MINCE global space and BDS C

[johnsonjh]

2-Sep-82 10:26:00,3244;000000000000
Date:     2 September 1982 1226-edt
From:     Brian N. Hess              <Hess.Unicorn at MIT-MULTICS>
Subject:  Mince globals
To:       amethyst-users at MIT-MC

Mike --

A quote from the Amethyst Users Group newsletter, September, 1981:
There is one question I would like to answer that everyone keeps asking.
There are in fact several ways for people to get more global space in
Mince for use with personal extensions.

First a little background.  We use the "-e" option in the C compiler to
save space in the resulting programs.  This forces the global address to
appear at the specified address regardless of the size of the code.  L2
and CLINK both get this address, along with the size of the global data
area from the first .CRL file linked.  Mince does dynamic storage
allocation in the region between the global data and the stack, thus the
size of the global data must be accurate.  Further, there is more global
data than there might appear to be -- MINCE.CRL is compiled with a
different version of MINCE.GBL than is distributed, and has extra global
data for the buffer abstraction.

The simplest way to add storage is to have this storage be in the form
of string constants.  The compiler allocates string constants in such a
way that you can write over them so long as you don't exceed their
allocated length.  This is a fair although uncomfortable programming
practice which all of the compilers I know of (...) support.  This can
be done as follows:

Create a function such as:

          Storage()
          {
          return("This returns a string of exactly fifty characters    ");
          }

This function can now be used in another place to get a pointer to use
for storage as in:

          Munge(arg)                    /* Store arg in storage */
          char *arg
          {
          if (strlen(arg) > 50) Error("String too long");
          else strcpy(Storage(),arg);
          Echo("String now is: ");
          TNLPrnt(Storage());
          }

Another way to get extra storage is to create some room between the end
of the code and the start of the global data area by elimination of some
code or use of the L*.CRL files.  The routines can simply KNOW that this
space is there and use it.  This can get tricky to maintain since the
end of the code is likely to change every time you work on the program.
One good thing to do is to have your routine check the location of the
end of the code with the BDS-C function codend().

The final way to ge more storage is a little-known feature of L2.  L2,
unlike CLINK, does not require the first function linked to be the main
function.  Thus, you can compile BINDINGS.C with extra global
definitions and link BINDINGS first and then MINCE.  The trick is that
you have to leave extra space (more than 1000 and less than 1100 bytes;
1100 works [and I counted once and got 1086 -- bnh]) for the globals
that you cannot see.  The only way to know this number exactly is to put
a function in Mince that prints the address of the last visible global
and also the value returned by the BDS-C function that gives the address
of the end of the global area.
                            
Good luck,
Brian