Keyboard: DKS4

       Name: DKS4                                                    [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Scan the keyboard to see if a specific key is being pressed
  Deep dive: The key logger
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * RDKEY calls DKS4
             * scan_xin calls DKS4
             * scan_y calls DKS4


 Arguments:

   X                    The internal number of the key to check (see p.142 of
                        the Advanced User Guide for a list of internal key
                        numbers)


 Returns:

   A                    If the key in A is being pressed, A contains the
                        original argument A, but with bit 7 set (i.e. A + 128).
                        If the key in A is not being pressed, the value in A is
                        unchanged

   X                    Contains the same as A


.DKS4

 LDA #%00000011         \ Set A to %00000011, so it's ready to send to SHEILA
                        \ once interrupts have been disabled

 SEI                    \ Disable interrupts so we can scan the keyboard
                        \ without being hijacked

 STA VIA+&40            \ Set 6522 System VIA output register ORB (SHEILA &40)
                        \ to %00000011 to stop auto scan of keyboard

 LDA #%01111111         \ Set 6522 System VIA data direction register DDRA
 STA VIA+&43            \ (SHEILA &43) to %01111111. This sets the A registers
                        \ (IRA and ORA) so that:
                        \
                        \   * Bits 0-6 of ORA will be sent to the keyboard
                        \
                        \   * Bit 7 of IRA will be read from the keyboard

 STX VIA+&4F            \ Set 6522 System VIA output register ORA (SHEILA &4F)
                        \ to X, the key we want to scan for; bits 0-6 will be
                        \ sent to the keyboard, of which bits 0-3 determine the
                        \ keyboard column, and bits 4-6 the keyboard row

 LDX VIA+&4F            \ Read 6522 System VIA output register IRA (SHEILA &4F)
                        \ into X; bit 7 is the only bit that will have changed.
                        \ If the key is pressed, then bit 7 will be set,
                        \ otherwise it will be clear

 LDA #%00001011         \ Set 6522 System VIA output register ORB (SHEILA &40)
 STA VIA+&40            \ to %00001011 to restart auto scan of keyboard

 CLI                    \ Allow interrupts again

 TXA                    \ Transfer X into A

 RTS                    \ Return from the subroutine