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52 lines
1.3 KiB
NASM
52 lines
1.3 KiB
NASM
mov ah, 0x0e
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; attempt 1
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; Fails because it tries to print the memory address (i.e. pointer)
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; not its actual contents
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mov al, "1"
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int 0x10
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mov al, the_secret
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int 0x10
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; attempt 2
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; It tries to print the memory address of 'the_secret' which is the correct approach.
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; However, BIOS places our bootsector binary at address 0x7c00
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; so we need to add that padding beforehand. We'll do that in attempt 3
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mov al, "2"
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int 0x10
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mov al, [the_secret]
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int 0x10
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; attempt 3
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; Add the BIOS starting offset 0x7c00 to the memory address of the X
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; and then dereference the contents of that pointer.
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; We need the help of a different register 'bx' because 'mov al, [ax]' is illegal.
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; A register can't be used as source and destination for the same command.
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mov al, "3"
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int 0x10
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mov bx, the_secret
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add bx, 0x7c00
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mov al, [bx]
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int 0x10
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; attempt 4
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; We try a shortcut since we know that the X is stored at byte 0x2d in our binary
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; That's smart but ineffective, we don't want to be recounting label offsets
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; every time we change the code
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mov al, "4"
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int 0x10
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mov al, [0x7c2d]
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int 0x10
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jmp $ ; infinite loop
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the_secret:
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; ASCII code 0x58 ('X') is stored just before the zero-padding.
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; On this code that is at byte 0x2d (check it out using 'xxd file.bin')
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db "X"
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; zero padding and magic bios number
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times 510-($-$$) db 0
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dw 0xaa55
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