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Q24935: Effects of Min and Max Memory Allocation During C Program Load

Article: Q24935
Product(s): See article
Version(s): 4.00 5.00 5.10 6.00 6.00a
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickc s_link s_exemod s_exehdr | mspl13_c
Last Modified: 6-FEB-1991

Changing the minimum and maximum allocation values in an .EXE file
will affect the way in which memory is allocated for a program.

Memory is dynamically allocated for a program when it is loaded. DOS
initially attempts to allocate FFFFH paragraphs memory. This will
always fail, returning the size of the largest free block. If this
block is smaller than the minimum allocation and the load size, a
no-memory error will occur. If this block is larger than the sum of
the maximum allocation and the load size, DOS will allocate the amount
specified by the load size and the maximum allocation. Otherwise, the
largest block of memory available is allocated.

An example of when the maximum allocation should be changed would be a
program, such as a menu driver, that spawns other programs. Since this
program would stay resident in memory, the maximum allocation should
be set to the minimum allocation to keep as much memory available as
possible for the spawned programs.

In small memory model, the C start-up code will return all memory
allocated to it back to DOS, except for a 64K block reserved for the
default data group. If only 4K of this 64K will be used by the menu
program, the entire 64K should not be kept. There are several ways of
changing the maximum allocation to 4K -- you can link with the /CP:1
option or you can modify an existing .EXE file with the EXEHDR or
EXEMOD utilities.

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