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Microsoft KB Archive/102325

From BetaArchive Wiki

How to Allocate Memory From the MS-DOS Upper Memory Area


The information in this article applies to:

  • The C Run-Time (CRT), included with:
    • Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
    • Microsoft C/C++ for MS-DOS, version 7.0
    • Microsoft Visual C++, versions 1.0, 1.5


In MS-DOS version 5.0, an application can dynamically allocate blocks of memory from the MS-DOS upper-memory area. To do so, the application calls Function 5803h, Set Upper-Memory Link, before it calls the malloc() function in the C run-time library.


The first 640KB of MS-DOS memory is commonly called "conventional memory." The address space immediately above that, in the address range from 640K to 1024K is the Upper Memory Area (UMA).

When an application developed in the compact or large memory model uses malloc() to dynamically allocate memory, malloc() acquires memory in the MS-DOS far heap to satisfy the request. By default, MS-DOS makes only conventional memory available to an application because, by default, MS-DOS does not link conventional memory with the 384K of UMA memory immediately above conventional memory.

MS-DOS version 5.0 introduces the ability to satisfy allocation requests with the UMA on machines that have an 80386 or greater processor and that have physical memory installed in the address space from 640K to 1024K. Both HIMEM.SYS and EMM386 must be installed, and the CONFIG.SYS file must contain the line DOS=UMB.

The code example below demonstrates establishing the link to the MS-DOS UMA and allocating memory from the UMA.

NOTE: This example does not work when an application other than MS-DOS, such as 386MAX, manages the UMA.

By default, the MS-DOS memory allocation strategy involves searching memory from the available memory block with the lowest address. If the upper-memory link is established, MS-DOS searches the UMA only after all conventional memory has been exhausted. You can change this allocation strategy using the MS-DOS Function 5801h, Set Allocation Strategy. For example, your application can choose to search only the UMA.

If you change the upper-memory link state or the original MS-DOS memory allocation strategy, you must restore them before terminating your application.

Sample Code

 * Compiler options needed: /AL or /AC

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include <malloc.h>
#include <dos.h>

void main(void)
   int i;
   char in;
   union REGS inregs, outregs;

   printf("Link upper-memory area? Y/N: ");
   in = getche();
   while (in != 'Y' && in != 'y' && in != 'N' && in != 'n')
      printf("\nLink upper-memory area? Y/N: ");
      in = getche();

   if (in == 'Y' || in == 'y')
      // Link upper-memory area = 0x5803;
      inregs.x.bx = 0x0001;
      int86(0x21, &inregs, &outregs);
      if (outregs.x.cflag)
         printf("\n** Set Upper-Memory Link error **\n");

   // malloc() as many 1KB blocks as possible
   for (i = 0; ; i++)
      if (!malloc(1024))
   printf("\n%d KB successfully allocated\n", i);

   // Unlink UMA = 0x5803;
   inregs.x.bx = 0x0000;
   int86(0x21, &inregs, &outregs);
   if (outregs.x.cflag)
      printf("** Set Upper-Memory Link error **\n");

Running the application produced the following output. The MEM /C command showed that the test machine had 582K available conventional memory and 76K free in the UMA.


Link upper-memory area? Y/N: n
503 KB successfully allocated
Link upper-memory area? Y/N: y
578 KB successfully allocated 

Additional query words: kbinf 1.00 1.50 6.00 6.00a 6.00ax 7.00

Keywords : kb16bitonly
Issue type :
Technology : kbVCsearch kbAudDeveloper kbCRT

Last Reviewed: May 8, 2001
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