May June 1980 - Commodore Computers

MAY/JUNE. 1980. 198O. ISSUE 4. 4 COMPUTE. 57
Machine Language
Tools
TOOlS Jim m Butterfield
The newcomer to machine language programming
will need to LO provide himself with certain toolss before
he gets started.
It's important to ro have the right computer. The
original ROM which came with early PETs can be
used with machine e language, but it's clumsy for this
kind of use. If you haven't done so already, make the
switch to upgrade ROM: you'll u ' get a built-in
Machine Language MoniLOr Monitor and the capability to dig
your way out of a program crash.
Get the official reference: 6502 Programming
Manual. It may be obtained from Commodore
(MOS Technology), Synertek or Rockwell, who are
the three manufacturers urers of 6502 microprocessors. It's
not a teaching leaching book, but a reference - although
many of the earlyy chapters are sett out in tutorial
style.
If you see other books that seem to suit your
needs, you may buy them, of course. Avoid books that
deal with "somebody else's" machine - you won't
learn much about the PET from books on KIM, , SYM, ,
Apple, OSI, AIM or Atari. At a later stage, you
might like to buy y such books in order to try your
hand att converting the programs LO to PET - or just
to get an idea as to what's in other machines.
General books on the 6502 arc are good. Remember
that you'll have to fit what these books say intoo
the framework of thee PET architecture. Leventhal's
6502 Assembly Language Programming (Osbornel
(Osborne/
McGraw Hill, , Inc.) is full of programming examples;
but they will all need to be trimmed up LO to fit the
PET. Don't let that worry youu - it's not hard, , and
it's good practice.
You have very little need for hardware unless
you have a special project in mind. If your machine
has upgrade ROM, , try building or buying g an uncrashing
connector (see COMPUTE, P issue 1, page 89).
Your early y programs will certainly "crash" and
you'll save a lot of reloading or retyping if you
un-
can re-awaken your machine. Most non-PET
machines have a Reset button butron that will do the same
job, which is very convenient except when you hit
it accidentally.
The Machine Language Monitor is very vcry useful.
It's built into upgrade ROMs. Original ROM
machines cann load a MLM [\1 from tape, but t thiss is
much less convenient especially y since it uses up
memory space that you u might want for your own
purposes. You may enhance the e usefulness of your
MLM by obtainingg more elaborate versions. Many
of these are availablee from the PET Programs Onn
Tape Exchange (P.O. Box 516, , Montgomeryv Montgomcryville, PA
18936). (Edilor (Editor's Note: Nole: Sec See Table of COl/leI/Is. Contents. RCL) R If you're stayingg with original ROM, , you
might like Bill Seiler's Seller's NEWMON which has many
extra functions including a tinyy assembler. Another
useful package is Jim Russo's high monitor with
machine language single-step; this is sometimes
called MT6671 and sometimes DISS/STEP. STEP.
The upgrade ROM monitor is built in, , but
there are a couple of enhancement packages that add
extra functions: SUPERMON and EXTRAMON.
Both of these contain powerful l extras: disassembler,
tiny y assembler, memory hunt, , and single-stepping.
If you get deeply into machine language programming,
you'll ' want to obtain a full assembler. These
are available able from commercial sources. Don't get
an assembler right away, especially if you're
thinking of adding memory or a disk system. . Your
beginning programs will be quite small, , and easy to
hand assemble; what's more, you'll get a better
feel for how the machine works by doing it all
yourself. Later, as your programs stan start to exceed
thirty lines of code or so, you'll find that an
assembler will be a big help. . This is especially
true when youu want to make changes to [Q a previously
written program. Since few of us write
perfect programs every time, the capability of oi making
changes easilyy is a major advantage that assemblers
give you.
.
The most important tool you need for machine-
language programming ming is one you've already got: your
brain. Remember that each machine language instruction
is simple and logical; a program gets complex
onlyy when youu put many instructions together. Be
prepared to change your style when you go to
machinee language; a lot t off thinking and planning
should take place before you write your first line of
code.
If bits and logical operations are not familiar to
you. u, you'll need to do some advance brushing g up.
Learn why you u can n store values lucs only y from 0 to 255
in a memory location; how w a negative number is
represented reprcsen in binary; how to relate binary, hexadecilTla
mall and decimal numbers; and how the logical functions
(AND,, OR, R , and EOR) work. There are rc plenty y of
introductory texts ts around to help. 0©