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⌂ ∞ Elite on the BBC Micro and NES

Keyboard: scan_fire

[Elite-A, I/O processor]

       Name: scan_fire                                               [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Implement the scan_fire command (scan the joystick's fire button)

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * tube_table calls scan_fire




 This routine is run when the parasite sends a scan_fire command. It checks the
 joystick's fire button and returns a value to the parasite with bit 4 clear if
 the joystick's fire button is being pressed, or bit 4 set if it isn't.



.scan_fire

 LDA #&51               \ Set 6522 User VIA output register ORB (SHEILA &60) to
 STA VIA+&60            \ the Delta 14B joystick button in the middle column
                        \ (high nibble &5) and top row (low nibble &1), which
                        \ corresponds to the fire button

 LDA VIA+&40            \ Read 6522 System VIA input register IRB (SHEILA &40)

 AND #%00010000         \ Bit 4 of IRB (PB4) is clear if joystick 1's fire
                        \ button is pressed, otherwise it is set, so AND'ing
                        \ the value of IRB with %10000 extracts this bit

 JMP tube_put           \ Send A back to the parasite and return from the
                        \ subroutine using a tail call