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add functions to get the mbr/vbr in stage2

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Elaina Claus
2025-09-11 19:05:50 -04:00
parent 58fc4a08dd
commit a8b24c7b01
1 changed files with 56 additions and 0 deletions
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56
src/stage2/stage2.nasm
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@@ -193,6 +193,62 @@ hcf:
;
; ##############################
; int read_mbr(int boot_drive, void* dst)
; destination buffer needs 512 bytes of space
read_mbr:
__CDECL16_PROC_ENTRY
.func:
__BOCHS_MAGIC_DEBUG
; read mbr on boot drive to memory (for the partition table)
movzx ax, byte [bp - 4]
push ax ; drive_num (2)
push 0x01 ; count (2)
push dword 0x0 ; lba (4)
push word [bp - 6] ; offset = dst
push __STAGE2_SEGMENT ; this segment
__CDECL16_CALL read_disk_raw, 12
; TODO: this needs error checking, zero checking, check the sig a bunch of stuff...
.endf:
__CDECL16_PROC_EXIT
.error:
ERROR STEVIA_DEBUG_ERR
; int read_vbr(int boot_drive, void* mbr, void* buf)
read_vbr:
__CDECL16_PROC_ENTRY
.func:
__BOCHS_MAGIC_DEBUG
; read vbr on boot partition to memory (for fat bpb/ebpb)
.calc_part_offset:
mov bx, [bp - 6]
add bx, 0x1B8 ; offset to partition table in mbr
mov cx, 4 ; only checking 4 entries
.find_active_L0:
mov al, byte [bx + PartEntry_t.attributes]
test al, 0x80 ; 0x80 == 1000_0000b
jnz read_vbr.active_found
add bx, 0x10 ; add 16 bytes to offset
loop read_vbr.find_active_L0
jmp read_vbr.error
.active_found:
movzx ax, byte [bp - 4]
push ax ; drive_num (2)
push 0x01 ; count (2)
mov eax, dword [bx + PartEntry_t.lba_start]
push dword eax ; lba (4)
push word [bp - 6] ; offset = dst
push __STAGE2_SEGMENT ; this segment
__CDECL16_CALL read_disk_raw, 12
; vbr (with fat bpb/ebpb) is at the buffer now
.endf:
__CDECL16_PROC_EXIT
.error:
ERROR STEVIA_DEBUG_ERR
; set ds and es segments back to the base of the loader
%ifnmacro __TINY_DS_ES
%macro __TINY_DS_ES 0