FA-89845 / Bytecode virtual machines / Member archive

Relative-jump VM: JMP displacement measured from the opcode byte · case 05

Backward loops land two bytes early and re-execute the wrong instruction; forward jumps fall short.

Member previewVariant 5 · 3 implementations · 9 checks per implementation

Case contract

Flat int bytecode, byte addressed: 0 HALT, 1 PUSH imm, 2 ADD, 3 SUB (second-from-top minus top), 4 JMP rel, 5 JZ rel (pops condition, branches when it is 0), 6 DUP, 7 SWAP, 8 PRINT (pop to out). Relative displacements are measured from the address after the one-byte operand. Every executed instruction including HALT costs one unit of fuel; with fuel exhausted report status fuel. Return status, pc, printed values and stack; stack underflow, bad pc and bad opcode are statuses.

Why this case matters

Branch displacement bases, operand order and fuel accounting are classic interpreter-loop defects.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: countdown loop prints n..1	`{"out": [6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6], "pc": 0, "stack": [5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5], "status": "fuel"}`	`{"out": [6, 5, 4, 3, 2, 1], "pc": 12, "stack": [0], "status": "halt"}`	Failed

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This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

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