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⌂ ∞ ▽ Elite on the 6502

Keyboard: U%

[Elite-A, Flight]

       Name: U%                                                      [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Clear the key logger

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DEATH calls U%
             * DOKEY calls U%
             * RES2 calls U%




 Returns:

   A                    A is set to 0

   Y                    Y is set to 0



.U%

 LDA #0                 \ Set A to 0, as this means "key not pressed" in the
                        \ key logger at KL

 LDY #16                \ We want to clear the 16 key logger locations from
                        \ KY1 to KY20, so set a counter in Y

.DKL3

 STA KL,Y               \ Store 0 in the Y-th byte of the key logger

 DEY                    \ Decrement the counter

 BNE DKL3               \ And loop back for the next key, until we have just
                        \ cleared KL+1. We don't want to clear the first key
                        \ logger location at KL, as the keyboard table at KYTB
                        \ starts with offset 1, not 0, so KL is not technically
                        \ part of the key logger (it's actually used for logging
                        \ keys that don't appear in the keyboard table, and
                        \ which therefore don't use the key logger)

 RTS                    \ Return from the subroutine