bootnorm.asm: work arount Xi8088 and Book8088 BIOS bug, see...

FDOS/kernel#156

source/fdisk/bootnorm.asm

@@ -105,9 +105,14 @@ dap:
 
 read_boot_sector:
  .check_lba:
- mov bx, 0x55aa ; magic value changed after call
- mov ah, 0x41
- int 0x13 ; test for ext. INT13 LBA support
+ push ds ; temporarly set DS to 0x40 (BIOS data area)
+ mov ax, 0x40 ; to work around a BIOS bug
+ mov ds, ax ; https://github.com/FDOS/kernel/issues/156
+ mov bx, 0x55aa ; magic value shoud be changed after call
+ mov ah, 0x41 ; query INT 13h LBA capabilities
+ stc
+ int 0x13
+ pop ds
  jc .read_chs ; no support if carry set
  cmp bx, 0xaa55 ; no support if 55aa not swapped 
  jne .read_chs
@@ -118,15 +123,17 @@ read_boot_sector:
  mov [dap.lba_low], ax
  mov ax, [di + 10]
  mov [dap.lba_high], ax
- mov ax, 0x4200
+ mov ax, 0x4200 ; LBA read function
  mov si, dap
+ stc
  int 0x13
  ret
  .read_chs:
  mov ax, 0x0201 ; read one sector
  mov bx, 0x7c00 ; to 0:7c00
  mov cx, [di + 2]
  mov dh, [di + 1]
+ stc
  int 0x13
  ret