08a1a9033a
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.
48 lines
1.9 KiB
ArmAsm
48 lines
1.9 KiB
ArmAsm
# Stage 1 for optical media -- the El Torito "no emulation" boot sector.
|
|
#
|
|
# This exists because bootasm.S cannot boot a CD. It loads stage 2 by talking
|
|
# straight to the primary IDE controller's ATA registers, and a CD is not an
|
|
# ATA disk: it answers to the ATAPI packet interface instead, and under
|
|
# El Torito it may not even be the drive at 0x1F0. Rather than teach a
|
|
# 512-byte sector two protocols, optical media get their own first stage.
|
|
#
|
|
# It has almost nothing to do, because El Torito's "load segment / sector
|
|
# count" fields do the reading for us: tools/mkiso.sh asks for
|
|
# 1 + STAGE2_SECTORS virtual sectors at 0x7C00, so by the time this runs the
|
|
# BIOS has already placed stage 2 at 0x7E00 -- exactly where bootasm.S would
|
|
# have put it. The only real work is recording *which* medium we came from,
|
|
# since stage2main.c has to read the kernel back with the matching protocol.
|
|
#
|
|
# The flag goes next to the verbose-boot flag, one byte past it, for the same
|
|
# reason that one sits where it does: it is clear of the 256-byte VBE mode
|
|
# info block at 0x30000 that stage2.S fills in a moment from now.
|
|
|
|
#define STAGE2_ADDR 0x7E00
|
|
#define BOOT_MEDIA_FLAG_SEG 0x3000
|
|
#define BOOT_MEDIA_FLAG_OFF 0x101
|
|
#define BOOT_MEDIA_CD 1
|
|
|
|
.code16
|
|
.global start
|
|
start:
|
|
cli
|
|
xorw %ax, %ax
|
|
movw %ax, %ds
|
|
movw %ax, %ss
|
|
movw $0x7c00, %sp
|
|
|
|
# "You were booted from a CD." stage2main.c reads this to decide between
|
|
# ATA and ATAPI; without it the loader would read the kernel off whatever
|
|
# happens to be sitting at 0x1F0, which on a CD boot is usually nothing.
|
|
movw $BOOT_MEDIA_FLAG_SEG, %ax
|
|
movw %ax, %es
|
|
movw $BOOT_MEDIA_FLAG_OFF, %di
|
|
movb $BOOT_MEDIA_CD, %al
|
|
movb %al, %es:(%di)
|
|
|
|
# stage2.S expects zeroed segments and an inherited stack, same as it gets
|
|
# from bootasm.S.
|
|
xorw %ax, %ax
|
|
movw %ax, %es
|
|
|
|
ljmp $0, $STAGE2_ADDR
|