Utility routines: ZES2k

       Name: ZES2k                                                   [Show more]
       Type: Subroutine
   Category: Utility routines
    Summary: Zero-fill a specific page
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * TTX66K calls ZES2k


 Zero-fill from address (X SC) + Y to (X SC) + 1.


 Arguments:

   Y                    The offset from (X SC) where we start zeroing, counting
                        down to 1; if Y = 0, then the whole page is reset


 Returns:

   Z flag               Z flag is set

   A                    A is 0

   Y                    Y is 0


.ZES2k

 LDA #0                 ; Load A with the byte we want to fill the memory block
                        ; with - i.e. zero

 STX SC+1               ; We want to zero-fill page X, so store this in the
                        ; high byte of SC, so the 16-bit address in SC and
                        ; SC+1 is now pointing to the SC-th byte of page X

.ZEL1k

 STA (SC),Y             ; Zero the Y-th byte of the block pointed to by SC,
                        ; so that's effectively the Y-th byte before SC

 DEY                    ; Decrement the loop counter

 BNE ZEL1k              ; Loop back to zero the next byte

 RTS                    ; Return from the subroutine