The E8 is a custom 8-bit CPU architecture designed in Logisim Evolution, paired with a functional software emulator written in C. Together, they provide a complete platform for exploring low-level computing: from gate-level hardware design through instruction execution, memory access, and real-time RGB video output.

The architecture uses a fixed 32-bit instruction word, a small general-purpose register file, an accumulator for arithmetic, external RAM, and dedicated video registers for drawing to a display. Programs can be assembled into binary test files and executed on the emulator to observe register state, memory, and graphical output in action.

The E8 CPU is implemented as a digital logic circuit in Logisim Evolution. The schematic below shows the full datapath, control logic, and peripheral connections.

The Logisim project file is available at circuit/e8.circ.

Every instruction occupies 32 bits, structured as four 8-bit fields:

┌────────────┬────────────┬────────────┬────────────┐
│  Opcode    │  Operand 1 │  Operand 2 │  Operand 3 │
│  [8 bits]  │  [8 bits]  │  [8 bits]  │  [8 bits]  │
└────────────┴────────────┴────────────┴────────────┘

Registers A through E serve dual roles: they participate in general computation and act as the dedicated interface for the RGB video output peripheral.

All arithmetic instructions read a register value and an immediate operand, then store the result in ACC.

The E8 includes a built-in RGB framebuffer accessible through two dedicated instructions and five shared registers.

Clears the entire display, resetting all pixels to the background state. No operands are required.

SCREEN_DRAW is a special instruction that does not take explicit operands in the instruction word. Instead, it reads pixel data directly from the general-purpose registers at execution time:

A typical drawing sequence looks like this:

1. Use MOV to set the desired R, G, and B values into registers A, B, and C.
2. Use MOV to set the target pixel coordinates into registers D (X) and E (Y).
3. Execute SCREEN_DRAW — the emulator reads the current contents of A–E and renders a single pixel at (D, E) with colour (A, B, C).
4. Repeat steps 1–3 to build up an image, or call SCREEN_CLEAR to wipe the canvas before redrawing.

Because the video registers are ordinary general-purpose registers, the same values can participate in arithmetic and control-flow logic before being sent to the display — enabling programmatic graphics, animations, and colour effects entirely in E8 assembly.

The C emulator loads assembled programs (.bin files), executes instructions cycle-by-cycle, and renders output to a display window. Sample test programs are included in the tests/ directory.

See LICENSE for licensing details.