FA-89529 / Instruction set emulation / Member archive

Signed comparison ignores overflow · case 04

ge gives the wrong answer when the subtraction overflowed.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Input [cond]. Evaluate the condition for all 16 flag states, ordered as NZCV from 0000 to 1111 (N is the most significant), returning 1/0. eq Z; ne !Z; cs C; cc !C; mi N; pl !N; vs V; vc !V; hi C and !Z; ls !C or Z; ge N==V; lt N!=V; gt !Z and N==V; le Z or N!=V; al always. Carry here is set when an unsigned subtraction did not borrow.

Why this case matters

Predicated execution in ARM-style emulators depends on exact condition predicates; unsigned conditions are easily confused with ISAs where carry means borrow.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
condition ge over all flags	`[1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0]`	`[1, 0, 1, 0, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1, 0, 1]`	Failed

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