FA-89240 / Digital logic simulation / Member archive

Unknown select corrupts every decoder line · case 05

A decoder with one unknown select bit drives x on lines that cannot be selected.

Member previewVariant 5 · 3 implementations · 9 checks per implementation

Case contract

Input [kind, sel, data]: sel is MSB-first over 0/1/x/z; an x or z select bit makes both values of that bit candidates. mux: the output is the common value of all candidate data inputs if they agree and are 0 or 1, else 'x'. dec: data[0] is the enable; enable 0 gives all '0'; otherwise candidate lines are '1' only when there is a single candidate and the enable is 1, else 'x'; non-candidate lines are '0'.

Why this case matters

Unknown-select pessimism rules in RTL simulation decide whether X on a control input corrupts datapath outputs.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
decoder unknown select bit	`["x", "x", "x", "x"]`	`["0", "0", "x", "x"]`	Failed

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