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⌂ ∞ ▽ Elite on the BBC Micro and NES

Keyboard: RDKEY

[6502 Second Processor version]

       Name: RDKEY                                                   [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Scan the keyboard for key presses by sending an OSWORD 240 command
             to the I/O processor

    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
             * DOKEY calls RDKEY
             * LL9 (Part 12 of 12) calls RDKEY
             * PAS1 calls RDKEY
             * PAUSE2 calls RDKEY
             * TT217 calls RDKEY




 This routine sends an OSWORD 240 command to the I/O processor to ask it to
 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). The results are copied from the I/O
 processor into the key logger buffer at KTRAN.

 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

 LDA #240               \ Set A in preparation for sending an OSWORD 240 command

 LDY #HI(buf)           \ Set (Y X) to point to the parameter block at buf
 LDX #LO(buf)

 JSR OSWORD             \ Send an OSWORD 240 command to the I/O processor to
                        \ scan the keyboard and joysticks, and populate the key
                        \ logger buffer in KTRAN, which is the part of the buf
                        \ buffer just after the two OSWORD size bytes

 LDX KTRAN              \ Set X to the first byte of the updated KTRAN, which
                        \ contains the internal key number of the key being
                        \ pressed, or 0 if there is no key press

 TXA                    \ Copy X into A

 RTS                    \ Return from the subroutine