Q49399: Example Allocating Memory in MASM Released by BASIC’s SETMEM

Article: Q49399
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom H_MASM S_QuickASM | mspl13_basic
Last Modified: 5-SEP-1990

The two programs shown below demonstrate how a BASIC program can use
SETMEM to free memory for an assembly routine to get dynamic memory.

This information about interlanguage calling applies to QuickBASIC
versions 4.00, 4.00b, and 4.50 for MS-DOS, to Microsoft BASIC Compiler
versions 6.00 and 6.00b for MS-DOS and MS OS/2, and to Microsoft BASIC
Professional Development System (PDS) versions 7.00 and 7.10 for
MS-DOS and MS OS/2.

For more information about interlanguage calling between BASIC and
MASM, search in the Software/Data Library or the Microsoft Knowledge
Base for the following word:

   BAS2MASM

Code Example
------------

The following BASIC program is BMEMSET.BAS, which uses SETMEM to free
a block of memory for an assembly routine that uses a DOS interrupt to
get dynamic memory:

   DECLARE SUB AMem(BYVAL AllocSize AS INTEGER)
   CLS
   ' Decrease the size of the far heap so AMem can use a DOS
   ' interrupt to get dynamic memory
   BeforeCall% = SETMEM(-2048)
   CALL AMem(1024%)
   ' Return the memory to the far heap; use a larger value so
   ' all space goes back into the heap.
   AfterCall% = SETMEM(3500)
   LOCATE 2, 1
   IF AfterCall% <= BeforeCall% THEN
      PRINT "Memory not reallocated"
   ELSE
      PRINT "Memory was successfully reallocated"
   END IF
   END

The following program is AMEMSET.ASM, which allocates and deallocates
a block of memory freed by BASIC:

; The following handy .MODEL MEDIUM,BASIC directive is found in
; MASM 5.10 but not in earlier versions:
.MODEL MEDIUM, BASIC
.DATA
        Fail    DB 'Failed to allocate memory$'
        Success DB 'Successfully allocated memory$'
.CODE
        PUBLIC AMem
AMem    PROC
        push bp
        mov bp, sp                ; set stack frame
        push cx
        push es

        mov ax, [bp+6]            ; get number of bytes freed
        mov cl, 4                 ; divide by 16 to get number of
        shr ax, cl                ;  paragraphs of memory
        mov bx, ax
        mov ah, 48h               ; DOS interrupt to allocate block
        int 21h                   ;  of memory
        mov es, ax
        mov ah, 9
        jnc NoFail                ; carry flag clear if successful
        mov dx, OFFSET Fail       ; display failed message
        int 21h
        jmp Exit                  ; go back to BASIC

NoFail: mov dx, OFFSET Success    ; display success message
        int 21h
        mov ah, 49h
        int 21h

Exit:   pop es
        pop cx
        pop bp
        ret 2
AMem    ENDP
        END

To demonstrate these programs from an .EXE program, compile and link
as follows:

   BC BMEMSET.BAS;
   MASM AMEMSET.ASM;
   LINK BMEMSET AMEMSET;

BMEMSET.EXE produces the following output:

   Successfully allocated memory
   Memory was successfully reallocated

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