Sparse Invaders Source - Relocate.txt

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Sparse Invaders Source - Relocate.txt

   ;-------------------------------------------------------------------------------
   ;  Relocate
   ;  Written by Steve O'Leary
   ;
   ;  Copyright 2008,2009 Neil Beresford
   ;
   ;    This file is part of Sparse Invaders.
   ;    Sparse Invaders is free software: you can redistribute it and/or modify
   ;    it under the terms of the GNU General Public License as published by
   ;    the Free Software Foundation, either version 3 of the License, or
   ;    (at your option) any later version.
   ;
   ;    Sparse Invaders is distributed in the hope that it will be useful,
   ;    but WITHOUT ANY WARRANTY; without even the implied warranty of
   ;    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   ;    GNU General Public License for more details.
   ;
   ;    You should have received a copy of the GNU General Public License
   ;    along with Sparse Invaders.  If not, see <http://www.gnu.org/licenses/>.
   ;
   ;
   ;   Notes:
   ;
   ;-------------------------------------------------------------------------------
   
   .org $880        ; printer buffer
   
   Main:
   
   ; we have loaded the code at 1900 but it's been assembled to run from E00, so just copy
   ; the code from 1900 down to 900. We know it's length of the block we've loaded as Swift
   ; will tell you this from the properties of Sparse Invaders source file. You can 
   ; also find it out by exmaining the disk image created. 
   
   .alias LastAddress $1900+ $2200  ;  2200 is the size of our code,just setting address
                                    ; that we stop copying at when we reach it.
   
   
   
   CopyFrom:
   lda $1900
   CopyTo:
   sta $900
   inc CopyFrom+1                   ; lo byte
   bne NoCarry                      ; Not gone to zeror, so effectivly no carry
   
                                    ; If we get here however....
   inc CopyFrom+2                   ; add in the effective carry (note we use BEQ above to
                                    ; detect for rolling over from 255 to 0 as the command
                                    ; INC does not effect the carry flag. 
    
   NoCarry:                                
   
   ; we now do the same increment for the copyto position
                                    
   inc CopyTo+1                   ; lo byte
   bne NoCarry2                      ; Not gone to zeror, so effectivly no carry
   
                                    ; If we get here however....
   inc CopyTo+2                     ; add in the effective carry (note we use BEQ above to
                                    ; detect for rolling over from 255 to 0 as the command
                                    ; INC does not effect the carry flag. 
   
   NoCarry2:
   
   ; now wecheck if we've copied all the bytes
   lda CopyFrom+2                   ; check hi byte first
   cmp #>LastAddress
   BNE CopyFrom                      ; Go back to start of loop if not equal
   
   ; high byte is equal, check lo byte
   lda CopyFrom+1
   cmp #<LastAddress
   BNE CopyFrom                      ; if no equal, carry on copying
   
   ; we've finished copying all the bytes, run the code at the new address
   JSR $0900        ; start of code to run
   RTS