The U% subroutine - Elite on the BBC Micro

       Name: U%                                                      [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Clear the key logger
    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:
             * BR1 (Part 1 of 2) calls U%
             * RDKEY2 calls U%
             * TITLE calls U%


 Returns:

   A                    A is set to 0

   X                    X is set to 0


.U%

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

 LDX #17                \ We want to clear the 17 key logger locations from
                        \ KL to KY20, so set a counter in Y

.DKL3

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

 DEX                    \ Decrement the counter

 BNE DKL3               \ And loop back for the next key, until we have just
                        \ 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
Keyboard: U%

[BBC Master version]
