Structures and Macros

1
Structures and Macros

• Chapter 8

2
Structures

• Template or pattern that can be overlayed on an
  area of memory
• Multiple fields of different types
• May initialize fields to default values
• Structure definition
• STUNUMBER_SIZE 7
• LASTNAME_SIZE 20
• typStudent struc
• IdNum db STUNUMBER_SIZE 1 dup(0)
• Lastname db LASTNAME_SIZE 1 dup(0)
• Credits dw ?
• Status db ?
• typStudent ends

3
Declaring a Structure Variable

• Declare a typStudent Variable called srec
• .data
• srec typStudent ltgt
• You can override default values when declaring a
  structure variable
• .data
• srec typStudent lt1234,Jones,32,0gt
• myRec typStudent lt,,50,0gt

4
Using a Structure Variable

• Refer to individual field offsets by name
• .data
• mov dl, srec.IdNum
• mov ax, srec.Credits
• mov ax, bx.Credits
• mov dl, srecsi.Status
• (the last two examples require the use of the /Zm
  option when using ML.EXE)
• Alternate notation (for ML.EXE without /Zm)
• mov ax, (typStudent PTR bx).Credits

5
Student Structure Demo Program

• Array of typStudent Structures
• Demonstrates input/output operations on
  individual fields.
• Program listing struc.asm

6
Introducing Macros

• Symbolic name given to one or more assembly
  language statements
• A macro is invoked, not called. A copy of the
  macro is inserted directly into the program.
• The mPutchar macro (no parameters)
• mPutchar macro
• mov ah,2
• int 21h
• endm

7
Macros with Parameters

• Parameters make a macro more flexible, usable in
  different situations
• A macro is invoked, not called. A copy of the
  macro is inserted directly into the program.
• The mPutchar macro (with parameter)
• mPutchar macro char
• push ax
• push dx
• mov ah,2
• mov dl,char
• int 21h
• pop dx
• pop ax
• endm

8
Defining a Macro

• Macro definition syntax
• macroname macro parameter-1, parameter-2
• statement-list
• endm
• Required parameters
• mGotoRowCol macro rowREQ, columnREQ
• The mDisplayStr macro writes a string to standard
  output
• The mGotoRowCol macro locates the cursor on a
  specific row/column on the screen.

9
Macros that Allocate Storage

• Macro definition syntax
• macroname macro parameter-1, parameter-2
• statement-list
• endm
• Macro definition
• mAlloc macro varname,numbytes
• varname db numbytes( 0,0,0,0)
• endm
• Invoking the Macro
• mAlloc value1,20
• Generated Code
• value1 db 20 dup( ,0,0,0,0)

10
Creating Local Labels

• Local labels permit the assembler to generate a
  unique label each time the macro is invoked
• mRepeat macro char, count
•     local L1
•     mov  cx,count
• L1 mov  ah,2
•    mov  dl,char
•    int  21h
•    loop  L1
• endm

11
Nested Macros

• A macro containing other macros is called a
  nested macro
• mDisplayRowCol macro row,col,string
•   mGotoRowCol row,col
•   mDisplayStr string
• endm
• Sample call
• .data
• greeting db "Hello from row 10, column 15."
• .code
• mDisplayRowCol 10, 15, greeting

12
Using a Macro to Call a Procedure

• A macro makes a good shell for calling
  procedures. For example, the mWriteInt macro
  calls the Writeint procedure
• mWriteint macro value, radix
• push ax
• push bx
• mov ax,value
• mov bx,radix
• call Writeint
• pop bx
• pop ax
• endm

13
Conditional-Assembly Directives

• Checking for missing macro arguments
• mWriteint macro value,radix
• ifb ltvaluegt if blank,
• exitm exit macro
• endif
• push ax
• push bx
• ifb ltradixgt
• mov bx,10 default radix
• endif
• mov ax,value
• (etc.)

14
Displaying Messages During Assembly

• Write a string to standard output during assembly
• Useful for identifying invalid macro arguments.
  For example,
• out Invalid parameter passed to macro

15
Substitute Operator ()

• Concatenates text in the macro with a macro
  argument
• mDOSmsg macro example
• mDOSmsg macro num,string
• msgnum db DOS error string, 0
• endm
• Invoke the macro
• mDOSmsg 1,ltInvalid functiongt
• Generated code
• msg1 db DOS error Invalid function,0

16
Expression Operator ()

• Permits the calculated value of an expression to
  be passed as a macro argument
• The value, not the expression itself, is passed
• mDirectVideo Example
• mDirectVideo ((row 160) (col 2)), string1

17
Literal-Text Operator ( ltgt )

• Ties together a string of characters
• Can be used to pass a single macro argument
• Example containing embedded characters
• ltEfficiency is 50, fallinggt

18
A Simple Macro Library

• mWriteLiteral (write string literal)
• mCondCall (conditional Call)
• mCompJmp (compare and jump)
• mMult16 (memory multiply/16)
• mMove (memory to memory move)
• mLongLoop (long loop)

19
REPT, IRP, and IRPC Directives

• REPT repeats a block of statements based on a
  counter
• Can be used to create repeated data structures
• IRP creates a repeate block in which each
  repetition contains a different value
• IRP parameter,ltargument ,argumentgt
• statements
• ENDM

20
Linked List Example

• ListNode struc
• NodeData dw ? the node's data
• NextPtr dw ? pointer to next node
• ends
• .data
• COUNT 0
• NUMBER_OF_NODES 50
• Create multiple ListNode instances
• LinkedList label word
• rept NUMBER_OF_NODES
• COUNT COUNT 1
• ListNode lt COUNT, ( 2) gt
• endm

21
Extended Jump Macro

• Uses the IRP Directive to generate macros on an
  as needed basis
• irp cond,lta,ae,b,be,e,ne,z,nz,g,ge,l,le,c, \
• nc,o,no,p,np,s,nsgt
• jxcond macro dest
• local L1,L2
• jcond L1 condition true?
• jmp short L2 no exit
• L1 jmp dest yes jump to destination
• L2
• endm
• endm

22
The Texts MACRO Library

• Use this macro library by copying it to your
  programs directory and placing the following
  line before the .data directive
• INCLUDE MACROS.INC
• Listing of the MACROS.INC file