ReadKey PROC    (Not covered in the 4th edition)

   Performs a no-wait keyboard check and reads a single character if available.
   If the returned ASCII code is zero, special keys can be processed by checking the virtual scan code (ah) and virtual key code (dx).

Call args:  None.

Return arg: ZF=1 if no key is available,
            ZF=0 if a key is read into the following registers:
              AL  = key's Ascii code (is set to zero for special extended codes)
              AH  = Virtual scan code
              DX  = Virtual key code
              EBX = Keyboard flags (Alt,Ctrl,Caps,etc.)
            Upper halves of EAX and EDX are overwritten.

Example:

INCLUDE Irvine32.inc
INCLUDE Macros.inc

.code
main PROC  ; Read and display each key until <Esc> is input.

    mWriteLn "  ASCII     Virtual-scan  Virtual-key   Keyboard flags"
    call Crlf

LookForKey:
    mov  eax,50          ; sleep, to allow OS to time slice
    call Delay           ; (otherwise, some key presses are lost)

    call ReadKey         ; look for keyboard input
    jz   LookForKey      ; no key pressed yet

    mShow  al,h
    mShow  ah,h
    mShow  dx,h
    mShow  ebx,hnn

    cmp    dx,VK_ESCAPE  ; time to quit?
    jne    LookForKey    ; no, go get next key.

    exit
main ENDP
END main

Output when F1, Shift-F1, Ctrl-F1, Alt-F1, and Esc keys are input:

  ASCII     Virtual-scan  Virtual-key   Keyboard flags

  al = 00h    ah = 3Bh    dx = 0070h    ebx = 00000000h

  al = 00h    ah = 54h    dx = 0070h    ebx = 00000010h

  al = 00h    ah = 5Eh    dx = 0070h    ebx = 00000004h

  al = 00h    ah = 68h    dx = 0070h    ebx = 00000001h

  al = 1Bh    ah = 01h    dx = 001Bh    ebx = 00000000h