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⌂ ∞ ▽ Elite on the 6502

Version analysis of U%

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

       Name: U%
       Type: Subroutine
   Category: Keyboard
    Summary: Clear the key logger



 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

Code variation 1 of 1Related to an enhanced feature

Compared to the cassette and Electron versions, the enhanced versions have an extra key in the key logger, for "P" (which turns off the docking computer).

Tap on a block to expand it, and tap it again to revert.

Cassette, Electron
Flight, 6502SP
 LDY #15                \ We want to clear the 15 key logger locations from
                        \ KY1 to KY19, so set a counter in Y

 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