DOOM Port for MayteraOS - Debug Notes

The port is built from id Software's DOOM engine source, used under id's DOOM Source Code License (not the GPL); the per-file id Software notices are kept intact in the source. See Licenses for details. It requires a user-supplied DOOM.WAD on the boot disk; no WAD game data is included with or distributed by MayteraOS.

Status at the time: NOT WORKING

Issue: Z_Malloc fails with zone size=0

Problem Summary

DOOM's zone memory allocator (z_zone.c) is receiving 0 bytes from I_ZoneBase(), causing all Z_Malloc calls to fail.

Root Cause Analysis

The Bug

I_ZoneBase() in i_system.c returns mb_used * 1024 * 1024 bytes, but mb_used is 0 at call time.

Why mb_used is 0

1. mb_used is defined in i_system.c as int mb_used = 32;
2. m_misc.c has config entry {"mb_used", &mb_used, 32}
3. M_LoadDefaults() runs BEFORE Z_Init() and sets mb_used = defaults[i].defaultvalue
4. However, something is still setting it to 0

Suspected Issues

• The config default array in m_misc.c may not be applying correctly
• There may be another place resetting mb_used
• Compiler optimization might be causing issues with extern variable linkage

Files Modified

kernel/games/doom/i_system.c

int mb_used = 32;  // Changed from 6

byte* I_ZoneBase (int* size) {
    *size = mb_used*1024*1024;
    byte *ptr = (byte *)malloc(*size);
    kprintf("[DOOM] I_ZoneBase: requested %d bytes, got ptr=%p\n", *size, ptr);
    return ptr;
}

kernel/games/doom/m_misc.c

{"mb_used", &mb_used, 32},  // Changed default from 2 to 32

kernel/games/doom/z_zone.c

Added debug output:

kprintf("[DOOM] Z_Malloc FAIL: requested %d bytes, zone size=%d\n", size, mainzone->size);

kernel/mm/heap.c

#define HEAP_INITIAL_SIZE   (64 * MB)  // Changed from 16MB

kernel/gui/desktop.c

Added serial command handler (not fully working):

static void process_serial_commands(void);
// Called from desktop_run() main loop

What Works

• WAD file loading (the user-supplied 11MB DOOM.WAD loads successfully)
• M_Init() starts
• R_Init() starts loading lumps
• Kernel heap allocation (64MB initial)

What Fails

• Z_Init() gets 0 bytes from I_ZoneBase()
• All subsequent Z_Malloc() calls fail
• DOOM enters error loop

Suggested Fix (Not Yet Tested)

Hardcode the zone size directly in I_ZoneBase to bypass mb_used:

byte* I_ZoneBase (int* size) {
    *size = 32 * 1024 * 1024;  // Hardcode 32MB
    byte *ptr = (byte *)malloc(*size);
    kprintf("[DOOM] I_ZoneBase: allocated %d bytes at %p\n", *size, ptr);
    return ptr;
}

Serial Command Feature

Added but not fully working at the time. Issue: the QEMU serial socket did not receive input correctly.

Location: kernel/gui/desktop.c - process_serial_commands()

The intent at the time was a serial command to launch DOOM plus a command listing. That handler was never shipped in this form; today DOOM is launched from the desktop (Applications menu), not by a dedicated serial command.

Backups

Timestamped backups of the modified files (d_main.c, desktop.c, heap.c, i_system.c, z_zone.c) were taken before the change, per the project backup workflow.

Next Steps to Fix

1. Hardcode zone size in I_ZoneBase (bypass mb_used)
2. Or: Add kprintf in M_LoadDefaults to trace mb_used value
3. Or: Initialize mb_used in Z_Init before calling I_ZoneBase
4. Test with DOOM1.WAD (shareware, smaller) if the user's registered WAD still fails

Test Procedure

1. Start the test VM
2. Wait ~90 seconds for GUI to load
3. Click DOOM icon in Applications menu
4. Check the captured serial log for [DOOM] lines