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https://github.com/cfenollosa/os-tutorial.git
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47 lines
1.5 KiB
NASM
47 lines
1.5 KiB
NASM
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; load 'dh' sectors from drive 'dl' into ES:BX
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disk_load:
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pusha
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; reading from disk requires setting specific values in all registers
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; so we will overwrite our input parameters from 'dx'. Let's save it
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; to the stack for later use.
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push dx
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mov ah, 0x02 ; ah <- int 0x13 function. 0x02 = 'read'
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mov al, dh ; al <- number of sectors to read (0x01 .. 0x80)
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mov cl, 0x02 ; cl <- sector (0x01 .. 0x11)
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; 0x01 is our boot sector, 0x02 is the first 'available' sector
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mov ch, 0x00 ; ch <- cylinder (0x0 .. 0x3FF, upper 2 bits in 'cl')
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; dl <- drive number. Our caller sets it as a parameter and gets it from BIOS
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; (0 = floppy, 1 = floppy2, 0x80 = hdd, 0x81 = hdd2)
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mov dh, 0x00 ; dh <- head number (0x0 .. 0xF)
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; [es:bx] <- pointer to buffer where the data will be stored
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; caller sets it up for us, and it is actually the standard location for int 13h
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int 0x13 ; BIOS interrupt
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jc disk_error ; if error (stored in the carry bit)
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pop dx
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cmp al, dh ; BIOS also sets 'al' to the # of sectors read. Compare it.
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jne sectors_error
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popa
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ret
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disk_error:
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mov bx, DISK_ERROR
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call print
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call print_nl
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mov dh, ah ; ah = error code, dl = disk drive that dropped the error
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call print_hex ; check out the code at http://stanislavs.org/helppc/int_13-1.html
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jmp disk_loop
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sectors_error:
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mov bx, SECTORS_ERROR
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call print
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disk_loop:
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jmp $
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DISK_ERROR: db "Disk read error", 0
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SECTORS_ERROR: db "Incorrect number of sectors read", 0
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