Question

I was writing a bootsector based on a FAT12 filesystem. After a while I came up with a problem.

.FindBootSectorTwo:
    push SI
    push DI
    push CX
    mov CX, 11
    mov SI, BootloaderSTG2
    rep cmpsb
    pop CX
    pop DI
    pop SI
    je LOAD_BOOTLOADER_STG2
    nop
    add DI, 32
    loop .FindBootSectorTwo
    jmp ERROR_NOT_FOUND

This label searches for a file called BLS2.SYS and if it finds it, it jumps to LOAD_BOOTLOADER_STG2 label. The beginning of LOAD_BOOTLOADER_STG2 label:

LOAD_BOOTLOADER_STG2:
    push SI
    push CX
    push DI
    ;sub DI, 32
    mov SI, DI
    mov CX, 11
    call Printname
    pop DI
    pop CX
    pop SI
    pop DS

If I try to subtract 32 from DI, it gives me the right file name — BLS2.SYS, but if I don’t , I get only a bunch of spaces. What could be the problem here? Is this is a bug, that the program finds the wrong file, or I just don’t understand something here? I’m launching this code on Bochs, on a x86 machine. P.S. here’s the LOAD_ROOT label together with .FindBootSectorTwo.

LOAD_ROOT: ; in this label I search for an entry to Root directory, where my file is. When I find the root directory, I load it at: 0x0800:0x0000
mov [BS_DriveNumber], DL
mov AX, 0x0000
mov SS, AX
mov SP, 0x7C00
mov DS, AX
mov ES, AX
xor DX, DX
movzx AX, [BPB_NumberOfFATs]
mov BX, [BPB_SectorsPerFAT]
mul BX ; BX = NumFATS * SecPerFAT
mov BX, AX
add BX, [BPB_ReservedSectors] ; BX now contains LBA value aka NumFATS * SecPerFAT + ResvSecs
push DI
push ES
mov AX, [BPB_RootEntries]
shl AX, 5
xor DX, DX
div WORD[BPB_BytesPerSector] ; AL = (RootEntries * 32) / BytesPerSec
mov AH, 0x0002
push AX
mov AX, 0x0800
mov ES, AX ; ES = 0x0800
pop AX
mov DI, 0x0000
call ReadSectors
mov CX, [BPB_RootEntries]
push DS
push AX
mov AX, 0x0800
mov ES, AX
mov DS, AX
pop AX
mov DI, 0x0000
.FindBootSectorTwo: ; here I search for a file called BLS2.SYS (at 0x0800:0x0000), when I find it, the program jumps to LOAD_BOOTLOADER_STG2
    push SI
    push DI
    push CX
    mov CX, 11
    mov SI, BootloaderSTG2
    rep cmpsb
    pop CX
    pop DI
    pop SI
    je LOAD_BOOTLOADER_STG2
    nop
    add DI, 32
    loop .FindBootSectorTwo
    jmp ERROR_NOT_FOUND

If you need any additional peaces of code — tell me. I’ll give you whatever you need.

Answer 1

The code is subtracting 32 from DI so points to the beginning of the file name string. Then you can print it accordingly.