Q47122: Example of Passing a Variable-Length String to Assembly

Article: Q47122
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 5-SEP-1990

The program shown below demonstrates how to pass a variable-length
string by far reference to an assembly language routine.

This information applies to QuickBASIC versions 4.00, 4.00b, and 4.50,
to Microsoft BASIC Compiler versions 6.00 and 6.00b for MS-DOS and MS
OS/2, and to near variable-length strings in Microsoft BASIC
Professional Development System (PDS) versions 7.00 and 7.10 for
MS-DOS and MS OS/2.

The following program is PSTRING.BAS, which passes a string to an
assembly language routine using the VARSEG and SADD functions. SADD
gives the actual address of the string, whereas VARPTR gives the
address of the string descriptor.

This example cannot be used in BASIC PDS 7.00 or 7.10 with far strings
(BC /Fs) or with QBX.EXE (which always uses far strings). For more
information about far strings, see Chapter 13 of "Microsoft BASIC 7.0:
Programmer's Guide" for versions 7.00 and 7.10.

DECLARE SUB PSTRING(BYVAL STRSEG AS INTEGER, BYVAL STROFF AS INTEGER)
A$ = "Hello World"
PRINT "Before call: ";
PRINT A$
CALL PSTRING(VARSEG(A$), SADD(A$))
PRINT "After call : ";
PRINT A$

The following separately compiled routine is PSTR.ASM:

; The following handy .MODEL MEDIUM,BASIC directive is found in MASM
; 5.10 but not in earlier versions:
.MODEL MEDIUM, BASIC
.CODE

pstring     PROC sseg:WORD, soff:WORD
            push bx                  ; save bx register, dx, and es
            push dx
            push es

            mov ax, sseg             ; get segment of string
            mov es, ax               ; put into segment register
            mov bx, soff
                                     ; 65 is ASCII value for letter 'A'.
            mov BYTE PTR es:[bx], 65 ; Move the 'A' to the first character
                                     ;  in the string.
            pop es
            pop dx
            pop bx                   ; restore (pop) es, dx, and bx
            ret
pstring     ENDP
            end

Compile and link the program as follows:

   BC PSTRING;
   MASM PSTR;

   LINK PSTRING PSTR;

When run, PSTRING should print the following:

   Before call: Hello World
   After call : Aello World

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