8 bit computer, building up the ALU

8 bit computer, building up the ALU


Building up the heart of the computer, the two big chips are 74LS181’s (4 bit ALU’s, hence there’s two of them). The two little chips below them are the 74LS245 tri-state buffer, that connects it to the bus in a controlled fashion and the even smaller one on the right is a 74LS00 logic chip to summarise and invert some control signals.

The ALU’s output can be transferred to the bus when needed by the use of the proper bit in the control word driven by the micro code.

Two registers

The top and bottom rows are two register circuits, The A register and the B register. Both of them are comprised of a 74LS245 tri-state buffer and two 74LS173 4 bit registers. These registers will be connected to the bus through the tri-state buffer for input and output They will also be connected permanently to the ALU, so it can calculate one one or both of them.

Wire colours

Data bus wires are in blue, Vcc in red, Ground in black, control wires in yellow, chip-to-chip connections in grey or brown. At the bottom you can see the bus connection, and on top there’s three break-out connectors for the A register, the ALU output and the B register (left to right).