Introducing Macros Introducing Macros

Macros Macros

• Introducing Macros

• Defining Macros

• Invoking Macros

• Macro Examples

• Nested Macros

• Example Program: Wrappers

Introducing Macros Introducing Macros

• A macro¹ is a named block of assembly language statements.

• Once defined, it can be invoked (called) one or more times.

• During the assembler's preprocessing step, each macro call is expanded into a copy of the macro.

• The expanded code is passed to the assembly step, where it is checked for correctness.

Defining Macros Defining Macros

• A macro must be defined before it can be used.

• Parameters are optional.

• Each parameter follows the rules for identifiers. It is a string that is assigned a value when the macro is invoked.

• Syntax:

macroname MACRO [parameter-1, parameter-2,...]

statement-list ENDM

mNewLine

mNewLine Macro Example Macro Example

mNewLine MACRO ; define the macro call Crlf

ENDM .data .code

mNewLine ; invoke the macro

This is how you define and invoke a simple macro.

The assembler will substitute "call crlf" for "mNewLine".

mPutChar

mPutChar Macro Macro

mPutchar MACRO char push eax

mov al,char call WriteChar pop eax

ENDM

Writes a single character to standard output.

Definition:

.code

mPutchar 'A'

Invocation:

1 push eax 1 mov al,'A'

1 call WriteChar

1 pop eax

Expansion:

viewed in the listing file

Invoking Macros

Invoking Macros

• When you invoke a macro, each argument you pass matches a declared parameter.

• Each parameter is replaced by its corresponding argument when the macro is expanded.

• When a macro expands, it generates assembly language source code.

• Arguments are treated as simple text by the preprocessor.

Invoking Macros

Invoking Macros

parameter macro

text

argument

replaces

declared inside consists passes of

macro invocation

statement

generates assembly

code

Relationships between macros, arguments, and parameters:

mWriteStr

mWriteStr Macro Macro

mWriteStr MACRO buffer push edx

mov edx,OFFSET buffer call WriteString

pop edx ENDM

.data

str1 BYTE "Welcome!",0 .code

mWriteStr str1

Provides a convenient way to display a string, by passing the string name as an argument.

mWriteStr

mWriteStr Macro Macro

1 push edx

1 mov edx,OFFSET str1 1 call WriteString

1 pop edx

The expanded code shows how the str1 argument replaced the parameter named buffer:

mWriteStr MACRO buffer push edx

mov edx,OFFSET buffer call WriteString

pop edx ENDM

Invalid Argument Invalid Argument

• If you pass an invalid argument, the error is caught when the expanded code is assembled.

• Example:

.code

mPutchar 1234h 1 push eax

1 mov al,1234h ; error!

1 call WriteChar

1 pop eax

Blank Argument Blank Argument

• If you pass a blank argument, the error is also caught when the expanded code is assembled.

• Example:

.code mPutchar

1 push eax

1 mov al,

1 call WriteChar

1 pop eax

Macro Examples Macro Examples

• mReadStr - reads string from standard input

• mGotoXY - locates the cursor on screen

• mDumpMem - dumps a range of memory

mReadStr mReadStr

mReadStr MACRO varName push ecx

push edx

mov edx,OFFSET varName

mov ecx,(SIZEOF varName) - 1 call ReadString

pop edx pop ecx ENDM

.data

firstName BYTE 30 DUP(?) .code

mReadStr firstName

The mReadStr macro provides a convenient wrapper around ReadString procedure calls.

mGotoXY mGotoXY

mGotoxy MACRO X:REQ, Y:REQ push edx

mov dh,Y mov dl,X call Gotoxy pop edx ENDM

The mGotoXY macro ets the console cursor position by calling the Gotoxy library procedure.

The REQ next to X and Y identifies them as required parameters.

mDumpMem mDumpMem

mDumpMem MACRO address, itemCount, componentSize push ebx

push ecx push esi

mov esi,address mov ecx,itemCount

mov ebx,componentSize call DumpMem

pop esi pop ecx pop ebx ENDM

The mDumpMem macro streamlines calls to the link library's DumpMem procedure.

mWrite mWrite

mWrite MACRO text LOCAL string

.data ;; data segment

string BYTE text,0 ;; define local string

.code ;; code segment

push edx

mov edx,OFFSET string call Writestring

pop edx ENDM

The mWrite macro writes a string literal to standard output. It is a good example of a macro that contains both code and data.

The LOCAL directive prevents string from becoming a global

Nested Macros Nested Macros

• The mWriteLn macro contains a nested macro (a macro

invoked by another macro).

mWriteLn MACRO text mWrite text

call Crlf ENDM

mWriteLn "My Sample Macro Program"

2 .data

2 ??0002 BYTE "My Sample Macro Program",0 2 .code

2 push edx

2 mov edx,OFFSET ??0002 2 call Writestring

2 pop edx 1 call Crlf

Your turn . . . Your turn . . .

• Write a nested macro that clears the screen, locates the cursor at a given row and column, asks the user to enter an account number, and inputs the account number. Use any macros shown so far.

• Use the following data to test your macro:

.data

acctNum BYTE 30 DUP(?) .code

main proc

mAskForString 5,10,"Input Account Number: ", \ acctNum

. . . Solution . . . Solution

mAskForString MACRO row,col,prompt,inbuf call Clrscr

mGotoXY col,row mWrite prompt mReadStr inbuf ENDM

Example Program: Wrappers Example Program: Wrappers

• Demonstrates various macros from this chapter

• Shows how macros can simplify argument passing

• View the source code

Conditional

Conditional - - Assembly Directives Assembly Directives

• Checking for Missing Arguments

• Default Argument Initializers

• Boolean Expressions

• IF, ELSE, and ENDIF Directives

• The IFIDN and IFIDNI Directives

• Special Operators

• Macro Functions

Checking for Missing Arguments Checking for Missing Arguments

• The IFB directive returns true if its argument is blank.

For example:

IFB <row> ;; if row is blank, EXITM ;; exit the macro ENDIF

mWriteString

mWriteString Example Example

mWriteStr MACRO string IFB <string>

ECHO --- ECHO * Error: parameter missing in mWriteStr ECHO * (no code generated)

ECHO --- EXITM

ENDIF push edx

mov edx,OFFSET string call WriteString

pop edx ENDM

Display a message during assembly if the string parameter is empty:

Default Argument

Default Argument Initializers Initializers

• A default argument initializer automatically assigns a value to a parameter when a macro argument is left blank. For example, mWriteln can be invoked either with or without a string argument:

mWriteLn MACRO text:=<" ">

mWrite text call Crlf ENDM

.code

mWriteln "Line one"

mWriteln

mWriteln "Line three"

Line one Line three

Sample output:

Boolean Expressions Boolean Expressions

A boolean expression can be formed using the following operators:

• LT - Less than

• GT - Greater than

• EQ - Equal to

• NE - Not equal to

• LE - Less than or equal to

• GE - Greater than or equal to

Only assembly-time constants may be compared using these operators.

IF, ELSE, and ENDIF Directives IF, ELSE, and ENDIF Directives

IF boolean-expression

statements [ELSE

statements]

ENDIF

A block of statements is assembled if the boolean expression evaluates to true. An alternate block of statements can be assembled if the expression is false.

Simple Example Simple Example

IF RealMode EQ 1 mov ax,@data mov ds,ax ENDIF

The following IF directive permits two MOV instructions to be assembled if a constant named RealMode is equal to 1:

RealMode can be defined in the source code any of the following ways:

RealMode = 1 RealMode EQU 1

The IFIDN and IFIDNI Directives The IFIDN and IFIDNI Directives

• IFIDN compares two symbols and returns true if they are equal (case-sensitive)

• IFIDNI also compares two symbols, using a case- insensitive comparison

• Syntax:

IFIDNI <symbol>, <symbol>

statements ENDIF

Can be used to prevent the caller of a macro from passing an

IFIDNI Example IFIDNI Example

mReadBuf MACRO bufferPtr, maxChars IFIDNI <maxChars>,<EDX>

ECHO Warning: Second argument cannot be EDX ECHO **************************************

EXITM ENDIF . . ENDM

Prevents the user from passing EDX as the second argument to the mReadBuf macro:

Special Operators Special Operators

• The substitution (&) operator resolves ambiguous references to parameter names within a macro.

• The expansion operator (%) expands text macros or converts constant expressions into their text

representations.

• The literal-text operator (<>) groups one or more characters and symbols into a single text literal. It

prevents the preprocessor from interpreting members of the list as separate arguments.

• The literal-character operator (!) forces the

preprocessor to treat a predefined operator as an

Substitution (&) Substitution (&)

ShowRegister MACRO regName .data

tempStr BYTE " &regName=",0 .

.

.code

ShowRegister EDX ; invoke the macro

Text passed as regName is substituted into the literal string definition:

tempStr BYTE " EDX=",0

Macro expansion:

Expansion (%) Expansion (%)

mGotoXY %(5 * 10),%(3 + 4)

The preprocessor generates the following code:

1 push edx 1 mov dl,50 1 mov dh,7 1 call Gotoxy 1 pop edx

Forces the evaluation of an integer expression. After the

expression has been evaluated, its value is passed as a macro argument:

Literal

Literal - - Text (<>) Text (<>)

mWrite "Line three", 0dh, 0ah mWrite <"Line three", 0dh, 0ah>

The first macro call passes three arguments. The second call passes a single argument:

Literal

Literal - - Character (!) Character (!)

BadYValue TEXTEQU Warning: <Y-coordinate is > 24>

The following declaration prematurely ends the text definition when the first > character is reached.

The following declaration continues the text definition until the final > character is reached.

BadYValue TEXTEQU <Warning: Y-coordinate is !> 24>

Macro Functions

Macro Functions

• A macro function returns an integer or string constant

• The value is returned by the EXITM directive

• Example: The IsDefined macro acts as a wrapper for the IFDEF directive.

IsDefined MACRO symbol IFDEF symbol

EXITM <-1> ;; True ELSE

EXITM <0> ;; False ENDIF

ENDM

Notice how the assembler defines True and False.

Macro Functions

Macro Functions

• When calling a macro function, the argument(s) must be enclosed in parentheses

• The following code permits the two MOV statements to be assembled only if the RealMode symbol has been defined:

IF IsDefined( RealMode ) mov ax,@data

mov ds,ax ENDIF

Defining Repeat Blocks Defining Repeat Blocks

• WHILE Directive

• REPEAT Directive

• FOR Directive

• FORC Directive

• Example: Linked List

WHILE Directive WHILE Directive

• The WHILE directive repeats a statement block as long as a particular constant expression is true.

• Syntax:

WHILE constExpression statements

ENDM

WHILE Example WHILE Example

.data val1 = 1 val2 = 1

DWORD val1 ; first two values DWORD val2

val3 = val1 + val2

WHILE val3 LT 0F0000000h DWORD val3

val1 = val2 val2 = val3

val3 = val1 + val2 ENDM

Generates Fibonacci integers between 1 and F0000000h at assembly time:

REPEAT Directive REPEAT Directive

• The REPEAT directive repeats a statement block a fixed number of times.

• Syntax:

REPEAT constExpression

statements ENDM

ConstExpression, an unsigned constant integer expression, determines the number of repetitions.

REPEAT Example REPEAT Example

iVal = 10 REPEAT 100

DWORD iVal

iVal = iVal + 10 ENDM

The following code generates 100 integer data definitions in the sequence 10, 20, 30, . . .

How might we assign a data name to this list of integers?

Your turn . . . Your turn . . .

rows = 10 columns = 5 .data

iVal = 10

REPEAT rows * columns DWORD iVal

iVal = iVal + 10 ENDM

What will be the last integer to be generated by the following loop? 500

FOR Directive FOR Directive

• The FOR directive repeats a statement block by iterating over a comma-delimited list of symbols.

• Each symbol in the list causes one iteration of the loop.

• Syntax:

FOR parameter,<arg1,arg2,arg3,...>

statements ENDM

FOR Example FOR Example

Window STRUCT

FOR color,<frame,titlebar,background,foreground>

color DWORD ? ENDM

Window ENDS

The following Window structure contains frame, title bar, background, and foreground colors. The field definitions are created using a FOR directive:

Generated code:

Window STRUCT frame DWORD ? titlebar DWORD ?

background DWORD ? foreground DWORD ?

FORC Directive FORC Directive

• The FORC directive repeats a statement block by

iterating over a string of characters. Each character in the string causes one iteration of the loop.

• Syntax:

FORC parameter, <string>

statements ENDM

FORC Example FORC Example

FORC code,<ABCDEFG>

Group_&code WORD ? ENDM

Suppose we need to accumulate seven sets of integer data for an experiment. Their label names are to be Group_A, Group_B, Group_C, and so on. The FORC directive creates the variables:

Generated code:

Group_A WORD ? Group_B WORD ? Group_C WORD ? Group_D WORD ? Group_E WORD ?

Example: Linked List

Example: Linked List

• We can use the REPT directive to create a singly linked list at assembly time.

• Each node contains a pointer to the next node.

• A null pointer in the last node marks the end of the list

data link data link data link null

Linked List

Linked List

• Each node in the list is defined by a ListNode structure:

ListNode STRUCT

NodeData DWORD ? ; the node's data

NextPtr DWORD ? ; pointer to next node ListNode ENDS

TotalNodeCount = 15 NULL = 0

Counter = 0

Linked List

Linked List

• The REPEAT directive generates the nodes.

• Each ListNode is initialized with a counter and an

address that points 8 bytes beyond the current node's location:

.data

LinkedList LABEL DWORD REPEAT TotalNodeCount

Counter = Counter + 1

ListNode <Counter, ($ + Counter * SIZEOF ListNode)>

ENDM

The value of $ does not change—it remains fixed at the location of the LinkedList label.

Linked List

Linked List

00000000 00000001 00000008 00000008 00000002 00000010 00000010 00000003 00000018 00000018 00000004 00000020

The following hexadecimal values in each node show how each NextPtr field contains the address of its following node.

NextPtr offset contents

Linked List

Linked List

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Sample output:

View the program's source code