common/clearwram.asm

   1 ;
   2 ;    Copyright (C) 2004, 2005 Kent Hansen.
   3 ;
   4 ;    This file is part of Neotoxin.
   5 ;
   6 ;    Neotoxin is free software; you can redistribute it and/or modify
   7 ;    it under the terms of the GNU General Public License as published by
   8 ;    the Free Software Foundation; either version 2 of the License, or
   9 ;    (at your option) any later version.
  10 ;
  11 ;    Neotoxin is distributed in the hope that it will be useful,
  12 ;    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 ;    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 ;    GNU General Public License for more details.
  15 ;
  16 ;    You should have received a copy of the GNU General Public License
  17 ;    along with this program; if not, write to the Free Software
  18 ;    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  19 ;
  20
  21 .dataseg
  22
  23 ptr .dw
  24
  25 .codeseg
  26
  27 .public clear_wram
  28
  29 ; Clears WRAM at $6000-$7FFF
  30 ; Params: None
  31 ; Destroys: A, X
  32
  33 .proc clear_wram
  34     lda     #$7F        ; high byte of last WRAM page
  35     sta     ptr+1
  36     lda     #$00        ; low byte of last WRAM page
  37     sta     ptr
  38     tay
  39   - sta     [ptr],y     ; clear address
  40     iny                 ; cleared 256 bytes?
  41     bne     -           ; if not, clear another
  42     dec     ptr+1       ; decrement high byte of address
  43     ldx     ptr+1
  44 ;        cpx     #$60   ; HMM???
  45     cpx     #$70
  46     bcs     -           ; clear another page if >= $60
  47     rts
  48 .endp
  49
  50 .end