The 8-tape minimal accumulator CPU — ISA and programming guide

For mtm-cpu.automaton, an 8-tape Turing machine (MTM) that fetches, decodes and executes a stored program. It is a real CPU: the program is data on tape 1, and the machine walks it, decodes opcodes, and runs an ALU, a RAM, an index register and a call stack against it.

Specifications

Machine	8-tape deterministic Turing machine, 614 states, 19,191 transitions
Word size	5 bits, unsigned, wraps mod 32 (two's complement for SUB)
Registers	A accumulator, X index register, one flag
RAM	32 words, addresses 0–31, 5 cells per word, total 160 bits
Program	up to 256 instructions (8-bit absolute jump targets)
Call stack	8-bit return addresses, one frame deep in practice
Opcodes	19
Halting	reaching ! is the machine's single accepting state

The tapes

#	Role
1	ROM — the program, and the only tape the input is written on
2	A — accumulator, 5 bits
3	FLAG — f (set) or g (clear)
4	OPERAND — the decoded operand, then the unary tally the RAM walk consumes
5	RAM — 32 words × 5 cells
6	CALL counter — unary work tape for the return-address computation
7	X — index register, 5 bits
8	Return stack — 8-bit return addresses

Only tape 1 is written by you. The rest start blank and are set up by the machine's own initialisation sequence.

Closest real-life counterpart

A 5-bit MOS 6502. The correspondence is not superficial — it is the register file and the mnemonics:

• One accumulator plus one X index register, with TAX / TXA to move between them and LDX / STX for X-indexed memory. Those are 6502 mnemonics, and this machine's U V D E are exactly them.

• CMP sets the flag on A >= operand — that is the 6502's carry flag after a compare, not a zero flag, and the conditional branch is therefore a BCS.

• Accumulator-plus-index with immediate and indexed addressing, and JSR/RTS subroutines, is the 6502 programming model in miniature.

Architecturally it also sits in the PDP-8 / Intersil 6100 lineage — a single accumulator, memory-reference instructions, no general-purpose register file. What it lacks against both is interrupts, a stack pointer you can touch, indirect addressing through memory, and any I/O; and its jumps are unusually shaped (see below).

Program format

^ flag # seed # <instruction>... !

• flag is f or g — the flag's initial value.
• seed is 5 bits — the initial accumulator.
• Every instruction is OP # operand #, including the trailing #. Leaving it off is the single most common mistake; the machine will reject.
• ! is HLT, and reaching it is what makes the run accept.

Instructions are numbered from 0 for jump targets.

^f#00000#A#00011#!          A = 0 + 3, halt

Instruction set specification

Operand widths differ by group; the assembler handles this for you.

Arithmetic and logic — 5-bit immediate

Op	Mnemonic	Effect
A	ADD n	A = A + n (mod 32)
S	SUB n	A = A - n (mod 32)
N	AND n	A = A & n
O	ORA n	A = A | n
X	EOR n	A = A ^ n
C	CMP n	flag f if A >= n, else g. A unchanged

Memory — 5-bit address

Op	Mnemonic	Effect
L	LDA addr	A = RAM[addr]
T	STA addr	RAM[addr] = A

Index register — operand is the literal 0

Op	Mnemonic	Effect
U	TAX	X = A
V	TXA	A = X
E	STX	RAM[X] = A (indexed store)
D	LDX	A = RAM[X] (indexed load)
W	JMPX	jump to the instruction numbered X

Control flow

Op	Mnemonic	Operand	Effect
J	BCS d	one digit 0–9	if flag is f, skip d instructions
M	SKP d	one digit 0–9	skip d instructions, always
P	JMP n	8-bit	jump to absolute instruction n
K	JSR n	8-bit	push return address, jump to n
R	RTS	literal 0	pop and return
!	HLT	none	halt and accept

The two branch shapes are the thing to internalise. BCS/SKP are relative forward skips capped at 9 instructions, so they can only jump ahead and only a short way. JMP/JSR are absolute and can go anywhere, including backwards. So every loop is built the same way: a BCS forward to the exit, and a JMP backward to the top.

Writing programs

The idiom for a counted loop:

        .acc 0
loop:   ADD 1           ; 0
        CMP 3           ; 1   flag = f once A >= 3
        BCS done        ; 2   forward, short
        JMP loop        ; 3   backward, absolute
done:   HLT             ; 4

Multiply by repeated addition — same shape, different step:

        .acc 0
loop:   ADD 4
        CMP 12
        BCS done
        JMP loop
done:   HLT

A subroutine. JSR pushes the address of the instruction after it, so RTS resumes there; the caller has to jump over the subroutine body:

        .acc 0
        ADD 1           ; 0
        JSR sub         ; 1   returns to 2
        ADD 1           ; 2
        SKP 2           ; 3   hop over the body
sub:    ADD 10          ; 4
        RTS             ; 5
        HLT             ; 6

Indexed access, walking a small table:

        .acc 7
        TAX             ; X = 7
        STX             ; RAM[7] = A
        AND 0           ; clear A
        LDX             ; A = RAM[7]
        HLT

Things that will catch you

• The trailing # is mandatory on every operand.
• BCS/SKP cannot reach backwards and cannot skip more than 9. The assembler will refuse a label that is out of range and tell you to use JMP.
• CMP is >=, not ==. A loop that waits for equality will overshoot if the step does not divide the target; test A >= limit.
• Words wrap at 32. ADD 5 on 30 gives 3.
• RTS with an empty stack halts, it does not fault.
• A jump past the last instruction halts rather than running off the end.
• Instruction numbering starts at 0 and counts instructions, not characters.

Assembler

mtm-cpu-asm.mjs in this folder assembles and disassembles.

node mtm-cpu-asm.mjs asm prog.s          # assembly -> tape string
node mtm-cpu-asm.mjs dis '^f#00000#...'  # tape string -> assembly
node mtm-cpu-asm.mjs demo                # the worked examples above

Paste the tape string into the app's run box with mtm-cpu.automaton open. Directives are .acc n (initial accumulator) and .flag f|g; ; starts a comment; label: marks an instruction.

How to run

Automaton	Download mtm-cpu.automaton (10 MB)
AutomataStudio	Open in the browser, or get an offline copy
Assembler	Web assembler, or mtm-cpu-asm.mjs below

The complete process is straightforward:

1. Load the automaton. Download mtm-cpu.automaton, open AutomataStudio, and drag the file into it.

2. Write your program. Write it in a .s file (or straight into the web assembler) using the assembly language defined in this specification. Follow the machine's ISA, operand formats, directives, and control-flow rules described above.

   For example:

   .flag f
   .acc 0
   
   ADD 3
   HLT
   

3. Assemble the program. Run the assembler to convert the assembly program into the machine's tape-string format:

   node mtm-cpu-asm.mjs asm prog.s
   

   The assembler will produce a string beginning with the required program header and containing the encoded instructions, for example:

   ^f#00000#A#00011#! 
   

4. Copy the generated machine code. Copy the complete tape string produced by the assembler. Do not modify it manually—the separators, operand encoding, and trailing # characters are part of the machine's input format.

5. Paste it into Automata Studio. In Automata Studio, open the Simulate String section for the loaded mtm-cpu.automaton. Paste the generated tape string into the input/language field.

6. Run the machine. Start the simulation by pressing Play. Automata Studio will execute the 8-tape Turing machine automatically. The machine will initialize its internal tapes, fetch and decode the program from tape 1, execute each instruction, and perform the required ALU, RAM, index-register, and control-flow operations.

7. Inspect the result. Once the simulation runs, the resulting machine state/output is displayed in Automata Studio below the simulation area. This is where you can observe the result produced by your program and inspect the machine's execution.

In short, the workflow is:

mtm-cpu.automaton
       ↓
Load into Automata Studio
       ↓
Write assembly program (.s)
       ↓
Run mtm-cpu-asm.mjs
       ↓
Generated tape string / machine code
       ↓
Copy the string
       ↓
Paste into Automata Studio → Simulate String
       ↓
Press Play
       ↓
Machine executes the program
       ↓
Inspect the result below the simulation

The important distinction is that you never manually construct the machine's tape encoding. Write the program in the provided assembly language, let mtm-cpu-asm.mjs assemble it (or use the web based interface), and use the resulting tape string as the input to mtm-cpu.automaton.

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Made by Shreyan Chaubey. Released under the MIT License.