ALU - Logical Operations

A project log for World's first 32bit Homebrew CPU

Creating the world's first 32bit homebrew CPU using 74' series logic.

Phil Wright 10/22/2017 at 23:090 Comments

RISC-V specifies three different logical operations, AND, OR and XOR.

Each operation needs 8 chips for the actual operation, such as using the 74HC08 for the logical AND of the 32bit A and B inputs. Plus, another 4 x 74HC241 for the output buffer. All ALU operations are output to the same bus and so each operation needs an output buffer that ensures only one result is output at a time. Unfortunately, 12 IC's is too many to squeeze onto the 100mm x 80mm boards that I use. So we are forced into splitting it over two boards, one that does the lower 16bits and the other does the upper 16 bits.

We end up with a stack of 6 boards where long female headers providing the A and B inputs and the output result traversing the entire stack. Control lines determine which operation, if any, is performed.