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⌂ ∞ ▽ Elite on the 6502

Controllers: ReadControllers

[NES version, Bank 7]

       Name: ReadControllers                                         [Show more]
       Type: Subroutine
   Category: Controllers
    Summary: Read the buttons on the controllers and update the control
             variables
  Deep dive: Bolting NES controllers onto the key logger

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




.ReadControllers

 LDA #1                 ; Write 1 then 0 to the controller port at JOY1 to tell
 STA JOY1               ; the controllers to latch the button positions, so we
 LSR A                  ; can then read them in the ScanButtons routine
 STA JOY1

 TAX                    ; Call ScanButtons with X = 0 to scan controller 1 and
 JSR ScanButtons        ; update the controller variables

 LDX numberOfPilots     ; Set X to numberOfPilots, which will be 0 if only one
                        ; pilot is configured in the pause options, or 1 if two
                        ; pilots are configured

 BEQ RTS3               ; If X = 0 then only one pilot is configured, so jump to
                        ; RTS3 to return from the subroutine, as we do not need
                        ; to scan controller 2

                        ; Otherwise X = 1 and two pilots are configured, so fall
                        ; through into ScanButtons with X = 1 to scan controller
                        ; 2 and update the control variables