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Q60967: Dynamic Arrays, Far Strings Overwrite COMMAND.COM

Article: Q60967
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900228-4 B_BasicCom | mspl13_basic
Last Modified: 14-MAY-1990

If the far heap is used for any reason during the execution of a BASIC
program, the transient part of COMMAND.COM may have to be reloaded
after its execution. COMMAND.COM occupies an area of the far heap that
some features of Microsoft's BASIC compilers use. If COMMAND.COM does
need to be reloaded and cannot be found in the current path, the
system prompts you to insert a disk containing it.

This information applies to Microsoft QuickBASIC Compiler versions
4.00, 4.00b, and 4.50, to Microsoft BASIC Compiler versions 6.00 and
6.00b, and to Microsoft BASIC Professional Development System (PDS)
version 7.00.

Page 14 of "Advanced MS-DOS Programming, Second Edition" by Ray Duncan
(Microsoft Press, 1988) explains this behavior of COMMAND.COM, which
applies not only to BASIC, but all applications:

   When an application program terminates, the resident portion of
   COMMAND.COM does a checksum of the transient module to determine
   whether it has been destroyed and fetches a fresh copy from the
   disk if necessary.

Many features of Microsoft's BASIC compilers require use of the far
heap, and thus are common destroyers of COMMAND.COM. The most common
is when an executable file uses a run-time module, BRUN45.EXE for
example. Another is the use of dynamic arrays, which are stored in the
far heap, although using small enough arrays will not overwrite
COMMAND.COM. In BASIC PDS 7.00, the far strings option (/Fs) also uses
the far heap, even if no strings are used in the program.

Consider this sample program:

   'NOCODE.BAS
   'This program contains no code

If NOCODE.BAS is compiled without the /o switch (specifying use of the
run-time library) or compiled with the /Fs switch under BASIC PDS
7.00, COMMAND.COM has to be reloaded after execution, even though no
code or data is actually present.

Consider another example:

   'HUGEARAY.BAS
   REM $DYNAMIC
   DIM Array(1 TO 200, 1 TO 200) AS INTEGER

Since HUGEARAY.BAS dimensions a huge, dynamic array, it  overwrites
COMMAND.COM. If the array were much smaller, COMMAND.COM would remain
intact. Note that even larger dynamic arrays that aren't huge (over
64K) can cause COMMAND.COM to be overwritten.

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