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"How to Pass Parameters Between Basic and Assembly" (Part 1/2)

This article was previously published under Q51501
This article has been archived. It is offered "as is" and will no longer be updated.
The article below gives Part 1 of 2 of a complete tutorial andexamples for passing all types of parameters between compiled Basicand Assembly Language.

The examples in BAS2MASM (but not the tutorial section) are alsoavailable in this database as multiple separate ENDUSER articles,which can be found as a group by querying on the word BAS2MASM.


This document explains how Microsoft Basic compiled programs can passparameters to and from Microsoft Macro Assembler (MASM) programs. Thisdocument assumes that you have a fundamental understanding of Basicand assembly language.

Microsoft Basic supports calls to routines written in Microsoft MacroAssembler, FORTRAN, Pascal, and C. This document describes thenecessary syntax for calling Microsoft assembly-language proceduresand contains a series of examples demonstrating the interlanguagecalling capabilities between Basic and assembly language. The sampleprograms apply to the following Microsoft products:

  1. Microsoft QuickBasic versions 4.00, 4.00b, and 4.50 for MS-DOS
  2. Microsoft Basic Compiler versions 6.00 and 6.00b for MS-DOS and MS OS/2
  3. Microsoft Basic Professional Development System (PDS) versions 7.00 and 7.10 for MS-DOS and MS OS/2
  4. Microsoft Macro Assembler (MASM) versions 5.00 and 5.10 for MS-DOS and MS OS/2
  5. Microsoft QuickAssembler versions 2.01 and 2.51 (which are integrated as part of Microsoft QuickC Compiler with QuickAssembler versions 2.01 and 2.51) for MS-DOS
Microsoft Basic can be linked with all versions of MASM orQuickAssembler. However, we recommend that you use the latest versionof MASM or QuickAssembler with the examples in this application note.

For more information about interlanguage calling, refer to the"Microsoft Mixed-Language Programming Guide," which is available withC 5.00 and 5.10 and MASM 5.00 and 5.10.
                     MAKING MIXED-LANGUAGE CALLS                     ===========================Mixed-language programming always involves a call; specifically, itinvolves a function or subprogram call. For example, a Basic mainmodule may need to execute a specific task that you would like toprogram separately. Instead of calling a Basic subprogram, however,you can call an assembly-language procedure.Mixed-language calls require multiple modules. Instead of compilingall of your source modules with the same compiler, you use differentcompilers. In the example mentioned above, you would compile the main-module source file with the Basic compiler, assemble another sourcefile (written in assembly language) with the assembler, and then linktogether the two object files.There are two types of routines that can be called. Their principaldifference is that some return values, and others do not. (Note: Inthis document, "routine" refers to any function or subprogramprocedure that can be called from another module.)   Note: Basic DEF FN functions and GOSUB subroutines cannot be called   from another language.   Basic has a much more complex environment and initialization   procedure than assembly language. Because of this, Basic must be   the initial environment that the program starts in, and from there,   assembly-language routines can be called (which can in turn call   Basic routines). This means that a program cannot start in assembly   language and then call Basic routines.               THE Basic INTERFACE TO ASSEMBLY LANGUAGE               ========================================The Basic DECLARE statement provides a flexible and convenientinterface to assembly language. When you call a routine, the DECLAREstatement syntax is as follows:   DECLARE FUNCTION <name> [ALIAS "aliasname"][CDECL][<parameter-   list>]The <name> is the name of the function or subprogram that you want tocall as it appears in the Basic source file. The following are therecommended steps for using the DECLARE statement when callingassembly language:1. For each distinct assembly-language routine you plan to call, put a   DECLARE statement in your Basic source file before the routine is   called.2. If you are calling a MASM routine with a name longer than 31   characters, use the ALIAS feature. The use of ALIAS is explained   below.3. Use the parameter list to determine how each parameter is to be   passed. The use of the parameter list is explained below.4. Once the routine is properly declared, call it just as you would a   Basic subprogram or function.NAMING-CONVENTION REQUIREMENTS==============================The term "naming convention" refers to the way that a compiler altersthe name of the routine before placing it into an object file.It is important that you adopt a compatible naming convention when youissue a mixed-language call. If the name of the called routine isstored differently in each object file, then the linker will not beable to find a match. Instead, it will report an unresolved external.Microsoft compilers place machine code into object files, but theyalso place into object files the names of all routines and commonblocks that need to be accessed publicly. (Note: Basic variables arenever public symbols.) That way, the linker can compare the name of aroutine called in one module to the name of a routine defined inanother module, and recognize a match.Basic and MASM use the same naming conventions. They both translateeach letter of public names to uppercase. Basic drops the typedeclaration character (%, &, !, #, $). Basic recognizes the first 40characters of a routine name, while MASM recognizes the first 31characters of a name.CALLING-CONVENTION REQUIREMENTS===============================The term "calling convention" refers to the way that a languageimplements a call. The choice of calling convention affects the actualmachine instructions that a compiler generates to execute (and returnfrom) a function, procedure, or subroutine call.The use of a calling convention affects programming in two ways:1. The calling routine uses a calling convention to determine in what   order to pass arguments (parameters) to another routine. The   convention can usually be specified in a mixed-language interface.2. The called routine uses a calling convention to determine in what   order to receive the parameters that were passed to it. In most   languages, this convention can be specified in the routine's   heading. Basic, however, always uses its own convention to receive   parameters.Basic's calling convention pushes parameters onto the stack in theorder in which they appear in the source code. For example, the Basicstatement CALL Calc(A, B) pushes argument A onto the stack before itpushes B. This convention also specifies that the stack is restored bythe called routine just before returning control to the caller. (Thestack is restored by removing parameters.)USING ALIAS===========The use of ALIAS may be necessary because assembly language places thefirst 31 characters of a name into an object file, whereas Basicplaces up to 40 characters of a name into an object file.  Note: You do not need the ALIAS feature to remove type  declaration characters (%, &, !, #, $). Basic automatically  removes these characters when it generates object code. Thus,  Fact% in Basic matches FACT in assembly language.The ALIAS keyword directs Basic to place aliasname into the objectfile, instead of <name>. The Basic source file still contains calls to<name>. However, these calls are interpreted as if they were actuallycalls to aliasname. This is used when a Basic name is longer then 31characters and must be called from assembly language, or the assemblylanguage routine name contains characters that are illegal in a Basicsubroutine name.For example:  DECLARE FUNCTION  QuadraticPolynomialFunctionLeastSquares%                    ALIAS "QUADRATI" (a, b, c)In the example above, QUADRATI, the aliasname, contains the firsteight characters of the name QuadraticPolynomialFunctionLeastSquares%.This causes Basic to place QUADRATI into the object file, therebymimicking MASM's behavior.USING THE PARAMETER LIST========================The <parameter-list> syntax is displayed below, followed byexplanations of each field:   [BYVAL | SEG] <variable> [AS <type>]...,  Note: You can use BYVAL or SEG, but not both.Use the BYVAL keyword to declare a value parameter. In each subsequentcall, the corresponding argument will be passed by value.   Note: Basic provides two ways of "passing by value." The usual   method of passing by value is to use an extra set of parentheses,   as in the following:      CALL HOLM((A))   This method actually creates a temporary value, whose address is   passed. In contrast, BYVAL provides a true method of passing by   value, because the value itself is passed, not an address. Only by   using BYVAL will a Basic program be compatible with an   assembly-language routine that expects a value parameter.Use the SEG keyword to declare a far reference parameter. In eachsubsequent call, the far (segmented) address of the correspondingargument will be passed.You can choose any legal name for <variable>, but only the typeassociated with the name has any significance to Basic. As with othervariables, the type can be indicated with a type declaration character(%, &, !, #, $) or the implicit declaration.You can use the "AS type" clause to override the type declaration of<variable>. The type field can be INTEGER, LONG, SINGLE, DOUBLE,STRING, a user-defined type, or ANY, which directs Basic to permit anytype of data to be passed as the argument.For example:   DECLARE FUNCTION Calc2! (BYVAL a%, BYVAL b%, BYVAL c!)In the example above, Calc2! is declared as an assembly-languageroutine that takes three arguments: the first two are integers passedby value, and the last is a single-precision real number passed byvalue.ALTERNATIVE Basic INTERFACES============================You can specify parameter-passing methods without using a DECLAREstatement or by using a DECLARE statement and omitting the parameterlist.1. You can make the call with the CALLS statement. The CALLS statement   causes each parameter to be passed by far reference.2. You can use the BYVAL and SEG keywords in the actual parameter list   when you make the call, as follows:      CALL Fun2(BYVAL Term1, BYVAL Term2, SEG Sum)In the example above, BYVAL and SEG have the same meaning that theyhave in a Basic DECLARE statement. When you use BYVAL and SEG thisway, however, you need to be careful because neither the type nor thenumber of parameters will be checked as they would be in a DECLAREstatement.              SETTING UP THE ASSEMBLY-LANGUAGE PROCEDURE              ==========================================The linker cannot combine the assembly-language procedure with thecalling program unless compatible segments are used and the procedureitself is declared properly. The following points may be helpful:1. If you have version 5.00 of the Macro Assembler, use the .MODEL   directive at the beginning of the source file; this directive   automatically causes the appropriate return to be generated (NEAR   for small or compact model, FAR otherwise). Modules called from   Basic should be declared as .MODEL MEDIUM. If you have a version of   the assembler earlier than 5.00, declare the procedure FAR.2. If you have version 5.00 or later of the Microsoft Macro Assembler   (MASM), use the simplified segment directives .CODE to declare the   code segment and .DATA to declare the data segment. (Having a code   segment is sufficient if you do not have data declarations.) If you   are using an earlier version of the assembler, the SEGMENT, GROUP,   and ASSUME directives must be used.3. The procedure label must be declared public with the PUBLIC   directive. This declaration makes the procedure available to be   called by other modules. Also, any data you want to make public to   other modules must be declared as PUBLIC.4. Global data or procedures accessed by the routine must be declared   EXTRN. The safest way to use EXTRN is to place the directive   outside any segment definition (however, near data must go inside   the data segment).PRESERVING REGISTERS====================There are several registers that need to be preserved in a mixed-language program. These registers are as follows:   CX, BX   BP, SI, DI, SP   CS, DS, SS, ESThe direction flag should also be preserved.ENTERING THE ASSEMBLY-LANGUAGE PROCEDURE========================================The following two instructions begin the procedure:   push   bp   mov    bp, spThis sequence establishes BP as the "framepointer." The framepointeris used to access parameters and local data, which are located on thestack. SP cannot be used for this purpose because it is not an indexor base register. Also, the value of SP may change as more data ispushed onto the stack. However, the value of the base register BP willremain constant throughout the procedure, so that each parameter canbe addressed as a fixed displacement off of BP.The instruction sequence above first saves the value of BP because itwill be needed by the calling procedure as soon as the currentprocedure terminates. Then BP is loaded with the value of SP tocapture the value of the pointer at the time of entry to theprocedure.ALLOCATING LOCAL DATA (OPTIONAL)================================An assembly-language procedure can use the same technique forimplementing local data that is used by high-level languages. To setup local data space, decrease the contents of SP in the thirdinstruction of the procedure. (To ensure correct execution, you shouldalways increase or decrease SP by an even amount.) Decreasing SPreserves space on the stack for the local data. The space must berestored at the end of the procedure, as shown below:   push   bp   mov    bp, sp   sub    sp, spaceIn the text above, space is the total size in bytes of the local data.Local variables are then accessed as fixed, negative displacements offof BP.For example:   push   bp   mov    bp, sp   sub    sp, 4      .      .      .   mov    WORD PTR [bp-2], 0   mov    WORD PTR [bp-4], 0The example above uses two local variables, each of which is 2 bytesin size. SP is decreased by 4, since there are 4 bytes of local data.Later, each of the variables is initialized to 0 (zero). Thesevariables are never formally declared with any assembler directive;the programmer must keep track of them manually.Local variables are also called dynamic, stack, or automaticvariables.EXITING THE PROCEDURE=====================Several steps may be involved in terminating the procedure:1. If any of the registers SS, DS, SI, etc., have been saved, these   must be popped off the stack in the reverse order that they were   saved.2. If local data space was allocated at the beginning of the   procedure, SP must be restored with the instruction MOV SP, BP.3. Restore BP with POP BP. This step is always necessary.4. Finally, if you are not using CDECL and the C calling conventions,   return to the calling program with the RET <n> instruction (where   <n> is the number of bytes to pop off the stack) to adjust the   stack with respect to the parameters that were pushed by the   caller.ASSEMBLY-LANGUAGE CALLS TO Basic================================No Basic routine can be executed unless the main program is in Basic,because a Basic routine requires the environment to be initialized ina way that is unique to Basic. MASM will not perform this specialinitialization.However, a program can start up in Basic, call an assembly-languagefunction that does most of the work of the program, and then callBasic subprograms and functions as needed.The following rules are recommended when you call Basic from assemblylanguage:1. Start up in a Basic main module. You must use the DECLARE statement   to provide an interface to the assembly-language module.2. In the assembly-language module, declare the Basic routine as   EXTRN.3. Make sure that all data is passed as a near pointer. Basic can pass   data in a variety of ways, but is unable to receive data in any   form other than near reference.   Note: With near pointers, the program assumes that the data is   in the default data segment. If you want to pass data that is   not in the default data segment, then first copy the data to a   variable that is in the default data segment.   Note: Microsoft Basic Professional Development System (PDS)   version 7.10 allows a Basic routine to be passed parameters by   value.THE MICROSOFT SEGMENT MODEL===========================If you use the simplified segment directives by themselves, you do notneed to know the names assigned for each segment. However, versions ofthe Macro Assembler earlier than 5.00 do not support these directives.With earlier versions of the assembler, you should use the SEGMENT,GROUP, ASSUME, and ENDS directives equivalent to the simplifiedsegment directives.The following table shows the default segment names created by the.MODEL MEDIUM directive used with Basic. Use of these segments ensurescompatibility with Microsoft languages and will help you access publicsymbols. This table is followed by a list of three steps, illustratinghow to make the actual declarations, and a sample program.   Directive   Name         Align     Combine   Class     Group   ---------   ----         -----     -------   -----     -----   .CODE       name_TEXT    WORD      PUBLIC    'CODE'   .DATA       _DATA        WORD      PUBLIC    'DATA'    DGROUP   .CONST      CONST        WORD      PUBLIC    'CONST'   DGROUP   .DATA?      _BSS         WORD      PUBLIC    'BSS'     DGROUP   .STACK      STACK        PARA      STACK     'STACK'   DGROUPThe directives in the table refer to the following kinds of segments:   Directive      Description of Segment   ---------      ----------------------   .CODE          The segment containing all the code for the module.   .DATA          Initialized data.   .DATA?         Uninitialized data. Microsoft compilers store                  uninitialized data separately because it can be more                  efficiently stored than initialized data. (Note:                  Basic does not use uninitialized data.)   .FARDATA and   .FARDATA?      Data placed here will not be combined with the                  corresponding segments in other modules. The segment                  of data placed here can always be determined,                  however, with the assembler SEG operator.   .CONST         Constant data. Microsoft compilers use this segment                  for such items as string and floating-point                  constants.   .STACK         Stack. Normally, this segment is declared in the                  main module for you and should not be redeclared.The following steps describe how to use this table to createdirectives:1. Refer to the table to look up the segment name, align type, combine   type, and class for your code and data segments. Use all of these   attributes when you define a segment. For example, the code segment   is declared as follows:      _TEXT    SEGMENT   WORD PUBLIC 'CODE'   The name _TEXT and all the attributes are taken from the table.2. If you have segments in DGROUP, put them into DGROUP with the GROUP   directive, as in the following:      GROUP    DGROUP    _DATA     _BSS3. Use ASSUME and ENDS as you would normally. Upon entering routines   called directly from Basic, DS and SS will both point to DGROUP.The following example shows an assembly-language program without thesimplified segment directives from version 5.00 of the Microsoft MacroAssembler:  test_TEXT  SEGMENT WORD PUBLIC 'CODE'             ASSUME   cs:test_TEXT             PUBLIC Power2  Power2     PROC             push bp             mov bp, sp             mov ax, [bp+6]             mov cx, [bp+8]             shl ax, cl             pop bp             ret 4  Power2     ENDP  test_TEXT  ENDS             END                        COMPILING AND LINKING                        =====================After you have written your source files and resolved the issuesraised in the above sections, you are ready to compile individualmodules and then link them together.Before linking, each program module must be compiled or assembled withthe appropriate compiler or assembler.                         ACCESSING PARAMETERS                         ====================PARAMETER-PASSING REQUIREMENTS==============================Microsoft compilers support three methods for passing a parameter:   Method         Description   ------         -----------   Near reference Passes a variable's near (offset) address. This                  method gives the called routine direct access to the                  variable itself. Any change the routine makes to the                  parameter will be reflected in the calling routine.   Far reference  Passes a variable's far (segmented) address. This                  method is similar to passing by near reference,                  except that a longer address is passed.   By value       Passes only the variable's value, not address. With                  this method, the called routine knows the value of                  the parameter, but has no access to the original                  variable. Changes to the value parameter have no                  effect on the value of the parameter in the calling                  routine, once the routine terminates.Because there are different parameter-passing methods, please note thefollowing:1. Make sure that the called routine and the calling routine use the   same method for passing each parameter (argument). In most cases,   you will need to check the parameter-passing defaults used by each   language, and possibly make adjustments. Each language has keywords   or language features that allow you to change the parameter-passing   method.2. You may want to use a particular parameter-passing method rather   then using the default for the language.Basic ARGUMENTS===============The default for Basic is to pass all arguments by near reference. Thiscan be overridden by using the SEG directive or CALLS instead of CALL.Both of these methods cause Basic to pass both the segment and offset.These methods can be used only to call a non-Basic routine becauseBasic receives all parameters by near reference.   Note: Although Basic can pass parameters to other languages by far   reference by using the SEG directive or CALLS, Basic routines can   be CALLed only from other languages when parameters are passed by   near reference. You cannot DECLARE or CALL a Basic routine with   parameters that have SEG or BYVAL attributes. SEG and BYVAL are   only used for parameters of non- Basic routines.   Note: Basic PDS version 7.10 allows a Basic routine to be passed   parameters by value.Basic STACK FRAME=================The following diagram illustrates the Basic stack frame as it appearsupon entry to the assembly-language routine:          +--------------------+        A |   Arg 1 address    | <-- BP + 8          |--------------------|        B |   Arg 2 address    | <-- BP + 6          |--------------------|          |   Return address   |     BP + 4          |      (4 bytes)     |     BP + 2          |--------------------|          |      Saved BP      | <-- BP          +--------------------+           Low AddressesASSEMBLY-LANGUAGE ARGUMENTS===========================Once you have established the procedure's framepointer, allocatedlocal data space (if desired), and pushed any registers that need tobe preserved, you can write the main body of the procedure. To writeinstructions that can access parameters, consider the general pictureof the stack frame after a procedure call, as illustrated in thefollowing figure:          High Addresses                    +------------------+                    |    Parameter     |                    |------------------|                    |    Parameter     |                    |------------------|                    |        .         |                    |        .         |                    |        .         |       Stack grows  |------------------|     Parameters above       downward with|    Parameter     |     this generated       each push or |------------------|     automatically       by call      |  Return Address  | <-- the compiler.                    |------------------|                    |     Saved BP     | <-- Framepointer (BP)                    |------------------|     points here.                    | Local Data Space |     These parameters                    |------------------|     would be generated                    |     Saved SI     |     by your assembly-                    |------------------|     language code.                    |     Saved DI     | <-- SP points to last                    +------------------+     item placed on                                             stack.          Low AddressesThe stack frame for the procedure is established by the followingsequence of events:1. The calling program pushes each of the parameters on the stack,   after which SP points to the last parameter pushed.2. The calling program issues a CALL instruction, which causes the   return address (the place in the calling program to which control   will ultimately return) to be placed on the stack.  This address   may be either 2 bytes long (for near calls) or 4 bytes long (for   far calls).  SP now points to this address.  (Note: When dealing   with Basic, the return address will always be a far address [4   bytes].)3. The first instruction of the called procedure saves the old value   of BP, with the instruction push bp.  SP now points to the saved   copy of BP.4. BP is used to capture the current value of SP, with the instruction   MOV BP, SP.  Therefore, BP now points to the old value of BP.5. Whereas BP remains constant throughout the procedure, SP may be   decreased to provide room on the stack, for local data or saved   registers.In general, the displacement (off of BP) for a parameter X is equal tothe following:   2  + size of return address      + total size of parameters between X and BPFor example, consider a FAR procedure (all Basic procedures are FAR)that has received one parameter, a 2-byte address. The displacement ofthe parameter would be as follows:   Argument's displacement     = 2 + size of return address                               = 2 + 4                               = 6The argument can thus be loaded into BX with the followinginstruction:   mov bx, [bp+6]Once you determine the displacement of each parameter, you may want touse string equates or structures so that the parameters can bereferenced with a single identifier name in your assembly-languagesource code. For example, the parameter above at bp+6 can beconveniently accessed if you put the following statement at thebeginning of the assembly-language source file:   Arg1   EQU  [bp+6]You could then refer to this parameter as Arg1 in any instruction. Useof this feature is optional.PASSING Basic ARGUMENTS BY VALUE================================An argument is passed by value when the called routine is firstdeclared with a DECLARE statement, and the BYVAL keyword is applied tothe argument. For example:   DECLARE SUB AssemProc (BYVAL a AS INTEGER)PASSING Basic ARGUMENTS BY NEAR REFERENCE=========================================The Basic default is to pass by near reference. Use of SEG, BYVAL, orCALLS changes this default.PASSING Basic ARGUMENTS BY FAR REFERENCE========================================Basic passes each argument in a call by far reference when CALLS isused to invoke a routine. Using SEG to modify a parameter in apreceding DECLARE statement also causes a Basic CALL to passparameters by far reference.   Note: CALLS cannot be used to call a routine that is named in a   DECLARE statement. For this reason, the use of the SEG directive is   the preferred method of passing variables by far reference.                              DATA TYPES                              ==========NUMERICAL FORMATS=================Numerical data formats are the simplest kinds of data to pass betweenassembly language and Basic. The following chart shows the equivalentdata types in each language:   Basic        Assembly Language   -----        -----------------   x%, INTEGER  DW   ...          DB, DF, DT    <-- These are not available in Basic.   x&, LONG     DD   x!, SINGLE   DD   x#, DOUBLE   DQUSER-DEFINED TYPES==================The elements in a user-defined type are stored contiguously in memory,one after the other. When a Basic user-defined type appears in anargument list, Basic passes the address of the beginning element ofthe user-defined type.The routine that receives the user-defined type must know the formatof the type beforehand. The assembly-language routine should thenexpect to receive a pointer to a structure of this type.Basic STRING FORMATS====================Near Variable-Length Strings----------------------------Variable-length strings in Basic have 4-byte string descriptors:        +-------------------------------------+        |      Length      | Address (offset) |        +-------------------------------------+                   (2 bytes)          (2 bytes)The first field of the string descriptor contains a 2-byte integerindicating the length of the actual string text. The second fieldcontains the address of the text. This address is an offset into thedefault data area (DGROUP) and is assigned by Basic's string-spacemanagement routines. These management routines need to be available toreassign this address whenever the length of the string changes, yetthe routines are available only to Basic. Therefore, an assembly-language routine should not alter the length or address of a Basicvariable-length string.   Note: Fixed-length strings do not have a string descriptor.Passing Variable-Length Strings from Basic------------------------------------------When a Basic variable-length string (such as A$) appears in anargument list, Basic passes a string descriptor rather than the stringdata itself.   Warning: When you pass a string from Basic to assembly language,   the called routine should under no circumstances alter the length   or address of the string.The routine that receives the string must be aware that if any Basicroutine is called, Basic's string-space management routines may changethe location of the string data without warning. In this case, thecalling routine must note that the values in the string descriptor maychange.The Basic functions SADD and LEN extract parts of the stringdescriptor. SADD extracts the address of the actual string data, andLEN extracts the length. The results of these functions can then bepassed to an assembly-language routine.Basic should pass the result of the SADD function by value. Bear inmind that the string's address, not the string itself, will be passedby value. This amounts to passing the string itself by reference. TheBasic module passes the string address, and the other module receivesthe string address. The address returned by SADD is declared as typeINTEGER, but is actually equivalent to a near pointer.There are two methods for passing a variable-length string from Basicto assembly language. The first method is to pass the string addressand string length as separate arguments, using the SADD and LENfunctions. The second method is to pass the string descriptor itself,with a call statement such as the following:   CALL CRoutine(A$)The assembly-language routine should then expect to receive a pointerto a string descriptor of this type.Passing Near String Descriptors from Assembly Language------------------------------------------------------To pass an assembly-language string to Basic, first allocate a stringin assembly language. Then create a structure identical to a Basicstring descriptor. Pass this structure by near reference. Make surethat the string originates in assembly language, not in Basic.Otherwise, Basic may attempt to move the string around in memory.   Warning: Microsoft does not recommend creating your own string   descriptors in assembler functions because it is very easy to   inadvertently destroy portions of the data segment. The Basic   routine should not reassign the value or length of a string passed   from assembly language.The preferred method is to create the strings in Basic and then modifytheir contents in the assembler function without altering their stringdescriptors.Far Variable-Length Strings---------------------------Microsoft Basic Professional Development System (PDS) versions 7.00and 7.10 allow for the use of far strings. Information on using farstrings with other languages is covered in the "Microsoft Basic 7.0:Programmer's Guide," in Chapter 13, "Mixed-Language Programming withFar-Strings."Fixed-Length Strings--------------------Fixed-length strings in Basic are stored simply as contiguous bytes ofcharacters, with no terminating character. There is no stringdescriptor for a fixed-length string.To pass a fixed-length string to a routine, the string must be putinto a user-defined type. For example:   TYPE FixType       A AS STRING * 10   END TYPEThe string is then passed like any other user-defined type.ARRAYS======There are several special problems that you need to be aware of whenpassing arrays between Basic and assembly language:1. Arrays are implemented differently in Basic than in other   languages, so you must take special precautions when passing an   array from Basic to assembly language.2. Arrays are declared differently in assembly language and Basic.3. Because Basic uses an array descriptor, passed arrays must be   created in Basic.Passing Arrays from Basic-------------------------To pass an array to an assembly-language routine, pass only the baseelement, and the other elements will be contiguous from there.Passed Arrays Must Be Created in Basic--------------------------------------Basic keeps track of all arrays in a special structure called an arraydescriptor. The array descriptor is unique to Basic and is notavailable in any other language. Because of this, to pass an arrayfrom assembly language to Basic, the array must first be created inBasic, then passed to the assembly-language routine. The assembly-language routine may then alter the values in the array, but it cannotchange the length of the array.The array descriptor is similar in some respects to a stringdescriptor. The array descriptor is necessary because Basic may shiftthe location of array data in memory. Therefore, you can safely passarrays from Basic only if you follow three rules:1. Pass the array's address by applying the VARPTR function to the   first element of the array and passing the result by value. To pass   the far address of the array, apply both the VARPTR and VARSEG   functions and pass each result by value. The assembler gets the   address of the first element and considers it the address of the   entire array.2. The routine that receives the array must not, under any   circumstances, make a call back to Basic. If it does, then the   location of the array may change, and the address that was passed   to the routine will become meaningless.3. Basic can pass any member of an array by value. With this method,   the above precautions do not apply.Array Ordering--------------There are two types of ordering: row-major and column-major.Basic uses column-major ordering, in which the leftmost dimensionchanges fastest. When you use Basic with the BC command line, you canselect the /R compile option, which specifies that row-major order isto be used, rather than column-major order.COMMON BLOCKS=============You can pass individual members of a Basic COMMON block in an argumentlist, just as you can any data. However, you can also give anassembly-language routine access to the entire COMMON block at once.Assembly language can reference the items of a COMMON block by firstdeclaring a structure with fields that correspond to the COMMON blockvariables. Having defined a structure with the appropriate fields, theassembly-language routine must then get the address of the COMMONblock.To pass the address of the COMMON block, pass the address of the firstvariable in the block. The assembly-language routine should expect toreceive a structure by reference.For named COMMON blocks, there is an alternative method. In theassembly-language program, a segment is set up with the same name asthe COMMON block and then grouped with DGROUP, as follows:   BNAME SEGMENT COMMON 'BC_VARS'      x dw 1 dup (?)      y dw 1 dup (?)      z dw 1 dup (?)   BNAME ENDS   DGROUP GROUP BNAMEThe above assembler code matches with the following Basic code using anamed COMMON block:   DEFINT A-Z   COMMON /BNAME/ x,y,zPassing arrays through the COMMON block is done in a similar fashion.However, only static arrays can be passed to assembler through COMMON.   Note: Microsoft does not support passing dynamic arrays through   COMMON to assembler (since this depends upon a Microsoft   proprietary dynamic array descriptor format that changes from   version to version). Dynamic arrays can be passed to assembler only   as parameters in a CALL statement.When static arrays are used, the entire array is stored in the COMMONblock.Note that variables in COMMON following STRING*n variables, where n isodd, are aligned on the next even word boundary. Thus, you must definean extra dummy byte using db 1 in the assembler code followingSTRING*n variables (where n is odd). A dummy byte is not necessaryafter STRING*n variables when n is even.        HOW TO RETURN VALUES FROM ASSEMBLY-LANGUAGE FUNCTIONS        =====================================================Assembler "functions" are not called with the CALL statement; they areinvoked on the right-hand side of an equal sign (=) in compiled Basic.When calling an assembly-language function from Basic, either thepassed variable or a pointer to the passed variable is returned in theAX register, as shown in the following chart:   Data Type      How Value Is Returned   ---------      ---------------------   INTEGER        The value is placed in AX.   LONG           The high-order portion is placed in DX. The low-order                  portion is placed in AX.   SINGLE         The value is placed in the location provided by                  Basic. The segment is DS. Basic will push an extra                  parameter on the stack, after all the other                  parameters, that contains the offset of the memory                  location to share the return value. The offset                  located in BP+6 should be placed in AX before the                  function exits.   DOUBLE         The value is placed in the location provided by                  Basic. The segment is DS. Basic will push an extra                  parameter on the stack, after all the other                  parameters, that contains the offset of the memory                  location to share the return value. The offset should                  be placed in AX before the function exits.   VARIABLE-   LENGTH STRING  Pointer to a descriptor (offset in AX).   Note: Basic does not allow functions with a fixed-length-string   type or a user-defined type.                  DEBUGGING MIXED-LANGUAGE PROGRAMS                  =================================Microsoft CodeView is very useful when trying to debug mixed-languageprograms. With CodeView you can trace through the source code of bothassembly language and Basic and watch variables in both languages.To compile programs for use with CodeView, use the /Zi switch on thecompile line for both the assembler and the Basic compiler. Then whenlinking, use the /CO switch.CodeView is a multilanguage source code debugger supplied withMicrosoft Basic Compiler versions 6.00 and 6.00b; Microsoft BasicProfessional Development System (PDS) versions 7.00 and 7.10;Microsoft C Optimizing Compiler versions 5.00 and 5.10; MicrosoftMacro Assembler versions 5.00 and 5.10; and Microsoft FORTRAN Compilerversions 4.00 and 5.00.              COMPILING AND LINKING THE SAMPLE PROGRAMS              =========================================The following is a series of examples, demonstrating the interlanguagecalling capabilities between Basic and assembler.When compiling the sample Basic programs, use the following compileline:   BC /O Basicprogramname;When compiling the sample MASM programs, use the following compileline for MASM 5.00 or 5.10:   MASM Assemprogramname;Or, use the following compile line for QuickAssembler 2.01:   QCL Assemprogramname;To link the programs together, use the following LINK line:   LINK Basicprogramname Assemprogramname;   Note: All the examples using variable-length strings assume the use   of near variable-length strings. These examples will not work in   the QuickBasic Extended (QBX.EXE) environment, or when compiling   with the BC/FS directive, in Microsoft Basic Professional   Development System (PDS) versions 7.00 and 7.10.                   APPENDIX A: MISCELLANEOUS TOPICS                   ================================Basic SUPPORTS MASM 5.10 UPDATE .MODEL AND PROC EXTENSIONS==========================================================Microsoft Macro Assembler (MASM) version 5.10 includes several newfeatures (not found in MASM version 5.00 or earlier) that simplifyassembly-language routines linked with high-level-language programs.Two of these features are as follows:1. An extension to the .MODEL directive that automatically sets up   naming, calling, and return conventions for a given high-level   language. For example:      .MODEL MEDIUM,Basic2. A modification of the PROC directive that handles most of the   procedure entry automatically. The PROC directive saves specified   registers, defines text macros for passed arguments, and generates   stack setup code on entry and stack tear-down code on exit.Section 5 of the "Microsoft Macro Assembler Version 5.1 Update" manualdiscusses the new features.PROBLEM CALLING ASSEMBLER ROUTINE WITH LABEL ON END DIRECTIVE=============================================================A QuickBasic .EXE program will hang at run time if it is LINKed to anassembly-language routine that uses a label on the END directive. Thesame programs execute successfully when run inside the QB.EXE editorwith the assembly-language routine in a Quick library.Although versions of QuickBasic prior to version 4.00 allow a label onthe END directive in a LINKed assembly-language program, programs forversions 4.00 and 4.00b require you to have no label on the assembly-language END directive.When the linker creates an executable program, it successivelyexamines each .OBJ file and determines whether that file has aspecified entry point. The first .OBJ file that specifies an entrypoint is assumed by the linker to be the main program, and programexecution begins there.In the assembly-language routines, the purpose of a label with an ENDdirective is to indicate to the linker the program's starting addressor entry point (where program execution is to start). Therefore, if noentry point is found in the QuickBasic routine, program execution willbegin in the assembly-language routines (in effect, the Basic code istotally bypassed).In previous versions of the QuickBasic compiler, the QuickBasic objectcode contains an entry-point specifier. Therefore, by simply listingQuickBasic object files before the assembly-language object files onthe LINK command line, the linker recognizes that the QuickBasicprogram is the main program.However, in QuickBasic version 4.00, the entry-point information is nolonger in the object file; instead, it resides in the run-time module(for example, BCOM40.LIB or BRUN40.LIB). Because these files areLINKed after the Basic and assembly-language .OBJ files, if theassembly-language routine specifies an entry point, the linker willincorrectly assume that program execution is to begin in the assembly-language routine.Results of testing with previous versions of QuickBasic indicate thatthe programs run successfully both inside the editor and as .EXE fileswhen compiled with versions 2.00, 2.01, and 3.00 of the QuickBasiccompiler.There are two workarounds to correct this problem in version 4.00:1. Remove the label on the END directive (that is, remove the entry-   point specification in your assembly-language routine) and   reassemble.2. The assembler .OBJ module can be used successfully without removing   the label from the END directive. If the assembly-language routine   cannot be changed, place the assembly-language routine into a .LIB   file.ASSEMBLER ROUTINES MUST NOT ASSUME ES EQUALS DS===============================================If CALLed assembler routines do string manipulation and use the ESregister, then the results inside the QB.EXE editor may differ fromthe executable .EXE program if the assembler routines assume the ESand DS registers are equal.The ES and DS registers should not be assumed to be equal inQuickBasic versions 4.00 and later.Generally, the ES and DS registers are equal for the executableprogram; however, this is not always a valid assumption. The assemblerroutines must explicitly set ES equal to DS, as shown in the codeexample below.The following assembler code sets ES equal to DS:      push bp      mov bp, sp      push es         ;These three      push ds         ;lines set the      pop es          ;es register equal to ds        .        .             ;body of program        .      pop es          ;at end of program need to      pop bp          ;restore saved registers      retQUICK LIBRARY WITH 0 (ZERO) BYTES IN FIRST CODE SEGMENT=======================================================A Quick library containing leading zeros in the first CODE segment isinvalid, causing the message "Error in loading file <name> - Invalidformat" when you try to load it in QuickBasic. For example, this errorcan occur if an assembly-language routine puts data that isinitialized to 0 (zero) in the first CODE segment, and it issubsequently listed first on the LINK command line when you make aQuick library.  If you have this problem, do either of the following:1. Link with a Basic module first on the LINK command line.-or-2. In whatever module comes first on the LINK command line, make sure   that the first code segment starts with a nonzero byte.				
This article is continued in the following article in the MicrosoftKnowledge Base:

ARTICLE-ID: Q71275TITLE : "How to Pass Parameters Between Basic and Assembly" (Part 2/2)

Article ID: 51501 - Last Review: 01/11/2015 00:48:46 - Revision: 2.1

Microsoft QuickBasic 4.0, Microsoft QuickBASIC 4.0b, Microsoft QuickBasic 4.5 for MS-DOS, Microsoft BASIC Compiler 6.0, Microsoft BASIC Compiler 6.0b, Microsoft BASIC Professional Development System 7.0, Microsoft BASIC Professional Development System 7.1, Microsoft Macro Assembler 5.0, Microsoft Macro Assembler 5.1 Standard Edition

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