Prints the wrong filename

[benjii]

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

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	.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
		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:

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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.

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LOAD_ROOT:
	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:
		push SI
		push DI
		push CX
		mov CX, 11
		mov SI, BootloaderSTG2
		rep cmpsb
		pop CX
		pop DI
		pop SI
		je LOAD_BOOTLOADER_STG2
		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.

[BMW]

Why do you do mov di,0x0000?

At memory location 0x0000 is the interrupt vector table...
http://wiki.osdev.org/Memory_Map_%28x86%29

Comparing the filename with the IVT is no good

[bluemoon]

Why do you do mov di,0x0000?

At memory location 0x0000 is the interrupt vector table...
http://wiki.osdev.org/Memory_Map_%28x86%29

Comparing the filename with the IVT is no good

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mov AX, 0x0800
mov ES, AX ; ES = 0x0800
pop AX
mov DI, 0x0000

0800:0000 is not IVT.

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rep cmpsb

Do you mean repe cmpsb?