Scott Data Systems 1

A gate-level emulator of the computer from But How Do It Know? by John Clark Scott. It runs at https://scott-cpu.cs.montana.edu/.

The page builds the circuit and the drawing from the same description, so every wire on the screen is a net in the machine. The registers are named A, B, C and D, the way an 8-bit machine names them.

Use

Open index.html in a browser. No build step and no server are needed.

The editor is Monaco, loaded from a pinned CDN copy so the 13 MB editor is not checked in. With no network the page falls back to a plain text box and everything else keeps working, so a lecture survives a dead connection.

Control	Effect
START / STOP	Start and stop the clock. START stops on a jump to its own address
BACK TICK	Undo the last tick
TICK	One quarter of a step
SINGLE STEP	One stepper step
INSTR	All six steps of one instruction
RESET	Clear the registers. Memory stays
CLEAR MEM	Set all 256 bytes to zero

With the clock stopped:

• Click a register bit or a memory bit to flip it.
• Click a wire to hold it on. A second click lets it go.
• Click the clock for the next phase. Click the stepper for the next step.
• Held step wires are exclusive. A click on one releases the others.

Query parameters: ?prog=and or ?prog=sample loads a program. ?speed=20 sets the clock rate.

The assembly language

MOV A, 10       load a number          ADD A, B        A = A + B
MOV A, [B]      read memory at B       NOT A           also SHL and SHR
MOV [B], A      write memory at B      CMP A, B        set the flags only
JMP label       jump                   JE / JG / JGE   jump on the flags
HLT             stop                   DB 1, 2, 3      data bytes

The registers are A, B, C and D, and each holds a byte. A number is decimal, 0x hex or 0b binary. A label is written name: and then used as an address. A comment starts with ;, and a comma between operands is optional.

CMP sets the flags and a jump reads them, so a conditional jump must follow its CMP directly. JG and JGE compare unsigned, because the comparator reports unsigned "larger".

Press HELP in the page for the same guide.

Credits

Design of the computer copyright John Clark Scott 2009, https://buthowdoitknow.com/. The design may be used freely for personal and educational use while the copyright stays visible.

This emulator is an independent implementation written for CSCI 366 at Montana State University.

Deployment

GitHub Pages serves the main branch from the root. CNAME holds the custom domain.