Help with BIOS Disk Read Interrupt

Discussion in 'Computer Software and Operating Systems' started by Mr. Mysterio, May 12, 2015.

  1. Mr. Mysterio
    OP

    Mr. Mysterio Super Genius

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    Sep 16, 2014
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    Rosalina's Comet Observatory
    I've been trying to write a MBR that bootstraps code from the disk. I've been trying to use the BIOS interrupt 0x13 with ah=2, but it always fills the memory with zeros. The return is ah = 0 = success, and al = the correct number of read sectors. I'm using NASM. Here is my code:

    Code:
    [org 0x7c00]
    [BITS 16]
     
    ; Init stack.
    mov bp, 0x7B00
    mov sp, bp
     
    mov ah , 0x02 ; BIOS read sector function.
    mov al , 0x01 ; Read 1 sector.
    mov ch , 0x00 ; Cylinder 0.
    mov dh , 0x00 ; Head 0.
    mov cl , 0x02 ; Start from sector 2.
    mov bx, 0
    mov es, bx
    mov bx, 0x8000 ; Read data to 0x0000:0x8000. dl already contains the boot drive index.
    int 0x13
    jc disk_error ; Jump if error.
     
    mov bx, 0x8000 ; Start from 0x8000.
    mov ax, 0x200 ; Read 512 bytes.
    clc ; Do not prepend "0x".
    call printx ; Print bytes as hex.
     
    jmp $
    
    I've tried starting from sectors 0 through 5 and reading different amounts with no luck. I haven't been using an emulator, I've been directly testing it on a PC. I had this function working once in the past so I know it can work with my SD card, but my HDD died and killed that data. I haven't been able to get it working again. Please help!