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
             * DOKEY calls via RDKEY-1


 Scan the keyboard, starting with internal key number 16 ("Q") and working
 through the set of internal key numbers, returning the resulting key press in
 ASCII.

 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 ASCII code
                        of the key pressed, otherwise it contains 0

   A                    Contains the same as X

   Y                    Y is preserved


 Other entry points:

   RDKEY-1              Only scan the keyboard for valid BCD key numbers


 SED                    \ Set the D flag to enter decimal mode. Because
                        \ internal key numbers are all valid BCD (Binary Coded
                        \ Decimal) numbers, setting this flag ensures we only
                        \ loop through valid key numbers

.RDKEY

 TYA                    \ Store Y on the stack so we can retrieve it later
 PHA

 JSR FILLKL             \ Call FILLKL to scan the keyboard, update the key
                        \ logger and return any non-logger key presses in X

 PLA                    \ Retrieve the value of Y we stored above
 TAY

 LDA TRTB%,X            \ Fetch the internal key number for the key pressed

 STA KL                 \ Store the key pressed in KL

 TAX                    \ Copy the key value into X

 RTS                    \ Return from the subroutine