New users of the Tandy portables are often confused about how to load machine
language programs, what HIMEM and MAXRAM mean, and what relationship they have
to loading and running the .CO program. This file started out as a response to
those questions from a new user.


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Copyright 1989 Tony B. Anderson, All Rights Reserved



HIMEM - MAXRAM - What they are, and how they relate to use of .CO programs.


HIMEM is a movable partition which can be set to allow you to load and protect
a machine language program in a reserved area of RAM just below MAXRAM, in an
area between the HIMEM partition and MAXRAM.  Normally, HIMEM = MAXRAM, but
using the CLEAR,N,X command, you can move the HIMEM pointer to some address
below MAXRAM, "X", then load a machine language program between HIMEM and
MAXRAM, and the operating system will not overwrite the program by storing
variables, the file buffers, or the stack pointer in that area.  Such things
are normally stored just under the HIMEM address.  The "N" figure in the above
CLEAR statement specifies a number of bytes which should be reserved for
variable storage; usually it's 256, but it could be more, or less, depending on
what you want.

MAXRAM is a partition that separates RAM space available to the user from the
reserved area of RAM that is used by the computer's operating system - an area
that stores flags, buffers, pointers, the file directory, and other information
needed by the operating system.  Machine language programs - .CO programs - are
loaded and execute just under MAXRAM.

In order to load a machine language program, designated with the filename
extension ".CO", first you go into BASIC and test to see if HIMEM = MAXRAM. Use
the command PRINT HIMEM;MAXRAM and press the ENTER key.

If the two addresses are different, then you clear out any existing machine
language program with the command CLEAR 256,MAXRAM.  That resets the HIMEM
partition back to it's default value, in effect "unloading" any machine language
program that was already in that area.  Then you attempt to load the .CO program
you want to run with the command LOADM"(filename)".  I say "attempt", because
the program will not load because the HIMEM partition has not yet been set.

BASIC will return three addresses, "Top", "End", and "Exe".  The display might
look like this:

Top: 60905
End: 61103
Exe: 60920
? OM error

The "error" indicates the program didn't load.  It might be a different type
of error - no matter, it means the program didn't load.  If it did, you'd get
"Ok" on the next line below the Exe address.

Note what the "Top" address is - ignore the others, they aren't of interest to
you at this point.

Now type CLEAR 256,"address", putting the top address where I indicated
"address".  A typical clear might be CLEAR 256,58000, if 58000 was the "Top"
address given to you.

You can now LOADM the .CO program again, return to the menu, move the cursor
over the .CO filename and press ENTER to run the program, or you can type
RUNM"(filename)", which will load and run the program automatically.

HIMEM should remain where it was put, and the machine language program should
remain loaded for further use until you set HIMEM to something else.  Depends
on how the .CO program was written, and what other programs may do to the HIMEM
pointer, or what may be loaded into the reserved area, or what might be poked
into the reserved area.  It is possible, although not suggested, that several
different machine language programs might run in the same area, each one
overwriting the previous one.  This could lead to problems, so is not advised
unless you really know what you're doing.

Machine language programs which are designed to run in the computers alternate
screen buffer do not need to have an area reserved for them, and you do not
have to set HIMEM.  Once such programs are on your menu as a .CO program, all
you have to do is move the cursor over the filename and press the ENTER key.
They can also be LOADMed from BASIC, and run with a call to the "Exe" address,
or they can be RUNMed from BASIC.  Some of these programs can be used as
subroutines from other programs, check the documentation or check with the
author for information on those applications.

For additional information, refer to the Tandy BASIC Reference Manual
which came with your computer.  It's in the form of a dictionary, more or less,
and you can look up the words you want more information on, such as HIMEM,
MAXRAM, LOADM, RUNM, CLEAR, etc.