The position of the label declaration affecting the exit
I wrote a build program to print a string:
[org 0x7c00]
mov bx, HELLO_MSG
HELLO_MSG:
db "Hello World!", 0
mov ah, 0x0e
PRINT:
mov al, [bx]
cmp al, 0
je END
int 0x10
add bx, 0x1
jmp PRINT
END:
jmp $
times 510-($-$$) db 0
dw 0xaa55
when compiled with nasm, it generated the following binary
BB 12 7C B4 0E 8A 07 3C 00 74 07 CD 10 83 C3 01 EB F3 48 65 6C 6C 6C 6F 20 57 6F 72 6C 64 21 00 EB FE 00 00 .... 00 00 55 AA
Quit using qemu emulator
since it is clear that "ll" is replaced by other characters.
However, if I move the label HELLO_MSG
to the bottom of the code just above jmp $
, the output is correct. I cannot understand the reason for this.
EDIT: I noticed the following outputs when trying to use different strings instead of "Hello World" in the source code
Case: "Hellllo World" (note the "l")
trash letters only show up on those two bytes
Case: "We are gods"
strange error is gone!
Case: "We are gods!" (pay attention to "!" )
nothing is printed by adding '!' something terrible?
Case: "Hello World" (note! '!')
removing '!' did something terrible again?
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You put a line in the middle of the executable code. So ASCII values are treated as command opcodes and perform some operation that appears to be happening to overwrite some bytes.
You must put the line at the end after the statement jmp
so that it won't be executed. Alternatively, you can add a statement jmp
before jumping the line.
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