Warren’s Crazy Small CPU

(c) 2017 Warren Toomey, GPL3

This is the website for version 2 of my “crazy small CPU”, one that has the fewest 7400-style chips. The design has only eleven chips:

three 74LS161 4-bit registers for the A & B registers and to store the flags output from the ALU

one 74LS157 4-bit multiplexer, to choose from where to load the A & B registers

two 74LS161 4-bit register/counters which are the Program Counter (PC)

one NE556 timer which acts as either a one-shot clock or a periodic clock

one RAM chip for memory: only 256 4-bit words are used

one ROM chip that acts as an ALU with two 4-bit inputs, sixteen operations, a 4-bit output and also four result flags: negative, zero, overflow and carry (NZVC)

two ROM chips that provide the control logic: one holds literal values and the other enables the control lines for the other chips