The Assembly Process

Creating and running an executable file involves four steps:
 

Once you have written your assembly-language program, MASM provides several options for assembling it. The OPTION directive has several different arguments that let you control the way MASM assembles your programs.

Conditional assembly allows you to create one source file that can generate a variety of programs, depending on the status of various conditional-assembly statements.

Generating and Running Executable Programs

This section briefly lists all the actions that take place during each of the assembly steps. You can change the behavior of some of these actions in various ways, such as using macros instead of procedures, or using the OPTION directive or conditional assembly. The other chapters in this book include specific programming methods; this section simply gives you an overview.

Assembling

The ML.EXE program does two things to create an executable program. First, it assembles the source code into an intermediate object file. Second, it calls the linker, LINK.EXE, which links the object files and libraries into an executable program.

At assembly time, the assembler:

For information about conditional assembly, see "Conditional Directives" in this chapter; for macros, see Chapter 9. Further details about segments and offsets are included in Chapters 2 and 3. Assembly listings are explained in Appendix C.

Linking

Once your source code is assembled, the resulting object file is passed to the linker. At this point, the linker may combine several object files into an executable program. The linker:

Classes and groups are defined in "Defining Segment Groups" in Chapter 2. Segments and offsets are explained in Chapter 3, "Using Addresses and Pointers."

Loading

After loading the executable file into memory, the operating system:

For information about segment registers, the instruction pointer (IP), and the stack pointer (SP), see "Registers" earlier in this chapter. For more information on the PSP see Help or an MS-DOS reference.

Running

To run your program, MS-DOS jumps to the program’s first instruction. Some program operations, such as resolving indirect memory operands, cannot be handled until the program runs. For a description of indirect references, see "Indirect Operands" in Chapter 7.

Using the OPTION Directive

The OPTION directive lets you modify global aspects of the assembly process. With OPTION, you can change command-line options and default arguments. These changes affect only statements that follow the OPTION keyword.

For example, you may have MASM code in which the first character of a variable, macro, structure, or field name is a dot (.). Since a leading dot causes MASM 6.1 to generate an error, you can use this statement in your program:

OPTION DOTNAME

This enables the use of the dot for the first character.

Changes made with OPTION override any corresponding command-line option. For example, suppose you compile a module with this command line (which enables M510 compatibility):

ML /Zm TEST.ASM

The assembler disables M510 compatibility options for all code following this statement:

OPTION NOM510

The following lists explain each of the arguments for the OPTION directive. Where appropriate, an underline identifies the default argument. If you wish to place more than one OPTION statement on a line, separate them by commas.

Conditional Directives

MASM 6.1 provides conditional-assembly directives and conditional-error directives. Conditional-assembly directives let you test for a specified condition and assemble a block of statements if the condition is true. Conditional-error directives allow you to test for a specified condition and generate an assembly error if the condition is true.

Both kinds of conditional directives test assembly-time conditions, not run-time conditions. You can test only expressions that evaluate to constants during assembly. For a list of the predefined symbols often used in conditional assembly, see "Predefined Symbols," earlier in this chapter.

Conditional-Assembly Directives

The IF and ENDIF directives enclose the conditional statements. The optional ELSEIF and ELSE blocks follow the IF directive. There are many forms of the IF and ELSE directives. Help provides a complete list.

The following statements show the syntax for the IF directives. The syntax for other condition-assembly directives follow the same form.

IF expression1
ifstatements
[[ELSEIF expression2
elseifstatements]]
[[ELSE
elsestatements]]
ENDIF

The statements within an IF block can be any valid instructions, including other conditional blocks, which in turn can contain any number of ELSEIF blocks. ENDIF ends the block.

MASM assembles the statements following the IF directive only if the corresponding condition is true. If the condition is not true and the block contains an ELSEIF directive, the assembler checks to see if the corresponding condition is true. If so, it assembles the statements following the ELSEIF directive. If no IF or ELSEIF conditions are satisfied, the assembler processes only the statements following the ELSE directive.

For example, you may want to assemble a line of code only if your program defines a particular variable. In this example,

IFDEF buffer
buff BYTE buffer DUP(?)
ENDIF

the assembler allocates buff only if buffer has been previously defined.

MASM 6.1 provides the directives IF1, IF2, ELSEIF1, and ELSIF2 to grant assembly only on pass one or pass two. To use these directives, you must either enable 5.1 compatibility (with the /Zm command-line switch or OPTION M510) or set OPTION SETIF2:TRUE, as described in the previous section.

The following list summarizes the conditional-assembly directives:

The Directive Grants Assembly If

IF expression expression is true (nonzero)

IFE expressionexpression is false (zero)

IFDEF name name has been previously defined

IFNDEF namename has not been previously defined

IFB argument* argument is blank

IFNB argument*argument is not blank

IFIDN[I] arg1, arg2* arg1 equals arg2

IFDIF[I] arg1, arg2* arg1 does not equal arg2 The optional I suffix (IFIDNI and IFDIFI) makes comparisons insensitive to differences in case.

* Used only in macros.

Conditional-Error Directives

You can use conditional-error directives to debug programs and check for assembly-time errors. By inserting a conditional-error directive at a key point in your code, you can test assembly-time conditions at that point. You can also use conditional-error directives to test for boundary conditions in macros.

Like other severe errors, those generated by conditional-error directives cause the assembler to return a nonzero exit code. If MASM encounters a severe error during assembly, it does not generate the object module.

For example, the .ERRNDEF directive produces an error if the program has not defined a given label. In the following example, .ERRNDEF makes sure a label called publevel actually exists.

.ERRNDEF publevel
IF publevel LE 2
PUBLIC var1, var2
ELSE
PUBLIC var1, var2, var3
ENDIF

The conditional-error directives use the syntax given in the previous section. The following list summarizes the conditional-error directives. Note their close correspondence with the previous list of conditional-assembly directives.

The Directive Generates an Error

.ERR Unconditionally where it occurs in the source file. Usually placed within a conditional-assembly block.

.ERRE expression If expression is false (zero).

.ERRNZ expressionIf expression is true (nonzero).

.ERRDEF name If name has been defined.

.ERRNDEF nameIf name has not been defined.

.ERRB argument* If argument is blank.

.ERRNB argument*If argument is not blank.

.ERRIDN[I] arg1, arg2* If arg1 equals arg2.

.ERRDIF[I] arg1, arg2* If arg1 does not equal arg2. The optional I suffix (.ERRIDNI and .ERRDIFI) makes comparisons insensitive to case.

* Used only in macros

Two special conditional-error directives, .ERR1 and .ERR2, generate an error only on pass one or pass two. To use these directives, you must either enable 5.1 compatibility (with the /Zm command-line switch or OPTION M510) or set OPTION SETIF2:TRUE, as described in the previous section.

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