This is my 8-bit CPU design implementing my DOT (Does One Thing) ISA. It supports 8 instructions; it has four 8-bit registers, 256 addressable 8-bit data memory locations, a bank of 4 selectable instruction memories supporting 16 8-bit instructions each, and external displays for monitoring the output.

Video Demonstration: YouTube

Note: When testing, please ensure that the clock starts at 0 and the reset button has been clicked before running the program to ensure accurate output. Data memory can be viewed after the program has run by sending 1 to DM_dump and then viewing your browser's console (Ctrl + Shift + K in Firefox and Ctrl + Shift + I in Chrome).

Instructions:

init pos, Rx, imm  ::  00 | pos | Rx | imm
This instruction overwrites Rx {00, 01, 10, 11} with value imm {00, 01, 10, 11} at bit position {11 = [7:6], 10 = [5:4], 01 = [3:2], 00 = [1:0]}.

addi Rx, imm  ::  0100 | Rx | imm
This instruction adds to Rx {00, 01, 10, 11} the sign-extended value imm {10 = -2, 11 = -1, 00 = 0, 01 = 1} and stores the result to Rx.

add Rx, Ry  ::  0101 | Rx | Ry
Rx = Rx + Ry

xor Rx, Ry  ::  0110 | Rx | Ry
Rx = Rx XOR Ry

sb Rx, (Ry)  ::  1000 | Rx | Ry
DM[Ry] = Rx

swid Rx, (Ry)  ::  1101 | Rx | Ry
DM[Ry + 0x80] = bitwidth(Rx)

check  ::  11100000
if (R0 < 0x64): PC goes back 5 instructions; else: PC++

halt  ::  11111111
ends the program