APPENDIX TWELVE



     Pass2   LDA     (SOURCE),Y      ;move RAM OS to proper
                                     address
             STA     (DEST),Y
             INY
             BNE     Pass2
             INC     SOURCE+1        ;move to next page
             INC     DEST+1
             BEQ     Enable          ;If complete
             LDA     DEST+1 
             CMP     #$D0
             BNE     Pass2           ;skip block at $D000
             LDA     #$D8
             STA     DEST+1
             BNE     Pass2           ;Until DEST = $000
     Enable  PLA
             STA     NMIEN           ;reestablish NMI mask
             PLP                     ;reenable IRQs
             RTS
             END     START

     A sophisticated program called "RamMaster," by Matt Ratcliff,
     is available free through the Gateway BBS in St. Louis, Missouri.
     It not only creates a RAM OS, but it has a trap to keep the OS as
     RAM even when you press RESET. It also allows you to switch
     BASIC in and out from DOS. Probably the most elegant solution
     is the XL BOSS board which allows you to switch in a RAM OS,
     the older 800 OS, and the XL OS, as well as turn BASIC on or off
     with a few keypresses. It's available from Allen MacroWare in
     Redondo Beach, California.

     When you change the OS ROM into RAM, you can change all
     but a small portion of the OS at 53248-55295 ($D000-$D7FF),
     since it's RAM. You could always write an OS, load it into RAM,
     disable the ROM, and load yours in. You can change the
     character sets in their original locations rather than having to
     move them and use more memory. You could rewrite the han-
     dlers, interrupts, and other routines--almost anything.

     This is exactly what the Translator disk does when it writes the
     800 OS into the XL. Boot the Translator and place a regular DOS
     disk in at the prompt so that BASIC READY comes up. Now type:

     10 FOR N=57344 TO 57351
     20 READ A:POKE N,A:NEXT N
     30 DATA 255,1,1,1,1,1,1,1

     You'll see a "graph pad" screen: You've POKEd directly into
     the character set at $E000, altering the first character (space).