banan-os/bootloader/bios/disk.S

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# FIXME: don't assume 512 byte sectors
.set SECTOR_SIZE_SHIFT, 9
.set SECTOR_SIZE, 1 << SECTOR_SIZE_SHIFT
.code16
.section .stage1
.global stage2_start
.global stage2_end
# check that drive has int13 ext
# dl: drive number
# returns only if drive does have the extension
drive_has_int13_ext:
pusha
movb $0x41, %ah
movw $0x55AA, %bx
int $0x13
jc .drive_has_int13_ext_no_int13_ext
popa
ret
.drive_has_int13_ext_no_int13_ext:
mov $no_int13_ext_msg, %si
jmp print_and_halt
# read sectors from disk
# bx:eax: lba start
# cx: lba count (has to less than 0x80)
# dl: drive number
# ds:di: physical address
# returns only on success
.global read_from_disk
read_from_disk:
pusha
call drive_has_int13_ext
# prepare disk read packet
mov $disk_address_packet, %si
movb $0x10, 0x00(%si) # packet size
movb $0x00, 0x01(%si) # always 0
movw %cx, 0x02(%si) # lba count
movw %di, 0x04(%si) # offset
movw %ds, 0x06(%si) # segment
movl %eax, 0x08(%si) # 32 bit lower lba
movw %bx, 0x0C(%si) # 16 bit upper lba
movw $0, 0x0E(%si) # zero
# issue read command
mov $0x42, %ah
int $0x13
jc .read_from_disk_failed
popa
ret
.read_from_disk_failed:
mov $read_from_disk_msg, %si
jmp print_and_halt
# Reads GPT header into gpt_header buffer
# dl: drive number
# return:
# ax: 1 if has GPT header, 0 otherwise
.global read_gpt_header
read_gpt_header:
pushw %bx
pushw %cx
pushw %di
xorw %bx, %bx
movl $1, %eax
movw $1, %cx
movw $gpt_header, %di
call read_from_disk
xorw %bx, %bx
movw $1, %ax
# check if header starts with 'EFI PART'
cmpl $0x20494645, (gpt_header + 0)
cmovnew %bx, %ax
cmpl $0x54524150, (gpt_header + 4)
cmovnew %bx, %ax
popw %di
popw %cx
popw %bx
ret
# Find bios boot partition from boot drive
# returns:
# bx:eax: first lba
# cx: sector count
find_stage2_partition:
# read boot disk GPT header
movb (boot_disk_number), %dl
call read_gpt_header
testb %al, %al
jz .find_stage2_partition_not_gpt
# eax := entry_count
movl (gpt_header + 80), %eax
test %eax, %eax
jz .find_stage2_partition_not_found
# edx:eax := eax * entry_size
mull (gpt_header + 84)
test %edx, %edx
jnz .find_stage2_partition_too_big_entries
# FIXME: read one entry array section at a time
# sector count := (arr_size + SECTOR_SIZE - 1) / SECTOR_SIZE
movl %eax, %ecx
shrl $SECTOR_SIZE_SHIFT, %ecx
# start lba
movl (gpt_header + 72), %eax
movw (gpt_header + 76), %bx
movw $gpt_entry_data, %di
movw $bios_boot_guid, %si
movb (boot_disk_number), %dl
call read_from_disk
# NOTE: 'only' 0xFFFF partitions supported,
# although read will fail with more than 0x80
movw (gpt_header + 80), %cx
.find_stage2_partition_loop_gpt_entries:
pushw %cx
movw $16, %cx
call memcmp
popw %cx
testb %al, %al
jnz .find_stage2_partition_found
# add entry size to entry pointer
addw (gpt_header + 84), %di
loop .find_stage2_partition_loop_gpt_entries
# fall through to not found case
.find_stage2_partition_not_found:
movw $no_bios_boot_partition_msg, %si
jmp print_and_halt
.find_stage2_partition_not_gpt:
movw $not_gpt_partition_msg, %si
jmp print_and_halt
.find_stage2_partition_too_big_entries:
movw $too_gpt_big_entries_msg, %si
jmp print_and_halt
.find_stage2_partition_found:
# first lba
movl 32(%di), %eax
movw 36(%di), %bx
# count := last lba - first lba + 1
movl 40(%di), %ecx
subl %eax, %ecx
incl %ecx
ret
# reads stage2 into memory
# dl: boot drive number
# returns only on success
.global read_stage2_into_memory
read_stage2_into_memory:
movb %dl, (boot_disk_number)
# push stage2 sector count
movl $stage2_end, %eax
subl $stage2_start, %eax
addl $(SECTOR_SIZE - 1), %eax
movl $SECTOR_SIZE, %ecx
xorl %edx, %edx
divl %ecx
pushl %eax
call find_stage2_partition
movb (boot_disk_number), %dl
popl %ecx # FIXME: validate that partition has enough sectors
movw $stage2_start, %di
call read_from_disk
ret
# 21686148-6449-6E6F-744E-656564454649
.align 4
bios_boot_guid:
.long 0x21686148 # little endian
.word 0x6449 # little endian
.word 0x6E6F # little endian
.word 0x4E74 # big endian
.quad 0x494645646565 # big endian
boot_disk_number:
.skip 1
read_from_disk_msg:
.asciz "read error"
no_int13_ext_msg:
.asciz "no INT13 ext"
no_bios_boot_partition_msg:
.asciz "no bios boot"
too_gpt_big_entries_msg:
.asciz "too big GPT array"
not_gpt_partition_msg:
.asciz "not GPT"
.section .stage2
# check if drive exists
# dl: drive number
# return:
# al: 1 if disk is usable, 0 otherwise
drive_exists:
pusha
movb $0x48, %ah
movw $disk_drive_parameters, %si
movw $0x1A, (disk_drive_parameters) # set buffer size
int $0x13
jc .drive_exists_nope
popa
movb $1, %al
ret
.drive_exists_nope:
popa
movb $0, %al
ret
# find root disk and populate root_disk_drive_number field
# NO REGISTERS SAVED
.global find_root_disk
find_root_disk:
movb $0x80, %dl
.find_root_disk_loop:
call drive_exists
testb %al, %al
jz .find_root_disk_not_found
# read GPT header
xorw %bx, %bx
movl $1, %eax
movw $1, %cx
movw $gpt_header, %di
call read_from_disk
# confirm header (starts with 'EFI PART')
cmpl $0x20494645, (gpt_header + 0)
jne .find_root_disk_next_disk
cmpl $0x54524150, (gpt_header + 4)
jne .find_root_disk_next_disk
# compare disk GUID
movw $root_disk_guid, %si
movw $(gpt_header + 56), %di
movw $16, %cx
call memcmp
testb %al, %al
jz .find_root_disk_next_disk
movw $root_disk_found_msg, %si
call puts; call print_newline
movb %dl, (root_disk_drive_number)
ret
.find_root_disk_next_disk:
incb %dl
jmp .find_root_disk_loop
.find_root_disk_not_found:
movw $root_disk_not_found_msg, %si
jmp print_and_halt
# finds root partition from root disk
# fills root_partition_entry data structure
# NOTE: assumes GPT header is in `gpt_header`
# NO REGISTERS SAVED
# return:
# dl: drive number
# ecx: sector count (capped at 0xFFFFFFFF)
# bx:eax: first sector
.global find_root_partition
find_root_partition:
pushl %ebp
movl %esp, %ebp
subl $16, %esp
# esp + 0: 8 byte entry array lba
movl (gpt_header + 72), %eax
movl %eax, 0(%esp)
movl (gpt_header + 76), %eax
movl %eax, 4(%esp)
# FIXME: check that bits 48-63 are zero
# esp + 8: 4 byte entries per sector
xorl %edx, %edx
movl $SECTOR_SIZE, %eax
divl (gpt_header + 84)
movl %eax, 8(%esp)
# esp + 12: 4 byte entries remaining
movl (gpt_header + 80), %eax
testl %eax, %eax
jz .find_root_partition_not_found
movl %eax, 12(%esp)
.find_root_partition_read_entry_section:
movl 0(%esp), %eax
movl 4(%esp), %ebx
movw $1, %cx
movb (root_disk_drive_number), %dl
movw $sector_buffer, %di
call read_from_disk
# ecx: min(entries per section, entries remaining)
movl 8(%esp), %ecx
cmpl 12(%esp), %ecx
jae .find_root_partition_got_entry_count
movl 12(%esp), %ecx
.find_root_partition_got_entry_count:
# update entries remaining
subl %ecx, 12(%esp)
# si: entry pointer
movw $sector_buffer, %si
.find_root_partition_loop_entries:
# temporarily save cx in dx
movw %cx, %dx
# check that entry is used
movw $16, %cx
movw $zero_guid, %di
call memcmp
test %al, %al
jnz .find_root_partition_next_entry
# compare entry guid to root guid
movw $16, %cx
addw $16, %si
movw $root_partition_guid, %di
call memcmp
subw $16, %si
testb %al, %al
jnz .find_root_partition_found
.find_root_partition_next_entry:
# restore cx
movw %dx, %cx
# entry pointer += entry size
addw (gpt_header + 84), %si
loop .find_root_partition_loop_entries
# entry not found in this sector
# increment 8 byte entry array lba
incl 0(%esp)
jnc .find_root_partition_no_overflow
incl 4(%esp)
.find_root_partition_no_overflow:
# loop to read next section if entries remaining
cmpl $0, 12(%esp)
jnz .find_root_partition_read_entry_section
.find_root_partition_not_found:
movw $root_partition_not_found_msg, %si
jmp print_and_halt
.find_root_partition_found:
# copy entry to buffer
movw $root_partition_entry, %di
movw $128, %cx
rep movsb
movw $root_partition_found_msg, %si
call puts; call print_newline
# ebx:eax := last lba
movl (root_partition_entry + 44), %ebx
movl (root_partition_entry + 40), %eax
# ebx:eax -= first lba - 1
subl (root_partition_entry + 36), %ebx
movl (root_partition_entry + 32), %ecx;
decl %ecx
subl %ecx, %eax
jnc .find_root_partition_count_sub_no_carry
decl %ebx
.find_root_partition_count_sub_no_carry:
# ecx: min(partition count, 0xFFFFFFFF)
movl $0xFFFFFFFF, %edx
movl %eax, %ecx
testl %ebx, %ebx
cmovnzl %edx, %ecx
# ebx:eax := first lba
# FIXME: confirm ebx bits 16:31 are zero
movl (root_partition_entry + 36), %ebx
movl (root_partition_entry + 32), %eax
movb (root_disk_drive_number), %dl
leavel
ret
# print information about root partition
.global print_root_partition_info
print_root_partition_info:
pushw %ax
pushw %bx
pushw %cx
pushw %si
movw $root_partition_info_start_msg, %si
call puts;
movw $16, %bx
movw $2, %cx
movw (root_partition_entry + 38), %ax; call print_number
movw (root_partition_entry + 36), %ax; call print_number
movw (root_partition_entry + 34), %ax; call print_number
movw (root_partition_entry + 32), %ax; call print_number
movb $'-', %al; call putc
movb $'>', %al; call putc
movw (root_partition_entry + 46), %ax; call print_number
movw (root_partition_entry + 44), %ax; call print_number
movw (root_partition_entry + 42), %ax; call print_number
movw (root_partition_entry + 40), %ax; call print_number
call print_newline
popw %si
popw %cx
popw %bx
popw %ax
ret
# These will be patched during bootloader installation
root_disk_guid:
.ascii "root disk guid "
root_partition_guid:
.ascii "root part guid "
zero_guid:
.skip 16, 0
root_disk_found_msg:
.asciz "Root disk found!"
root_disk_not_found_msg:
.asciz "Root disk not found"
root_partition_found_msg:
.asciz "Root partition found!"
root_partition_not_found_msg:
.asciz "Root partition not found"
root_partition_info_start_msg:
.asciz "Root partition: "
.section .bss
.align SECTOR_SIZE
gpt_header:
.skip SECTOR_SIZE
gpt_entry_data:
.skip SECTOR_SIZE
sector_buffer:
.skip SECTOR_SIZE
disk_address_packet:
.skip 16
disk_drive_parameters:
.skip 0x1A
.skip 2 # padding
root_disk_drive_number:
.skip 1
.skip 3 # padding
root_partition_entry:
.skip 128