FA-89833 / Instruction set emulation / Member archive

Writes to unmapped space do not drive the bus · case 03

A read of unmapped space after an unmapped write returns an older bus value.

Member previewVariant 3 · 3 implementations · 9 checks per implementation

Case contract

Input [rom_size, sram_enabled, ops] with ops ['r', addr] or ['w', addr, value] on a 16-bit bus. 0x0000-0x1FFF: 2 KiB RAM mirrored every 0x800. 0x2000-0x3FFF: 8 video registers mirrored every 8 bytes. 0x4000-0x4017: I/O registers. 0x4018-0x5FFF: unmapped. 0x6000-0x7FFF: SRAM when enabled, else unmapped. 0x8000-0xFFFF: ROM of rom_size bytes mirrored to fill the window, byte at offset o = (7o + (o >> 8) + 3) & 0xFF, writes ignored. Every write drives its value onto the data bus; reads of unmapped space return the last bus value, other reads drive the bus. Unwritten RAM/register/SRAM bytes read 0. Writes yield None, reads the value.

Why this case matters

Console and microcontroller emulators depend on exact address decoding; mirroring and open-bus behaviour are relied on by real software.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
open bus after unmapped write	`[null, 0, null, 19]`	`[null, 119, null, 19]`	Failed

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