Mr.Boom: the emulated 8086 registers in little-endian layout (2026-10-26)

Symptom. Debian’s ppc64 mrboom 5.5+ds-1 segfaulted on the PS3 within 15 s of every start: two played sessions, both on the welcome screen, after 13-14 s of play. dmesg: mrboom: segfault (11) at 1835c88a1 ... in mrboom, a load (lbzx) from an address far outside the program.

Evidence. The fault is in program(), at an LODSB in aff_lettre (the font code), with ESI 0x481dd440. Mr.Boom is the original DOS game translated from 8086 assembly to C: program() runs the translated code on emulated registers. ESI held a value no font offset could have.

Cause. mrboom.h gives the registers and the string instructions (LODSB, STOSW, …) a big-endian layout under MSB_FIRST, but only the libretro core gets that define, from retro_endianness.h. The SDL2 binary built on a big-endian machine used the little-endian layout: a byte written to AL landed in the top byte of EAX, so the font code’s offsets went out of range on its first frames of text.

Fix. Patch 0001 defines MSB_FIRST from the compiler’s byte order when the libretro header isn’t used (commit ec29378). In ppc64 qemu-user with the dummy SDL drivers, mrboom -e checks all 16 recorded demos against upstream’s frame numbers: SUCCESS, where Debian’s binary segfaults at once. Not yet run on the PS3: the next played session and mrboom -e -z there (docs/TESTS.md, batch mrboom-be).