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.
101 lines
3.4 KiB
ArmAsm
101 lines
3.4 KiB
ArmAsm
# Stage 1 — the actual MBR boot sector. Hard-limited to 510 bytes (+2-byte
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# 0x55AA signature) by the BIOS, so there's no room here for anything but
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# "load stage 2 off disk and jump to it". All the VBE/A20/protected-mode/
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# splash/kernel-loading work now lives in stage2.S + stage2main.c, which
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# aren't sector-size-constrained.
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#
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# How many sectors of stage2 to pull in. This, the padding the Makefile gives
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# stage2's binary, and the "+(STAGE2_SECTORS+1)" sector math in stage2main.c's
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# readseg() all have to agree on where the kernel starts on disk -- so the
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# Makefile passes -DSTAGE2_SECTORS and is the single authority. The value below
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# is only a fallback for reading this file standalone.
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#
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# It is 48, not the old 24, because stage2 now carries the boot logo
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# (include/logo.h, 8 KiB). The real ceiling is memory rather than this number:
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# stage2 loads at 0x7E00 and the multiboot kernel-search buffer starts at
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# 0x10000, so stage2 cannot exceed 0x8200 bytes = 65 sectors.
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#ifndef STAGE2_SECTORS
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#define STAGE2_SECTORS 48
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#endif
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#define STAGE2_ADDR 0x7E00
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.code16
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.global start
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start:
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cli
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xorw %ax, %ax
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movw %ax, %ds
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movw %ax, %es
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movw %ax, %ss
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movw $0x7c00, %sp
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# Read STAGE2_SECTORS sectors, starting at disk sector 1 (LBA), into
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# memory right after this boot sector (0x7E00). Raw IDE PIO (LBA28) —
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# same register convention as stage2main.c's own readsect(), just
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# inlined here since we can't call C code yet. Port numbers 0x1F2-0x1F7
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# are all > 0xFF, so they must go through %dx — `out $imm8,%al` only
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# encodes an 8-bit port and silently truncates anything larger.
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# "You were not booted from a CD." stage2main.c reads this byte to choose
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# between ATA and ATAPI when it loads the kernel, and nothing else clears
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# it -- so a warm reboot from a disc could otherwise leave the flag set and
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# send a perfectly ordinary hard-disk boot looking for an optical drive.
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# Placed one byte past the verbose flag; both sit clear of the 256-byte VBE
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# mode info block that stage2.S is about to write at 0x30000.
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movw $0x3000, %ax
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movw %ax, %es
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movw $0x101, %di
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movb $0, %al
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movb %al, %es:(%di)
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xorw %ax, %ax
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movw %ax, %es
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movw $STAGE2_ADDR, %di # ES:DI destination, ES=0
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movw $1, %si # first LBA sector to read
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movb $STAGE2_SECTORS, %bl
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.Lsector:
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.Lwait1:
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movw $0x1F7, %dx
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inb %dx, %al
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andb $0xC0, %al
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cmpb $0x40, %al
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jne .Lwait1
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movw $0x1F2, %dx
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movb $1, %al
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outb %al, %dx # sector count = 1
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incw %dx # 0x1F3
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movw %si, %ax
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outb %al, %dx # LBA [7:0]
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incw %dx # 0x1F4
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xorb %al, %al
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outb %al, %dx # LBA [15:8]
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incw %dx # 0x1F5
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outb %al, %dx # LBA [23:16]
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incw %dx # 0x1F6
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movb $0xE0, %al
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outb %al, %dx # drive 0, LBA mode
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incw %dx # 0x1F7
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movb $0x20, %al
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outb %al, %dx # READ SECTORS command
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.Lwait2:
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inb %dx, %al # %dx still 0x1F7
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andb $0xC0, %al
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cmpb $0x40, %al
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jne .Lwait2
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movw $256, %cx # 256 words = 512 bytes
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movw $0x1F0, %dx
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rep insw # reads into ES:DI, DI += 512 automatically
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incw %si
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decb %bl
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jnz .Lsector
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ljmp $0, $STAGE2_ADDR
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