Objective
Build an 8-bit computer on breadboards - clock, registers, ALU, RAM, program counter, control logic - to make computer architecture physical.
Design
Classic SAP-1-derived architecture on 74LS-series TTL: a single 8-bit bus, two general registers, a 4-bit program counter, and microcoded control via EEPROMs. Target clock: 1 kHz, adjustable down to single-step.
Build
Progress is chronicled in the build log below - module by module, failure by failure. The clock and registers are alive; the ALU is next on the bench.
Next Iteration
Once the ALU carries, the control-logic EEPROMs get burned and the first program - counting by threes - runs end to end.
Build Log
15 AUG 2026
Breadboard Computer - Day 01
Clock module built: 555-based astable with a range pot, plus a debounced single-step button and a run/step toggle. It ticks from 0.5 Hz to about 1.2 kHz. Watching an LED blink has never felt so load-bearing.
17 AUG 2026
Breadboard Computer - Day 02
A register: two 74LS173s and a 74LS245 behind LEDs. First load attempt wrote garbage - the load line was active the entire time because I’d wired enable low instead of the clock-gated load. One green wire later, the register holds what it’s told and nothing else.
21 AUG 2026
Breadboard Computer - Day 03
Second register plus the bus tie-ins. Discovered two modules happily driving the bus simultaneously; the fix was auditing every output-enable against the control table before power-up. New rule: no module joins the bus without its enable traced twice.
24 AUG 2026
Breadboard Computer - Day 04
Started the ALU. The 74LS283 adders sum, but subtraction via XOR-inverted B plus carry-in produced off-by-one chaos for an hour - the carry-in jumper was in the parts bin, not the board. Add and subtract both verified against a truth table by hand. Day ended with the flags register sketched on paper.