CODEWITHSUNDEEP
assemblyx86nasmbootloader
Arturo Salas
Arturo Salas

Reputation: 1

Flush Keyboard Buffer x86 assembly using int 16h

I have a doubt because my code is not working. I'm implementing a bootloader that runs the code below, when I try to load it only works in a few cases, like I explain below. My code is the next one:

ChangeGameState:
    mov cx, 00H      ;Here I make a delay
    mov dx, 3240H    ;.
    mov ah, 86H      ;.
    int 15h          ;.

    jmp DetectKeyPress

DetectKeyPress:
    mov ah, 01h 
    int 16h 
    jz noKeyInBuffer 
    xor ah, ah 
    int 16h 
    jmp exitKeyPress 

    noKeyInBuffer:
        xor ax, ax 

    exitKeyPress:   
        jmp ProcessKey

ProcessKey:
    cmp ah, 0
    je ExitKey
    cmp ah, 'd'
    je ChangeDir1
    cmp ah, 's'
    je ChangeDir2
    cmp ah, 'a'
    je ChangeDir3
    cmp ah, 'w'
    je ChangeDir4

ExitKey:
    jmp ChangeGameState

ChangeDir1:
    ;DO SOMETHING1
    jmp ChangeGameState

ChangeDir2:
    ;DO SOMETHING2
    jmp ChangeGameState

ChangeDir3:
    ;DO SOMETHING3
    jmp ChangeGameState

ChangeDir4:
    ;DO SOMETHING4
    jmp ChangeGameState

Now, when I try to press keys S, A and W it doesn't works, only if I press D key.. Any idea of what's going on??

Upvotes: 0

Views: 710

Answers (1)

Drew McGowen
Drew McGowen

Reputation: 11706

The problem is that int 16h returns the scancode in ah, and the ASCII character in al. So while your code is testing the scancode of the key, you should be testing the ASCII character.

So, in ProcessKey, change cmp ah, ... to cmp al, ....

Upvotes: 2

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