CHAPTER EIGHT: MASM: DIRECTIVES & PSEUDO-OPCODES (Part 9)

The Art of ASSEMBLY LANGUAGE PROGRAMMING

Chapter Eight (Part 8)	Table of Content	Chapter Eight (Part 10)

Another macro format (at least by Microsoft's definition) is the repeat macro. A repeat macro is nothing more than a loop that repeats the statements within the loop some specified number of times. There are three types of repeat macros provided by MASM:

repeat/rept
for/irp

and forc/irpc. The repeat/rept macro uses the following syntax:

Expression must be a numeric expression that evaluates to an unsigned constant. The repeat directive duplicates all the statements between repeat and endm that many times. The following code generates a table of 26 bytes containing the 26 uppercase characters:

The symbol ASCIICode is assigned the ASCII code for "A". The loop repeats 26 times each time emitting a byte with the value of ASCIICode. Also the loop increments the ASCIICode symbol on each repetition so that it contains the ASCII code of the next character in the ASCII table. This effectively generates the following statements:

Note that the repeat loop executes at assembly time not at run time. Repeat is not a mechanism for creating loops within your program; use it for replicating sections of code within your program. If you want to create a loop that executes some number of times within your program use the loop instruction. Although the following two code sequences produce the same result they are not the same:

The first code sequence above emits four machine instructions to the object code file. At assembly time the 80x86 CPU executes the statements between AddLp and the loop instruction ten times under the control of the loop instruction. The second code sequence above emits 20 instructions to the object code file. At run time the 80x86 CPU simply executes these 20 instructions sequentially with no control transfer. The second form will be faster since the 80x86 does not have to execute the loop instruction every third instruction. On the other hand the second version is also much larger because it replicates the body of the loop ten times in the object code file.

Unlike standard macros you do not define and invoke

repeat

macros separately. MASM emits the code between the repeat and endm directives upon encountering the repeat directive. There isn't a separate invocation phase. If you want to create a repeat macro that can be invoked throughout your program consider the following:

By placing the repeat macro inside a standard macro you can invoke the repeat macro anywhere in your program by invoking the REPTMacro macro. Note that you need two endm directives one to terminate the repeat macro one to terminate the standard macro.

Rept is a synonym for repeat.

Repeat

is the newer form MASM supports Rept for compatibility with older source files. You should always use the repeat form.

Another form of the repeat macro is the for macro. This macro takes the following form:

The angle brackets are required around the items in the operand field of the for directive. The braces surround optional items the braces should not appear in the operand field.

The for directive replicates the instructions between for and endm once for each item appearing in the operand field. Furthermore for each iteration the first symbol in the operand field is assigned the value of the successive items from the second parameter. Consider the following loop:

This loop emits six bytes containing the values zero one two ... five. It is absolutely identical to the sequence of instructions:

Remember the for loop like the repeat loop executes at assembly time not at run time.

For's second operand need not be a literal text constant; you can supply a macro parameter macro function result or a text equate for this value. Keep in mind though that this parameter must expand to a text value with the text delimiters around it.

Irp is an older obsolete synonym for for. MASM allows irp to provide compatibility with older source code. However you should always use the for directive.

The third form of the loop macro is the forc macro. It differs from the for macro in that it repeats a loop the number of times specified by the length of a character string rather than by the number of operands present. The syntax for the forc directive is

The statements in the loop repeat once for each character in the string operand. The angle brackets must appear around the string. Consider the following loop:

Irpc is an old synonym for forc provided for compatibility reasons. You should always use forc in your new code.

The while macro lets you repeat a sequence of code in your assembly language file an indefinite number of times. An assembly time expression that while evaluates before emitting the code for each loop determines whether it repeats. The syntax for this macro is

This macro evaluates the assembly-time expression; if this expression's value is zero the while macro ignores the statements up to the corresponding endm directive. If the expression evaluates to a non-zero value (true) then MASM assembles the statements up to the endm directive and reevaluates the expression to see if it should assemble the body of the while loop again.

Normally the while directive repeats the statements between the while and endm as long as the expression evaluates true. However you can also use the exitm directive to prematurely terminate the expansion of the loop body. Keep in mind that you need to provide some condition that terminates the loop otherwise MASM will go into an infinite loop and continually emit code to the object code file until the disk fills up (or it will simply go into an infinite loop if the loop does not emit any code).

Standard MASM macros are very flexible. If the number of actual parameters (those supplied in the operand field of the macro invocation) does not match the number of formal parameters (those appearing in the operand field of the macro definition) MASM won't necessarily complain. If there are more actual parameters than formal parameters MASM ignores the extra parameters and generates a warning. If there are more formal parameters than actual parameters MASM substitutes the empty string ("<>") for the extra formal parameters. By using the ifb and ifnb conditional assembly directives you can test this last condition. While this parameter substitution technique is flexible it also leaves open the possibility of error. If you want to require that the programmer supply exactly three parameters and they actually supply less MASM will not generate an error. If you forget to test for the presence of each parameter using ifb you could generate bad code. To overcome this limitation MASM provides the ability to specify that certain macro parameters are required. You can also assign a default value to a parameter if the programming doesn't supply one. Finally MASM also provides facilities to allow a variable number of macro arguments.

If you want to require a programmer to supply a particular macro parameters simply put ":req" after the macro parameter in the macro definition. At assembly time MASM will generate an error if that particular macro is missing.

Another possibility is to have the macro supply a default value for a macro if it is missing from the actual parameter list. To do this simply use the ":=<text>" operator immediately after the parameter name in the formal parameter list. For example the int 10h BIOS function provides various video services. One of the most commonly used video services is the ah=0eh function that outputs the character in al to the video display. The following macro lets the caller specify which function they want to use and defaults to function 0eh if they don't specify a parameter:

The last feature MASM's macros support is the ability to process a variable number of parameters. To do this you simply place the operator ":vararg" after the last formal parameter in the parameter list. MASM associates the first n actual parameters with the corresponding formal parameters appearing before the variable argument it then creates a text equate of all remaining parameters to the formal parameter suffixed with the ":vararg" operator. You can use the for macro to extract each parameter from this variable argument list. For example the following macro lets you declare an arbitrary number of two dimensional arrays all the same size. The first two parameters specify the number of rows and columns the remaining optional parameters specify the names of the arrays: