nexoniarz/SchOS
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boot: separate BIOS from UEFI

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boot/bios/ now holds the MBR, the El Torito stub and the real-mode stage
2; boot/uefi/ is empty and documented.

Separate directories rather than #ifdefs, because the two share almost
nothing. A BIOS loader lives in 16-bit real mode, reaches the firmware
through software interrupts, and walks itself up into long mode. A UEFI
loader is a 64-bit PE that the firmware has already placed in long mode
with paging on, calling services through a table of function pointers.
What they do share is the hand-off contract, which boot/README.md now
writes down: ELF segments placed, long mode, a 24-byte-entry memory map
at E820_MAP_ADDR, framebuffer geometry at VBE_INFO_ADDR.

That contract is the point of documenting it now. UEFI's GetMemoryMap
returns EFI_MEMORY_DESCRIPTORs, and the right move is to translate them
into the entries kernel/arch/x86_64/e820.c already parses rather than
teach the kernel a second format.

Build output paths under $(BUILD)/boot are unchanged; only sources moved.
This commit is contained in:
Nexoniarz 2026-08-09 16:00:03 +02:00
commit 08a1a9033a
19 changed files with 125 additions and 33 deletions
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22
Makefile
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@ -383,10 +383,10 @@ qemu-iso: schos.iso
# splash, and loads the kernel — not size-constrained the same way, just
# padded out to a fixed STAGE2_SECTORS*512 bytes so the kernel's starting
# sector on disk is predictable (kept in sync with STAGE2_SECTORS in
# boot/bootasm.S and boot/stage2main.c).
# boot/bios/bootasm.S and boot/bios/stage2main.c).
# -----------------------------------------------------------------------
# STAGE2_SECTORS used to be written out three times -- here, in boot/bootasm.S
# and in boot/stage2main.c -- and docs/layout.md carried a warning about keeping
# STAGE2_SECTORS used to be written out three times -- here, in boot/bios/bootasm.S
# and in boot/bios/stage2main.c -- and docs/layout.md carried a warning about keeping
# them in step. They are now all fed from this one variable via -D, so the
# warning describes a problem that can no longer happen. Both C/asm files keep
# an #ifndef fallback purely so they still read sensibly on their own.
@ -405,9 +405,9 @@ STAGE2_DEF = -DSTAGE2_SECTORS=$(STAGE2_SECTORS)
# that make carried on and padstage2.pl received nonsense.
STAGE2_BYTES = $$(( $(STAGE2_SECTORS) * 512 ))
$(BUILD)/boot/bootblock: boot/bootasm.S
$(BUILD)/boot/bootblock: boot/bios/bootasm.S
@mkdir -p $(BUILD)/boot
$(CC32) $(STAGE2_DEF) -fno-builtin -fno-pic -nostdinc -I./include -c boot/bootasm.S -o $(BUILD)/boot/bootasm.o
$(CC32) $(STAGE2_DEF) -fno-builtin -fno-pic -nostdinc -I./include -c boot/bios/bootasm.S -o $(BUILD)/boot/bootasm.o
$(LD) -m elf_i386 -nostdlib $(LD_LOWBASE) -n -N -e start -Ttext 0x7C00 \
-o $(BUILD)/boot/bootblocktmp.o $(BUILD)/boot/bootasm.o
$(OBJCOPY) -S -O binary -j .text $(BUILD)/boot/bootblocktmp.o $(BUILD)/boot/bootblock
@ -415,19 +415,19 @@ $(BUILD)/boot/bootblock: boot/bootasm.S
# Stage 1 for optical media. Same job as bootblock, far less of it: El Torito
# has already loaded stage2 alongside this sector by the time it runs, so it
# only records that we came from a CD and jumps. See boot/cdboot.S.
$(BUILD)/boot/cdboot: boot/cdboot.S
# only records that we came from a CD and jumps. See boot/bios/cdboot.S.
$(BUILD)/boot/cdboot: boot/bios/cdboot.S
@mkdir -p $(BUILD)/boot
$(CC32) -fno-builtin -fno-pic -nostdinc -I./include -c boot/cdboot.S -o $(BUILD)/boot/cdbootasm.o
$(CC32) -fno-builtin -fno-pic -nostdinc -I./include -c boot/bios/cdboot.S -o $(BUILD)/boot/cdbootasm.o
$(LD) -m elf_i386 -nostdlib $(LD_LOWBASE) -n -N -e start -Ttext 0x7C00 \
-o $(BUILD)/boot/cdboottmp.o $(BUILD)/boot/cdbootasm.o
$(OBJCOPY) -S -O binary -j .text $(BUILD)/boot/cdboottmp.o $@
./tools/sign.pl $@
$(BUILD)/boot/stage2: boot/stage2.S boot/stage2main.c include/logo.h
$(BUILD)/boot/stage2: boot/bios/stage2.S boot/bios/stage2main.c include/logo.h
@mkdir -p $(BUILD)/boot
$(CC32) $(STAGE2_DEF) -fno-builtin -fno-pic -nostdinc -I./include -c boot/stage2.S -o $(BUILD)/boot/stage2asm.o
$(CC32) $(STAGE2_DEF) -fno-builtin -fno-pic -O -nostdinc -I./include -c boot/stage2main.c -o $(BUILD)/boot/stage2main.o
$(CC32) $(STAGE2_DEF) -fno-builtin -fno-pic -nostdinc -I./include -c boot/bios/stage2.S -o $(BUILD)/boot/stage2asm.o
$(CC32) $(STAGE2_DEF) -fno-builtin -fno-pic -O -nostdinc -I./include -c boot/bios/stage2main.c -o $(BUILD)/boot/stage2main.o
$(LD) -m elf_i386 -nostdlib $(LD_LOWBASE) -n -N -e stage2start -Ttext 0x7E00 \
-o $(BUILD)/boot/stage2tmp.o $(BUILD)/boot/stage2asm.o $(BUILD)/boot/stage2main.o
$(OBJCOPY) -S -O binary -j .text -j .rodata -j .data $(BUILD)/boot/stage2tmp.o $(BUILD)/boot/stage2