FA-89689 / Instruction set emulation / Member archive

Protection fault reported as not-present · case 04

A permission violation on a present page has the present bit clear in its error code.

Member previewVariant 4 · 3 implementations · 9 checks per implementation

Case contract

Input [va, write, user, root, mem]; mem maps decimal-string physical addresses to 32-bit entries (missing = 0). VA splits 10/10/12 (directory index va>>22, table index (va>>12)&0x3FF, offset). The PDE is at root + 4*dir, the PTE at (PDE frame << 12) + 4*table. Bits: P=1, W=2, U=4, A=32, D=64; frame = entry >> 12. Not present -> ['fault', err]; write without W on both levels or user access without U on both levels -> ['fault', err | 1], where err = 2 if write plus 4 if user. On success set A on both entries and D on the PTE only for writes, returning ['ok', physical address, sorted [address, entry] of memory].

Why this case matters

MMU emulation must combine permissions across levels and update accessed/dirty bits exactly as guest operating systems expect.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
kernel write to read-only page	`["fault", 2]`	`["fault", 3]`	Failed

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