The code is subtracting 32 from DI so points to the beginning of the file name string. Then you can print it accordingly.
Can someone help understand why this prints the wrong file name?
Pregunta
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.
Solución
Licenciado bajo: CC-BY-SA con atribución
No afiliado a StackOverflow