Drawing the screen: ResetBuffers

       Name: ResetBuffers                                            [Show more]
       Type: Subroutine
   Category: Drawing the screen
    Summary: Reset the pattern and nametable buffers
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * ResetVariables calls ResetBuffers


 The pattern buffers are in a continuous block of memory as follows:

   * pattBuffer0 ($6000 to $67FF)
   * pattBuffer1 ($6800 to $6FFF)
   * nameBuffer0 ($7000 to $73BF)
   * attrBuffer0 ($73C0 to $73FF)
   * nameBuffer1 ($7400 to $77BF)
   * attrBuffer1 ($77C0 to $77FF)

 This covers $1800 bytes (24 pages of memory), and this routine zeroes the
 whole lot.


.ResetBuffers

 LDA #HI(pattBuffer0)   ; Set SC2(1 0) to pattBuffer0, the address of the first
 STA SC2+1              ; of the buffers we want to clear
 LDA #Lo(pattBuffer0)
 STA SC2

 LDY #0                 ; Set Y as a byte counter that we can use as an index
                        ; into each page of memory as we clear them

 LDX #$18               ; We want to zero memory from $6000 to $7800, so set a
                        ; page counter in X to count each page of memory as we
                        ; clear them

 LDA #0                 ; We are going to clear the buffers by filling them with
                        ; zeroes, so set A = 0 so we can poke it into memory

.rbuf1

 STA (SC2),Y            ; Zero the Y-th byte of SC2(1 0)

 INY                    ; Increment the byte counter

 BNE rbuf1              ; Loop back until we have zeroed a full page of memory

 INC SC2+1              ; Increment the high byte of SC2(1 0) so it points to
                        ; the next page in memory

 DEX                    ; Decrement the page counter in X

 BNE rbuf1              ; Loop back until we have zeroed all X pages of memory

 RTS                    ; Return from the subroutine