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Add protective BPB in bootsector and check for the presence of int 13h extensions before using them

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This commit is contained in:
mintsuki 2020-09-06 07:25:34 +02:00
parent b16f63458d
commit 8760c0b3d6
1 changed files with 29 additions and 19 deletions
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48
src/bootsect/bootsect.asm
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@ -1,10 +1,17 @@
org 0x7c00
bits 16
jmp start ; Workaround for some BIOSes that require this stub
nop
start:
jmp .skip_bpb ; Workaround for some BIOSes that require this stub
nop
; Some BIOSes will do a funny and decide to overwrite bytes of code in
; the section where a FAT BPB would be, potentially overwriting
; bootsector code.
; Avoid that by filling the BPB area with 0s
times 87 db 0
.skip_bpb:
cli
cld
jmp 0x0000:.initialise_cs
@ -12,10 +19,8 @@ start:
xor ax, ax
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
mov esp, 0x4000
mov sp, 0x4000
sti
; Some BIOSes don't pass the correct boot drive number,
@ -41,19 +46,26 @@ start:
; ****************** Load stage 1.5 ******************
mov si, Stage15Msg
call simple_print
; Make sure int 13h extensions are supported
mov ah, 0x41
mov bx, 0x55aa
int 0x13
jc err_reading_disk
cmp bx, 0xaa55
jne err_reading_disk
; If int 13h extensions are supported, then we are definitely running on
; a 386+. We have no idea whether the upper 16 bits of esp are cleared, so
; make sure that is the case now.
mov esp, 0x4000
mov eax, dword [stage15_sector]
mov ebx, 0x7e00
mov ecx, 1
mov bx, 0x7e00
mov cx, 1
call read_sectors
jc err_reading_disk
mov si, DoneMsg
call simple_print
jmp 0x7e00
err_reading_disk:
@ -73,10 +85,8 @@ halt:
; Data
LoadingMsg db 0x0D, 0x0A, 'Limine', 0x0D, 0x0A, 0x0A, 0x00
Stage15Msg db 'Loading stage 1.5...', 0x00
ErrReadDiskMsg db 0x0D, 0x0A, 'Disk read error, system halted', 0x00
ErrEnableA20Msg db 0x0D, 0x0A, 'A20 enable error, system halted', 0x00
DoneMsg db ' DONE', 0x0D, 0x0A, 0x00
ErrReadDiskMsg db 0x0D, 0x0A, 'Disk err', 0x00
ErrEnableA20Msg db 0x0D, 0x0A, 'A20 err', 0x00
times 0xda-($-$$) db 0
times 6 db 0
@ -101,8 +111,8 @@ stage15:
pop es
mov eax, dword [stage15_sector]
inc eax
xor ebx, ebx
mov ecx, 62
xor bx, bx
mov cx, 62
call read_sectors
pop es
jc err_reading_disk