FA-89338 / Digital logic simulation / Member archive

Pointers wrap at the FIFO depth · case 03

Full and empty become indistinguishable after one wrap.

Member previewVariant 3 · 3 implementations · 6 checks per implementation

Case contract

Input [depth, ops] with depth a power of two >= 4 and ops 'w','r','n'. Pointers are binary modulo 2*depth. Each step first computes flags from gray codes (g = b ^ (b >> 1)) using the other side's pointer as it was two steps earlier: full when gray(w) equals gray(r_sync) with its top two bits inverted, empty when gray(r) equals gray(w_sync). A write happens only if not full, a read only if not empty. Return [[full, empty] per step, [writes accepted, reads accepted]].

Why this case matters

Clock-domain-crossing FIFOs are simulated with synchronizer latency; flag comparisons on gray pointers are a common source of model bugs.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
fill until full	`[[[0, 1], [0, 1], [0, 0], [0, 0], [0, 0], [0, 1], [0, 0], [0, 0]], [8, 0]]`	`[[[0, 1], [0, 1], [0, 0], [0, 0], [1, 0], [1, 0], [1, 0], [1, 0]], [4, 0]]`	Failed

MEMBER ARCHIVE

The complete case is available to members.

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

Member access is invitation-based. Sign in with your invited account to inspect the sources.