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
 References: This subroutine is called as follows:
             * get_key calls RDKEY
             * scan_10in 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.

 If CTRL-P is pressed, then the routine calls the printer routine to print the
 screen, and returns 0 in A and X.


 Returns:

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


.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

 CMP #&37               \ If "P" was not pressed, jump to scan_test to return
 BNE scan_test          \ the key press

 LDX #1                 \ Set X to the internal key number for CTRL

 JSR DKS4               \ Scan the keyboard to see if the key in X (i.e. CTRL)
                        \ is currently pressed

 BPL scan_p             \ If it is not being pressed, jump to scan_p to return
                        \ "P" as the key press

 JSR printer            \ CTRL-P was pressed, so call printer to output the
                        \ screen to the printer

 LDA #0                 \ Set A to 0 to return no key press from the routine, as
                        \ we already acted on it

 RTS                    \ Return from the subroutine

.scan_p

 LDA #&37               \ Set A to the internal key number for "P", to return as
                        \ the result

.scan_test

 TAX                    \ Copy the key value into X

 RTS                    \ Return from the subroutine