117. Comparing programs with memory
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This is a routine along the same lines as the previous one, but is for
comparing a purely BASIC program in memory with a named program on disc. (It
does not work with cassette.) BASIC in this context also included programs
containing assembler code.

Since it is for comparing a program in memory with one on disc, the routine
cannot conveniently be in the the form of a program itself. So, it is stored
on a red function key ready for calling at any time. Before programming the
key, (I have used *KEY0 in this example), you should type *FX18 to clear all
the keys, as the routine takes up most of the available space on a B/B+. The
key definition can be saved on a B/B+ with *SAVE name FFFF0B00+100, and
reloaded at any time with *LOAD name . On a Master/Compact, save the
definition as a one-line program, and simply CHAIN it to redefine the key
when required. The routine is compatible with the 6502 2nd processor.

When called, the routine first prints out the length of the program in
memory, and then prompts for a filename. This will repeat until a valid
filename is found, and then the length of the disc file is also printed
before a byte-for-byte check is carried out. There is a similar routine in
BEEBUG, but it doesn't check for different lengths before laboriously
comparing byte-for-byte, and with BASIC-I it is liable to try and lengthen
the file if it is shorter than the program in memory!

Note that in order to squeeze everything on to one key definition, I have
had to use many abbreviations, and only vital spaces have been retained. I
have split the lines at convenient colons, but the definition is actually
typed in as one continuous line. It may help to omit the first term VDU21|M
initially, otherwise it tends to suppress any syntax error messages if you
make a typing mistake. When the routine is debugged, you can restore the
first term to give a neater effect on the screen.

 *K.0VDU21|MCLO.#0:P."  Length : &";~TOP-PA.:REP.I."Filename : "f$:
 ch=OP.f$:U.ch:P."  Length : &";~EXT#ch:M%=-1:
 REP.M%=M%+1:U.EXT#ch<>TOP-PA. ORBGET#ch<>M%?PA. OR EOF#ch:
 IF EOF#ch TH.P."Identical"ELSEP."Different":CLO.#0|F|M