O.S. SWAPPING DURING THE VBI
          ----------------------------
          
          By TML
          
          Last month I was working on a music demo and I hadn't enough RAM
          to use. I was also working on the last game of ANG: "Simple
          Minds". (You should get this game! Your editor is in it with a
          nice  picture). Anyway, for this game I needed almost all RAM
          for main routines, pictures and musix.
          
          I could put some pictures in the OS RAM area, but I should
          disable interrupts when I needed a new picture. Not so nice
          idea, because the music should play the same time... Flits! An
          idea! I should copy the music and player into the RAM/OS area
          and disable the OS during the VBI... Is it possible. I talked to
          Freddy about the subject. 'Hmmm... maybe... Hmmm... maybe
          not...' was his answer. I took my MAC65 and made some simple
          routines to test this idea. Yes! Eureka! An interrupt routine in
          the OS/RAM area!
          
          Finaly I took the Chaos composer, made some stupid musix, wrote
          it to disk, wrote the player to disk, replaced the load headers,
          wrote a copy routine to copy the music to $C000 and wrote the
          interrupt routine. Getting nervous, loading, running....
          MUSIC!!!
          
          So, in the future we can switch off the Operating System in the
          main loop (you have to disable interrupts!) or we can switch off
          the OS in an interrupt! I prefer the last option. I haven't
          tested it in other interrupts, but it should work. What can you
          put in the RAM area? Simple:
          
          -Music player
          -Music data
          -Main VBI routine
          -Color data
          -Sinus tables
          -etc.
          
          An example:
          
               LDA #7     ;INIT VBI ROUTINE
               LDY #>VBI
               LDX #>VBI
               JSR $E45C
          DO   JMP DO
          ;
          VBI  LDA #$FE   ;OS 'OFF'
               STA $D301
               JSR $C000  ;RAM AREA
               INC $D301
               JMP $E45C
          
          This small VBI takes 23 bytes of the available normal RAM, the
          main VBI may be 14 kilobytes and you still have all normal OS
          routines. Copy the main VBI routines including all data to $C000
          - $FFFF. Activate this routine and see...
          
          Don't forget to assemble your main VBI into right ram area and
          replace the loadheaders or re-write the loadroutines. I did this
          in the game Simple Minds. I wrote a sector reader and a half
          page copier. So, I needed 128 bytes RAM ($400-$47F) as buffer
          and 6 zero page pointers to copy the code into the right ram
          area. Switch off the OS before the copyroutine and switch it on
          at the end of the copyroutine. Write your routines to the right
          sectors on disk and ready!
          
          On side B of this magazine you will find a small package with
          some music, comfiles and MAC65 sources.
          
          DUMMY   .COM   : A runable example
          
          In the OSSWAP.LZH archieve....
          OSSWAP  .M65   : The source in MAC/65
          LORIEN1 .CMC   : Music data
          LORIEN1 .REP   : Music player
          
          Play with it and let me (Megazine) know if you have any other
          ideas to this 'new' idea.
          
          The Missing Link.