Utility routines: ZES2

       Name: ZES2                                                    [Show more]
       Type: Subroutine
   Category: Utility routines
    Summary: Zero-fill a specific page
    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:
             * TT26 calls ZES2


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


 Arguments:

   X                    The high byte (i.e. the page) of the starting point of
                        the zero-fill

   Y                    The offset from (X SC) where we start zeroing, counting
                        up to &FF

   SC                   The low byte (i.e. the offset into the page) of the
                        starting point of the zero-fill


 Returns:

   Z flag               Z flag is set


.ZES2

 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

.ZEL1

 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

 INY                    \ Increment the loop counter

 BNE ZEL1               \ Loop back to zero the next byte

 RTS                    \ Return from the subroutine