Wsrtjtyk

I'm in the middle of writing air hockey in tasm and I have encounter a problem which is how I get two keys/clicks at once because I need to get both click at once to move both players in one time and Im trying a lot but I don't think I have a way to doing it.
I heard that I need to read from the buffer directory and see what keys are there and read each one individually but I don't really know how to do this.

Do you mind working with scancodes?

I know this is not the simple, drop-in, solution you were looking for but I'm afraid there is none.

So I'm writing this in the hope that, if not you, some other troubled coder can find something useful.

I also know you write for TASM but I forgot that and started with NASM, converting however should be very easy (just add segments declarations, take out the segment register from the brackets and add PTR).

It seems that from an hardware perspective the keyboard repeats only the last pressed key¹, so if two players are pressing two keys, only one is actually sent by the keyboard.

However the software handle multiple keys at once everywhere, how do they do it?

The trick is that the keyboard sends two codes (scancodes) when a key is pressed: one when the key goes down (and many others while it is kept down) and one when it is released.

The software can thus tell when a key is down without further notice from the keyboard².

I cannot find any interrupt service that handle key up and down events.

The only solution is handling scancode directly.

While the 8042 chip is very simple, there is not need to get dirty with IO instruction.

Without explaining how the 8259A and the IRQ mapping works, it's enough to say that the interrupt 15h/AH=4fh is called with the scancode in AL when a key is pressed/released.

We can intercept that interrupt and check if the scancode is for a key down or a key up.

Release scancodes have bit7 set.

We can have an array of 128 bytes, each for any possible scancode value (bit0-6)³ and store in each element 0ffh if the scancode indicates a press or 00h if it indicates a release.

It is also useful keep a count of the processed scancodes, so a program can wait for new scancodes with trivial arithmetic.

Now, I imagine you are still confused.

I have written a demo.

The program below, for NASM, waits for you to press both a and d keys to exit.
Warning Since we are using scancodes, this is keyboard layout dependent!

BITS 16

ORG 100h        ;COM



 ;Setup ISR for the scancode 

 call init 



 ;Clear screen

 mov ax, 03h

 int 10h 



 ;Print command

 mov ah, 09h 

 mov dx, strCommand

 int 21h 



_main:

 ;Wait for a change in the scancode tables

 call wait_for_scancode

 ;Remove unused keystrokes

 call remove_keystrokes



 ;Check if a is pressed 

 mov al, 1eh           ;a

 call is_scancode_pressed

 jz _main 



 ;Check if 'd' is pressed 

 mov al, 20h           ;d 

 call is_scancode_pressed

 jz _main 



 ;Both are pressed, print bye and ...

 mov ah, 09h 

 mov dx, strDone

 int 21h 



 ;... restore the ISR and ...

 call dispose 



 ;... exit

 mov ax, 4c00h

 int 21h



 strCommand db "Press 'a' and 'd' to exit", 13, 10, 24h 

 strDone    db "Bye",13,10,13,10,24h 



 ;Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  L  

 ;  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll

 ;Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  Ll  L



 ;S C A N C O D E   F U N C T I O N S



 ;Set the ISR 

init:

  push ax  



  mov ax, cs

  mov WORD [old_isr_15 + 02h], ax          

  ;old_isr_15 is now a far pointer to new_isr_15



  call swap_isr_15          ;Swap the current isr15 with the one in old_isr_15



  pop ax 

  ret



  ;Restore the original ISR 

dispose:

  call swap_isr_15          ;Swap the current isr15 with the one in old_isr_15                  



  ret  



  ;Swap the pointer in the IVT for int 15h with the pointer in old_isr_15

swap_isr_15:

  push eax

  push es 



  xor ax, ax 

  mov es, ax 



  cli



  mov eax, DWORD [es: 15h*4]

  xchg eax, DWORD [old_isr_15]

  mov DWORD [es: 15h*4], eax 



  sti



  pop es 

  pop eax



  ret  



  ;Wait for a change in the scancode table

wait_for_scancode:

 cli                           ;Prevent the ISR from messing things up 



 ;At least one scancode processed?

 cmp WORD [new_scancode], 0 

 jne _wfs_found                ;Yes



 ;No, restore interrupt so the CPU can process the prending ones

 sti

jmp wait_for_scancode



 ;New scancode, decrement the count and restore interrupts

_wfs_found:

 dec WORD [new_scancode]

 sti 



 ret



  ;THe BIOS is still saving keystrokes, we need to remove them or they 

  ;will fill the buffer up (should not be a big deal in theory).

remove_keystrokes:

 push ax



 ;Check if there are keystrokes to read.

 ;Release scancodes don't generate keystrokes 

_rk_try:

 mov ah, 01h 

 int 16h

 jz _rk_end      ;No keystrokes present, done 



 ;Some keystroke present, read it (won't block)

 xor ah, ah 

 int 16h

jmp _rk_try



_rk_end:

 pop ax 

 ret 



 ;Tell if a scancode is pressed

 ;

 ;al = scancode  

 ;ZF clear is pressed 

is_scancode_pressed:

  push bx



  movzx bx, al 

  cmp BYTE [scancode_status + bx], 0 



  pop bx 

  ret 



 ;AL = scancode 

new_isr_15:

 ;Check for right function

 cmp ah, 4fh

 jne _ni15_legacy



 ;Save used regs

 push bx

 push ax





 movzx bx, al            ;BX = scancode 

 and bl, 7fh             ;BX = scancode value



 sar al, 07h             ;AL = 0ffh if scancode has bit7 set (release), 00h otherwise

 not al                  ;AL = 00h if scancode has bit7 set (release), 0ffh otherwise



 ;Save the scancode status

 mov BYTE [cs:bx + scancode_status], al 

 ;Increment the count

 inc WORD [cs:new_scancode]



 pop ax

 pop bx 



_ni15_legacy:   

 ;This is a far jump, in NASM is simply jmp FAR [cs:old_isr_15]

 ;Ended up this way for debug

 push WORD [cs: old_isr_15 + 02h] 

 push WORD [cs: old_isr_15] 

 retf



 ;Original ISR

old_isr_15                      dw new_isr_15, 0  



 ;Scan code status table

scancode_status     TIMES 128   db 0

 ;Scan code count 

new_scancode                    dw 0

All you need to use is:

• 
  init to set up the scancode snooping.


• 
  dispose to tear down the scancode snooping.


• 
  is_scancode_pressed to know if a key is being held down.

Everything else, including wait_for_scancode and remove_keystrokes is accessory and is there just to make your program behave very nice.

If you want to find the scancode associated with a key, you can use this other program that show you a table with the pressed scancodes (press ESC to exit).

For example, if I press a and d I get the values used in the demo

It is just a variant of the demo above.

¹I create this boot program (intended for NASM) to test my hypothesis, at least in my hardware.

² Effectively speaking the repeat feature is useful only in typing software, every other application, like games, that explicitly check for keys status don't need it.

³ There are actually extended (multi byte scancode) but they are ignored here. Stick with plain keys!