particular row apparently unrelated?

I investigated the chip to find out. Why are there 16 possible functions? 4 rows in the truth table. Each row can output 0 or 1. Arithmetic functions The 74181’s arithmetic operations are a combination of addition, subtraction, logic operations, and strange combinations such as “A PLUS AB PLUS 1”. It turns out that there is a rational system behind the operation set: they are simply the 16 logic functions added to A along with the carry-in.