Keyboard: RDKEY

       Name: RDKEY                                                   [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Scan the keyboard for key presses
    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * DK4 calls RDKEY
             * PAS1 calls RDKEY
             * PAUSE2 calls RDKEY
             * TITLE calls RDKEY
             * TT217 calls RDKEY


 Scan the keyboard, starting with internal key number 16 ("Q") and working
 through the set of internal key numbers (see p.142 of the Advanced User Guide
 for a list of internal key numbers).

 This routine is effectively the same as OSBYTE 122, though the OSBYTE call
 preserves A, unlike this routine.


 Returns:

   X                    If a key is being pressed, X contains the internal key
                        number, otherwise it contains 0

   A                    Contains the same as X


.RDKEY

 LDX #16                \ Start the scan with internal key number 16 ("Q")

.Rd1

 JSR DKS4               \ Scan the keyboard to see if the key in X is currently
                        \ being pressed, returning the result in A and X

 BMI Rd2                \ Jump to Rd2 if this key is being pressed (in which
                        \ case DKS4 will have returned the key number with bit
                        \ 7 set, which is negative)

 INX                    \ Increment the key number, which was unchanged by the
                        \ above call to DKS4

 BPL Rd1                \ Loop back to test the next key, ending the loop when
                        \ X is negative (i.e. 128)

 TXA                    \ If we get here, nothing is being pressed, so copy X
                        \ into A so that X = A = 128 = %10000000

.Rd2

 EOR #%10000000         \ EOR A with #%10000000 to flip bit 7, so A now contains
                        \ 0 if no key has been pressed, or the internal key
                        \ number if a key has been pressed

 TAX                    \ Copy A into X

 RTS                    \ Return from the subroutine