The U% subroutine - Elite on the 6502

       Name: U%                                                      [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Clear the key logger and reset a number of flight variables
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DEATH calls U%


 This routine zeroes the 17 key logger locations from KY1 to KY20 and the key
 variable at KL, and resets the 40 variable bytes from LSP to TYPE.

 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 KLO

 LDY #56                ; We want to clear the 16 key logger locations from KY1
                        ; to KY20, and we want to zero the 40 variable bytes
                        ; from LSP to TYPE, so set a counter in Y

.DKL3

 STA KLO,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
                        ; KLO+1

 STA KL                 ; Clear KL, which is used for logging keys that don't
                        ; appear in the keyboard table

 RTS                    ; Return from the subroutine

[X]

Label DKL3 is local to this routine

[X]

Variable KL in workspace UP

If a key is being pressed that is not in the keyboard table at KYTB, it can be stored here (as seen in routine DK4, for example)

[X]

Variable KLO in workspace KEYLOOK

The key logger in the BBC Micro version has a spare byte at the start for storing the last key press, so we also include a spare byte here so the KLO logger in the Commodore 64 version behaves in a similar way to the KL key logger in the BBC Micro

Keyboard: U%

[Commodore 64 version]