Q66104: Use of _far Keyword in Tiny Programs

Article: Q66104
Product(s): See article
Version(s): 6.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | docerr | mspl13_c
Last Modified: 17-DEC-1990

A tiny memory model program cannot have _far code. The following
documentation taken from online help is incorrect because it is not
possible to use the _far keyword with code addresses or static data
addresses in a tiny program. It is also not possible to use the _huge
keyword with static data addresses.

   The tiny-model option creates a program with the .COM extension.
   The program contains a single segment for both code and data. Code
   and data combined are limited to 64K.

   Both code and data items are accessed with near addresses. You can
   override the defaults with the _far keyword for code and the _far
   or _huge keyword for data.

The _far keyword does override the near addresses correctly, but the
linker will generate the following error message because a .COM file
cannot have segment references:

   LINK : fatal error L1127: far segment references not allowed with
   /TINY

It is possible to use _far and _huge keywords to create pointers for
use with dynamic memory or to set up pointers with absolute addresses,
such as a pointer to video memory. This is because these pointers do
not have segment values that depend on where the program is loaded in
memory. A .EXE file contains a table of all segment references that
are made by the program in its header. When the program is loaded, all
segment values in the table are adjusted for the programs load
location. Because .COM files do not have this header information, they
cannot contain segment references.

No Microsoft graphics library routines can be used when generating
.COM files because they are all compiled as far calls.

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