CODEWITHSUNDEEP
assemblykeyboardx86-16keystroke
Daniel Rivas
Daniel Rivas

Reputation: 256

How to read key without waiting for it, assembly 8086

I'm making an space invaders in Assembly 8086, for testing I'm using DOSBox. Let me show you my code:

;--------------------
;Update hero
;--------------------
update:
call vsync
call update_hero ; method to read keyboard
call set_video
call clear_screen
call draw_hero
jmp update

Now the procedure update_hero is:

update_hero proc
     mov ah, 01h
     int 16h
     cmp al, 97
     je left_pressed
     cmp al, 100
     jne none_pressed
     inc hero_x
     left_pressed:
          dec hero_x
     none_pressed:
     ret
update_hero endp

As you can see, I'm trying to read 'a' or 'd' for the movement, but, it's not working, can you help me to figure out why?

What I'm trying to do is read from the keyboard without waiting for it, that's why I'm using the subfunction ah, 01h.

Cheers.

Edit

I checked for the interrupts here, modified the code, and now it's working:

update_hero proc
    mov ah, 01h ; checks if a key is pressed
    int 16h
    jz end_pressed ; zero = no pressed

    mov ah, 00h ; get the keystroke
    int 16h

    begin_pressed:
        cmp al, 65
        je left_pressed
        cmp al, 97
        je left_pressed
        cmp al, 68
        je right_pressed
        cmp al, 100
        je right_pressed
        cmp al, 81
        je quit_pressed
        cmp al, 113
        je quit_pressed
        jmp end_pressed
        left_pressed:
            sub hero_x, 2
            jmp end_pressed
        right_pressed:
            add hero_x, 2
            jmp end_pressed
        quit_pressed:
            jmp exit
    end_pressed:

    ret
update_hero endp

Upvotes: 4

Views: 7339

Answers (3)

Dirk Wolfgang Glomp
Dirk Wolfgang Glomp

Reputation: 539

update_hero proc
 mov ah, 01h
 int 16h
 cmp al, 97
 je left_pressed
 cmp al, 100
 jne none_pressed
 inc hero_x

ret ; without the ret instruction "hero_x" will be decrease after increasing

 left_pressed:
      dec hero_x
 none_pressed:
 ret
update_hero endp

Upvotes: 0

JaiCraB
JaiCraB

Reputation: 1

check_key:

mov     ah, 1

int     16h

jz      .ret 

mov     cx, 0

xor     cl, ch

mov     ah, 0

int     16h

.new.key:

"al" is the new key presh

.ret:

 ret

Upvotes: 0

Bo Persson
Bo Persson

Reputation: 92211

You are just checking if there is a character available, but are not actually reading the character from the buffer. So the next time you check, it is still there.

From this page on BIOS functions http://webpages.charter.net/danrollins/techhelp/0230.HTM

INT 16H, AH=1 

Info: Checks to see if a key is available in the keyboard buffer, and
      if so, returns its keycode in AX.  It DOES NOT remove the
      keystroke from the buffer.

Upvotes: 3

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