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Improved comments about the manual symbol relocations.

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Minor cleanup.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@14382 a95241bf-73f2-0310-859d-f6bbb57e9c96
This commit is contained in:
Axel Dörfler 2005-10-14 11:49:35 +00:00
parent 8423f6f478
commit d32f04d20a
1 changed files with 24 additions and 21 deletions
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45
src/system/boot/platform/bios_ia32/shell.S
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@ -57,6 +57,7 @@ floppy_start:
.code32 // we need to create a 32-bit relocation entry for the linker...
.byte 0x67
movw sNumSectors - 0x10000, %di
// the loader symbols are located at offset 0x10000
.code16
xor %dh, %dh // head 0, don't change BIOS boot device
mov $0x2, %cx // sector 2
@ -74,7 +75,9 @@ start_loader:
// indicate that we were booted from CD/floppy
.code32
.byte 0x67
movb $1, gCDFloppyBoot - 0x7c00 // %ds is 0x7c0 right now
movb $1, gCDFloppyBoot - 0x7c00
// %ds is 0x7c0 right now, but the symbol were loaded
// to offset 0x10000
.code16
// set our environment and jump to the standard BFS boot block entry point
@ -97,27 +100,27 @@ load_sectors:
push %di
push %es
movb $8, %ah // get drive parameters - changes a lot of registers
movb $8, %ah // get drive parameters - changes a lot of registers
int $0x13
pop %es
pop %di
// ToDo: store the number of heads somewhere (it's in %dh)
pop %dx
and $63, %cx // mask out max. sector number (bit 0-5)
mov %cx, %si // and remember it
and $63, %cx // mask out max. sector number (bit 0-5)
mov %cx, %si // and remember it
pop %cx
pop %bx
load_track:
mov %di, %ax // limit the sector count to track boundaries
mov %di, %ax // limit the sector count to track boundaries
add %cl, %al
dec %ax
cmp %si, %ax
jbe matches_track_boundary
mov %si, %ax
matches_track_boundary:
inc %ax // take the current sector offset into account
inc %ax // take the current sector offset into account
sub %cl, %al
// make sure we don't cross a 64kB address boundary or else the read will fail
@ -126,7 +129,7 @@ matches_track_boundary:
mov %ax, %bp
add %bx, %bp
jnc respects_boundary
xor %ax, %ax // only read up to the 64kB boundary
xor %ax, %ax // only read up to the 64kB boundary
sub %bx, %ax
respects_boundary:
shr $9, %ax
@ -134,16 +137,16 @@ respects_boundary:
try_to_read:
pusha
movb $2, %ah // load sectors from drive
movb $2, %ah // load sectors from drive
int $0x13
jnc read_succeeded
xor %ax, %ax
int $0x13 // reset drive
int $0x13 // reset drive
popa
dec %bp
jz load_failed // if already retried often enough, bail out
jz load_failed // if already retried often enough, bail out
jmp try_to_read
read_succeeded:
@ -152,29 +155,29 @@ read_succeeded:
popa
xor %ah, %ah
add %ax, %cx // next sector start
sub %ax, %di // update sectors left to be read
add %ax, %cx // next sector start
sub %ax, %di // update sectors left to be read
shl $9, %ax // get byte offset
add %ax, %bx // update target address
shl $9, %ax // get byte offset
add %ax, %bx // update target address
jnz check_sector_start
mov %es, %ax // overflow to the next 64kB, %bx is already zero
mov %es, %ax // overflow to the next 64kB, %bx is already zero
add $0x1000, %ax
mov %ax, %es
check_sector_start:
mov %si, %ax // compare the sectors, not the cylinders
mov %si, %ax // compare the sectors, not the cylinders
cmp %al, %cl
jbe continue_reading
sub %si, %cx
inc %dh // next head
inc %dh // next head
cmp $1, %dh
// ToDo: check max. number of heads!
jbe check_sector_start
xor %dh, %dh // next cylinder
xor %dh, %dh // next cylinder
inc %ch
jmp check_sector_start
@ -188,8 +191,8 @@ load_failed:
call print_string
xor %ax, %ax
int $0x16 // wait for key
int $0x19 // and reboot
int $0x16 // wait for key
int $0x19 // and reboot
disable_floppy_motor:
xor %al, %al
@ -202,7 +205,7 @@ print_string:
xor %bx, %bx
print_char:
lodsb
orb %al, %al // are there still characters left?
orb %al, %al // are there still characters left?
jz no_more_chars
int $0x10
jmp print_char