Q37022: How to Determine the Amount of Available DOS Memory

Article: Q37022
Product(s): See article
Version(s): 5.00 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 12-DEC-1988

Question:

Is there a C run-time function that I can use to determine how much
RAM is available under DOS?

Response:

No function exists that specifically performs this task. Functions
"_freect" or "_memavl" only return available memory for dynamic
allocation in the near heap. The near heap is located above the stack
within the 64K byte or smaller default data segment called DGROUP.
Function _memavl indicates the amount of near space that never has
been allocated, whereas _freect allows you to detect what has been
freed.

However, you may use the function "_dos_allocmem" defined below
to determine the amount of far heap above the default data segment:

    #include <dos.h>
    unsigned _dos_allocmem(size, segment) ;
    unsigned size ;
    unsigned *segment ;

If the attempted memory allocation is NOT successful, _dos_allocmem
puts the maximum possible free memory size (in 16-byte paragraphs)
in the word pointed to by "segment". If you request to allocate
0xFFFF paragraphs of memory (which will fail in any case), you should
be able to find out the amount of available memory. (DOS function
call 48h performs the same task.)

Note: the value returned to *segment is in paragraphs too. If other
memory allocation functions are going to use this value, it should be
converted to bytes.

Refer to the "Microsoft C Version 5.10 Optimizing Compiler Run-time
Library Reference" for specific information on using _dos_allocmem.

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