Programming Techniques - Dropping PAGE to increase BASIC memory
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			J.G.Harston, 18-Sep-1997


Programs should adapt themselves to use the amount of memory available by
checking how much memory is between LOMEM and HIMEM, for instance:

      max%=(HIMEM-LOMEM-500)DIV4:DIM array%(max%)

will define an array of a size that will it into the available variable
space, leaving 500 bytes left over for other variables. Note also that you
should always compare HIMEM to LOMEM, not to TOP. By default LOMEM is at
TOP, but the variables can easily be put elsewhere, TOP is the complement
to PAGE, LOMEM is the complement to HIMEM.

Sometimes a program has to have a certain amount of workspace, and the
usual way to do this is to lower PAGE. However, You mustn't just force PAGE
down to an arbitary address that you think is best, you should move PAGE
down only if it needs to be, and only by the amount that is needed for the
program to run. The best way of doing this is to start the program with the
following:

MODE&87
IF HIMEM<&8000:A%=HIMEM-LOMEM-sizeneeded:IF A%<0:PAGE=PAGE+A%:CHAIN$&600

replacing sizeneeded with an appropriate amount for the program, and ensure
you use shadow screen modes (&80+n) in the rest of the program. Then, PAGE
will only be forced down if and only if it actually needs to be, and only
by an amount that it needs.

This starts by selecting the smallest possible screen mode, and if that
results in program space being used it calculates if PAGE needs to be
dropped, and if so it drops it by the required amount and reCHAINs the
program by using the contents of string buffer that has just been used to
load the program.

Some experimentation will be needed to work out the sizeneeded value. One
way is to start with &4C00 and progressively add &100 until the program
has enough memory to run.

or....


Find a value of PAGE where your program works with HIMEM at the non-shadow
screen value for the screen mode the program will use. In testing you may
need to temporarily put HIMEM=&3000 at the start of the program to force it
to work as though there is very little memory available. Then use

PRINT ~(&7C00-TOP) AND &FF00

and this should be the value to use for sizeneeded. If this is not enough,
then usually just adding another &100 to sizeneeded will do. You will then
end up with something like:

MODE&87:IF HIMEM<&8000:A%=HIMEM-LOMEM-&4E00:IF A%<0:PAGE=PAGE+A%:CHAIN$&600