114. Another relocate program
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Earlier, I gave a Procedure for tacking onto the end of a Basic program, so
that long programs could be run off disc, with a PAGE value down to &E00.
This works fine, but there are times when this is not convenient. If you are
adapting a very long and complex program from cassette to disc, it may not be
safe to go tacking procedures on the end. This is especially true if there is
data after the visible Basic part of the program, as any additions are likely
to overwrite it. A typical example would be a text adventure game like Castle
of Riddles, which is 65 blocks long, over half of which is data. This is too
long to even LOAD into memory with PAGE set at &1900.

The solution is to make the program a 2-part one, with the relocate routine
in the 1st half, thus avoiding any need to tamper with the main program. If
the program is short enough to run with PAGE at &1100, then the 1st program
need consist only of one line, ie 10 PAGE=&1100:CHAIN "Dragon2" . If the
program must run with PAGE lower than that, eg &E00, then the program below
will do the trick. It is so fast that it doesn't bother to work out the
length of the program, (tricky if you have to allow for any hidden data), but
simply relocates all available memory in 1/3 second. In line 10 you specify
the new value of PAGE, eg &E00, and the name of the main program. You could
of course replace the final term with an INPUT N$ statement, in which case
the one relocator will run any named program. PAGE is initially set to &1100
in line 10, in case the main program is too long to even LOAD with PAGE at
&1900. This doesn't affect the relocator itself, which churns on merrily,
blissfully unaware that PAGE has been altered, but it does define where the
main program will LOAD. In many cases, this 1st term in line 10 can be
omitted, but it doesn't do any harm to leave it in, (with the Acorn 8271
DFS's at any rate).

The program may already have a first part, which just sets up some of the
character shapes, and displays a logo on the screen. You could easily add
this routine in place of the CHAIN "Dragon2" type of command, followed by END
if necessary. Ignore any corruption of the logo; if you're clever enough,
you'll know how to preserve it!

This program looks rather weird, and it can be hard to spot mistakes. Try
missing out the VDU21 in line 60 at first, otherwise useful error messages
are suppressed.

 10 PAGE=&1100:D%=&E00:N$="Dragon2"
 20 !&70=D%:!&72=PAGE:!&74=HIMEM-PAGE
 30 P%=&76:[OPT2:LDX&74:LDY#0
 40 LDA(&72),Y:STA(&70),Y:INY:BNE&85:INC&71:INC&73
 50 DEX:BNE&7A:DEC&75:BNE&7A:RTS:]
 60 VDU21:*KEY9LOADN$|M*TAPE|MCALL&76|MPAGE=D%|MNEW|MOLD|MRUN|F|M
 70 *FX138,0,137