PRODUCT  :  Borland C++                           NUMBER  :  1008
  VERSION  :  3.0
       OS  :  DOS
     DATE  :  October 19, 1993                         PAGE  :  1/4

    TITLE  :  allocmem VS. malloc




     FUNCTION: Allocates DOS memory segment.

       SYNTAX: #include <dos.h>
               int allocmem(unsigned size, unsigned *segp);

       RETURN: allocmem returns -1 or success.  In the event of
               error, allocmem returns a number indicating the size
               in paragraphs of the largest available block.

        NOTES: This functions allocates a block of memory using
               DOS's memory allocation function, int 21h, function
               48h.  All allocated blocks are segment aligned.

  The reference guide states that malloc's and allocmem's cannot be
  used in harmony with one another.  While this is not necessarily
  true it is included to deter programmer's from stabbing
  themselves in the back due to a lack of knowlege about how the
  memory allocation routines actually work.

  When a Turbo C++ program runs, it always resides inside a single
  DOS memory block.  If more memory is needed to allocate a block
  from the far heap, the memory block is expanded to fit the need.

  ---------------  640K       --------------
  | DOS Free    |             | DOS Free   |
  | Memory      |             | Memory     |
  |             |             --------------
  |             |             | Far Heap   |  \
  ---------------             |            |   \
  | Far Heap    |             |            |    |
  | ............|             |............|    |- Program remains inside
  |   TC++      |             |  TC++      |    |  a single block which is
  |   Program   |             |  Program   |   /   resized to fit allocation.
  |             |             |            |  /
  ---------------             --------------
  |   DOS       |             |  DOS       |
  |             |             |            |
  ---------------             --------------
  Before farmalloc()          After farmalloc()

  If allocmem() is called, a separate block is allocated for the
  memory requested.  By default, this block will be the next higher
  block in memory above your program.  As a result, if you attempt













  PRODUCT  :  Borland C++                           NUMBER  :  1008
  VERSION  :  3.0
       OS  :  DOS
     DATE  :  October 19, 1993                         PAGE  :  2/4

    TITLE  :  allocmem VS. malloc




  to request more far heap memory using farmalloc at this point,
  the block your program resides in cannot be resized.  Due to this
  you will get a NULL back from the farmalloc call.

  ---------------  640K       --------------
  | DOS Free    |             | DOS Free   |
  | Memory      |             | Memory     |
  |             |             --------------
  |             |             | allocmem() |
  |             |             | block      |
  ---------------             -------------|
  | Far Heap    |             | Far Heap   |  \
  | ............|             |............|   |
  |   TC++      |             |  TC++      |   |- This block can no longer
  |   Program   |             |  Program   |   |  be expanded on farmalloc
  |             |             |            |  /   calls resulting in NULL
  ---------------             --------------      pointers.
  |   DOS       |             |  DOS       |
  |             |             |            |
  ---------------             --------------
  Before allocmem()           After allocmem()

  Note that anything performing a DOS allocation will have the same
  effect on your Turbo C++ program.  This includes the loading of
  TSR programs.

  We can now see that this will only be fatal to programs
  requesting memory from the far heap.  Therefore, allocations
  using malloc coming off of the near heap will not be affected.
  In addition we can see that if there were some way to reverse
  DOS's allocation scheme so that it would grab memory from the top
  down, that our farmalloc's could still work.

  It turns out that DOS interrupt 21h function 58h does just that.
  By issuing:

      _AX = 0x5801;
      _BX = 2;             /* selected last fit allocation */
      geninterrupt(0x21);

  DOS will then grab blocks in a top down fashion.  ALWAYS return
  DOS to a first fit selection following the call to allocmem by
  loading up bx with 0.













  PRODUCT  :  Borland C++                           NUMBER  :  1008
  VERSION  :  3.0
       OS  :  DOS
     DATE  :  October 19, 1993                         PAGE  :  3/4

    TITLE  :  allocmem VS. malloc




      _AX = 0x5801;
      _BX = 0;             /* select first fit allocation */
      geninterrupt(0x21);

  Follows are two program which illustrate the problem and the work
  around.

  Program #1 shows conflict in memory allocation schemes:

  #include <alloc.h>
  #include <dos.h>
  #include <stdio.h>

  main()
  {
      unsigned seg, count = 0;
      char far *ptr;

      allocmem( 100, &seg );  /* allocates next available block */
      printf(" Allocated block at %X:0000\n", seg);
      while ((ptr = farmalloc(50)) != NULL)
          count++;
      printf(" Allocated %u 50 byte pieces\n", count); /* can't allocate! */

      return 0;
  }

  Program #2 offers resolutions:
  #include <alloc.h>
  #include <dos.h>
  #include <stdio.h>

  main()
  {
      unsigned seg, count = 0;
      char far *ptr;

      _AX = 0x5801;
      _BX = 2;
      geninterrupt(0x21);    /* set allocation strategy to last fit */
      allocmem( 100, &seg ); /* grab highest block */
      _AX = 0x5801;
      _BX = 0;













  PRODUCT  :  Borland C++                           NUMBER  :  1008
  VERSION  :  3.0
       OS  :  DOS
     DATE  :  October 19, 1993                         PAGE  :  4/4

    TITLE  :  allocmem VS. malloc




      geninterrupt(0x21);    /* reset allocation strategy */
      printf(" Allocated block at %X:0000\n", seg);
      while ((ptr = farmalloc(50)) != NULL)
          count++;
      printf(" Allocated %u 50 byte pieces\n", count);

      return 0;
  }

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