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 page 142 of the "Advanced User
 Guide for the BBC Micro" by Bray, Dickens and Holmes 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