Lecture 18Lecture 18

Structures and MacrosStructures and Macros

Assembly Language for

Intel-Based Computers,

4th edition

Kip R. Irvine

2

Special OperatorsSpecial 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 ordinary character.

3

Substitution (&)Substitution (&)

ShowRegister MACRO regName.datatempStr BYTE " &regName=",0. ..codeShowRegister EDX ; invoke the macro

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

tempStr BYTE " EDX=",0

Macro expansion:

4

Expansion (%)Expansion (%)

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

The preprocessor generates the following code:

1 push edx1 mov dl,501 mov dh,71 call Gotoxy1 pop edx

Forces the evaluation of an integer expression. After the expression has been evaluated, its value is passed as a macro argument:

5

Expansion (%)Expansion (%)Instructs the preprocessor to expand all text macros and macro functions on the same line:

.data

Array DWORD 1, 2, 3, 4, 5, 6, 7, 8

.code

TempStr TEXTEQU % (SIZEOF array)

% ECHO The array contains TempStr bytes

6

Literal-Text (<>)Literal-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:

7

Literal-Character (!)Literal-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>

8

Macro Functions Macro Functions (1 of 2)(1 of 2)

• 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 symbolIFDEF symbol

EXITM <-1> ;; TrueELSE

EXITM <0> ;; FalseENDIF

ENDM

Notice how the assembler defines True and False.

9

Macro Functions Macro Functions (2 of 2)(2 of 2)

• 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,@datamov ds,ax

ENDIF

10

Defining Repeat BlocksDefining Repeat Blocks

• WHILE Directive• REPEAT Directive• FOR Directive• FORC Directive• Example: Linked List

11

WHILE DirectiveWHILE Directive

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

• Syntax:

WHILE constExpression

statements

ENDM

12

WHILE ExampleWHILE Example

.dataval1 = 1val2 = 1DWORD val1 ; first two valuesDWORD val2val3 = val1 + val2

WHILE val3 LT 0F0000000hDWORD val3val1 = val2val2 = val3val3 = val1 + val2

ENDM

Generates Fibonacci integers between 1 and F0000000h at assembly time:

13

REPEAT DirectiveREPEAT 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.

14

REPEAT ExampleREPEAT Example

iVal = 10REPEAT 100

DWORD iValiVal = 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?

15

Your turn . . .Your turn . . .

rows = 10columns = 5.dataiVal = 10REPEAT rows * columns

DWORD iValiVal = iVal + 10

ENDM

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

16

FOR DirectiveFOR 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

17

FOR ExampleFOR Example

Window STRUCTFOR 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 ?

Window ENDS

18

FORC DirectiveFORC 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

19

FORC ExampleFORC 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 ? Group_F WORD ? Group_G WORD ?

20

Example: Linked List Example: Linked List (1 of 5)(1 of 5)

• We can use the REPEAT 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

21

Linked List Linked List (2 of 5)(2 of 5)

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

ListNode STRUCTNodeData DWORD ? ; the node's dataNextPtr DWORD ? ; pointer to next node

ListNode ENDS

TotalNodeCount = 15NULL = 0Counter = 0

22

Linked List Linked List (3 of 5)(3 of 5)

• 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:

.dataLinkedList LABEL DWORDREPEAT TotalNodeCount

Counter = Counter + 1ListNode <Counter, ($ + Counter * SIZEOF ListNode)>

ENDM

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

23

Linked List Linked List (4 of 5)(4 of 5)

00000000 00000001

00000008

00000008 00000002

00000010

00000010 00000003

00000018

00000018 00000004

00000020

00000020 (etc.)

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

NextPtr

offset contents

24

Linked List Linked List (5 of 4)(5 of 4)

123456789101112131415

Sample output:

View the program's source code

Chapter 12Chapter 12

High-Level Language InterfaceHigh-Level Language Interface

Assembly Language for

Intel-Based Computers,

4th edition

Kip R. Irvine

26

Chapter OverviewChapter Overview

• Why Link ASM and HLL Programs?• General and Calling Conventions• External Identifiers

• Inline Assembly Code• __asm Directive• File Encryption Example

• Linking to C++ Programs• Linking to Borland C++• ReadSector Example

• Special Section: Optimizing Your Code• Loop Optimization Example• FindArray Example• Creating the FindArray Project

27

Why Link ASM and HLL Programs?Why Link ASM and HLL Programs?

• Use high-level language for overall project development• Relieves programmer from low-level details

• Use assembly language code• Speed up critical sections of code

• Access nonstandard hardware devices

• Write platform-specific code

• Extend the HLL's capabilities

28

General ConventionsGeneral Conventions

• Considerations when calling assembly language procedures from high-level languages:• Both must use the same naming convention (rules

regarding the naming of variables and procedures)

• Both must use the same memory model, with compatible segment names

• Both must use the same calling convention

29

Calling ConventionCalling Convention

• Identifies specific registers that must be preserved by procedures

• Determines how arguments are passed to procedures: in registers, on the stack, in shared memory, etc.

• Determines the order in which arguments are passed by calling programs to procedures

• Determines whether arguments are passed by value or by reference

• Determines how the stack pointer is restored after a procedure call

• Determines how functions return values

30

External IdentifiersExternal Identifiers

• An external identifier is a name that has been placed in a module’s object file in such a way that the linker can make the name available to other program modules.

• The linker resolves references to external identifiers, but can only do so if the same naming convention is used in all program modules.

31

Inline Assembly CodeInline Assembly Code

• Assembly language source code that is inserted directly into a HLL program.

• Compilers such as Microsoft Visual C++ and Borland C++ have compiler-specific directives that identify inline ASM code.

• Efficient inline code executes quickly because CALL and RET instructions are not required.

• Simple to code because there are no external names, memory models, or naming conventions involved.

• Decidedly not portable because it is written for a single platform.

32

_asm Directive in Microsoft Visual C++_asm Directive in Microsoft Visual C++

• Can be placed at the beginning of a single statement• Or, It can mark the beginning of a block of assembly

language statements• Syntax:

__asm statement

__asm {

statement-1

statement-2

...

statement-n

}

33

Commenting StylesCommenting Styles

mov esi,buf ; initialize index registermov esi,buf // initialize index registermov esi,buf /* initialize index register */

All of the following comment styles are acceptable, but the latter two are preferred:

34

You Can Do the Following . . .You Can Do the Following . . .

• Use any instruction from the Intel instruction set• Use register names as operands• Reference function parameters by name• Reference code labels and variables that were

declared outside the asm block • Use numeric literals that incorporate either

assembler-style or C-style radix notation • Use the PTR operator in statements such as inc

BYTE PTR [esi]• Use LENGTH, TYPE, and SIZE directives

35

You Cannot Do the Following . . .You Cannot Do the Following . . .

• Use data definition directives such as DB, DW, or BYTE

• Use assembler operators other than PTR• Use STRUCT• Use macro directives such as MACRO, REPT, ENDM• Reference segments by name.

• (You can, however, use segment register names as operands.)

36

Register UsageRegister Usage

• In general, you can modify EAX, EBX, ECX, and EDX in your inline code because the compiler does not expect these values to be preserved between statements

• Conversely, always save and restore ESI, EDI, and EBP.

37

File Encryption ExampleFile Encryption Example

• Reads a file, encrypts it, and writes the output to another file. • The TranslateBuffer function uses an __asm block to define

statements that loop through a character array and XOR each character with a predefined value.

Void TranslateBuffer (char *buf, unsigned count, unsigned char encryptChar)

{_asm {mov esi, bufmov ecx, countmov al, encryptChar

L1:xor [esi], alinc esiloop L1} // asm

}

38

Linking Assembly Language to C++Linking Assembly Language to C++

• Basic Structure - Two Modules• The first module, written in assembly language,

contains the external procedure• The second module contains the C/C++ code that

starts and ends the program • The C++ module adds the extern qualifier to the

external assembly language function prototype.• The "C" specifier must be included to prevent name

decoration by the C++ compiler:

extern "C" functionName( parameterList );

39

Name DecorationName Decoration

Also known as name mangling. HLL compilers do this to uniquely identify overloaded functions. A function such as:

int ArraySum( int * p, int count )

would be exported as a decorated name that encodes the return type, function name, and parameter types. For example:

int_ArraySum_pInt_int

The problem with name decoration is that the C++ compiler assumes that your assembly language function's name is decorated. The C++ compiler tells the linker to look for a decorated name.

C++ compilers vary in the way they decorate function names.

40

Linking to Borland C++Linking to Borland C++

• We will look at a C++ program that calls an external assembly language procedure named ReadSector• Reads a range of sectors from a disk drive

• Not possible with pure C++ code

• ASM code uses 16-bit MS-DOS functions

• Tools:• 16-bit version of Borland C++ 5.01

• Borland TASM 4.0 assembler (included with Borland C++)

41

ReadSector: Sample OutputReadSector: Sample Output

Sector display program.

Enter drive number [1=A, 2=B, 3=C, 4=D, 5=E,...]: 1

Starting sector number to read: 0

Number of sectors to read: 20

Reading sectors 0 - 20 from Drive 1

Sector 0 --------------------------------------------------------

.<.(P3j2IHC........@..................)Y...MYDISK FAT12 .3.

....{...x..v..V.U."..~..N..........|.E...F..E.8N$}"....w.r...:f..

|f;..W.u.....V....s.3..F...f..F..V..F....v.`.F..V.. ....^...H...F

..N.a....#.r98-t.`....}..at9Nt... ;.r.....}.......t.<.t..........

..}....}.....^.f......}.}..E..N....F..V......r....p..B.-`fj.RP.Sj

.j...t...3..v...v.B...v..............V$...d.ar.@u.B.^.Iuw....'..I

nvalid system disk...Disk I/O error...Replace the disk, and then

press any key....IOSYSMSDOS SYS...A....~...@...U.

42

Special Section: Optimizing Your CodeSpecial Section: Optimizing Your Code

• The 90/10 rule: 90% of a program's CPU time is spent executing 10% of the program's code

• We will concentrate on optimizing ASM code for speed of execution

• Loops are the most effective place to optimize code • Two simple ways to optimize a loop:

• Move invariant code out of the loop• Substitute registers for variables to reduce the number

of memory accesses

• Take advantage of high-level instructions such as XLAT, SCASB, and MOVSD.

43

Loop Optimization ExampleLoop Optimization Example

.datadays DWORD ?minutesInDay DWORD ?totalMinutes DWORD ?str1 BYTE "Daily total minutes: ",0

• We will write a short program that calculates and displays the number of elapsed minutes, over a period of n days.

• The following variables are used:

44

Sample Program OutputSample Program Output

Daily total minutes: +1440Daily total minutes: +2880Daily total minutes: +4320Daily total minutes: +5760Daily total minutes: +7200Daily total minutes: +8640Daily total minutes: +10080Daily total minutes: +11520..Daily total minutes: +67680Daily total minutes: +69120Daily total minutes: +70560Daily total minutes: +72000

45

No optimization.mov days,0mov totalMinutes,0

L1: ; loop contains 15 instructionsmov eax,24 ; minutesInDay = 24 * 60mov ebx,60mul ebxmov minutesInDay,eaxmov edx,totalMinutes ; totalMinutes += minutesInDayadd edx,minutesInDaymov totalMinutes,edxmov edx,OFFSET str1 ; "Daily total minutes: "call WriteStringmov eax,totalMinutes ; display totalMinutescall WriteIntcall Crlfinc days ; days++cmp days,50 ; if days < 50,jb L1 ; repeat the loop

Version 1Version 1

46

Move calculation of minutesInDay outside the loop, and assign EDX before the loop. The loop now contains 10 instructions.

mov days,0mov totalMinutes,0mov eax,24 ; minutesInDay = 24 * 60mov ebx,60mul ebxmov minutesInDay,eaxmov edx,OFFSET str1 ; "Daily total minutes: "

L1: mov ebx,totalMinutes ; totalMinutes += minutesInDayadd ebx,minutesInDaymov totalMinutes,ebxcall WriteString ; display str1 (offset in EDX)mov eax,totalMinutes ; display totalMinutescall WriteIntcall Crlfinc days ; days++cmp days,50 ; if days < 50,jb L1 ; repeat the loop

Version 2Version 2

47

Move totalMinutes to EAX, use EAX throughout loop. Use constant expresion for minutesInDay calculation. The loop now contains 7 instructions.

C_minutesInDay = 24 * 60 ; constant expressionmov days,0mov totalMinutes,0mov eax,totalMinutesmov edx,OFFSET str1; "Daily total minutes: "

L1: add eax,C_minutesInDay ; totalMinutes += minutesInDaycall WriteString ; display str1 (offset in EDX)call WriteInt ; display totalMinutes (EAX)call Crlfinc days ; days++cmp days,50 ; if days < 50,jb L1 ; repeat the loop

mov totalMinutes,eax ; update variable

Version 3Version 3

48

Substitute ECX for the days variable. Remove initial assignments to days and totalMinutes.

C_minutesInDay = 24 * 60 ; constant expressionmov eax,0 ; EAX = totalMinutesmov ecx,0 ; ECX = daysmov edx,OFFSET str1 ; "Daily total minutes: "

L1: ; loop contains 7 instructionsadd eax,C_minutesInDay ; totalMinutes += minutesInDaycall WriteString ; display str1 (offset in EDX)call WriteInt ; display totalMinutes (EAX)call Crlfinc ecx ; days (ECX)++cmp ecx,50 ; if days < 50,jb L1 ; repeat the loop

mov totalMinutes,eax ; update variablemov days,ecx ; update variable

Version 4Version 4

49

Using Assembly Language to Optimize C++Using Assembly Language to Optimize C++

• Find out how to make your C++ compiler produce an assembly language source listing• /FAs command-line option in Visual C++, for example

• Optimize loops for speed• Use hardware-level I/O for optimum speed• Use BIOS-level I/O for medium speed

50

FindArray ExampleFindArray Example

#include "findarr.h"

bool FindArray( long searchVal, long array[], long count ){ for(int i = 0; i < count; i++) if( searchVal == array[i] ) return true; return false;}

Let's write a C++ function that searches for the first matching integer in an array. The function returns true if the integer is found, and false if it is not:

51

Code Produced by C++ CompilerCode Produced by C++ Compiler

_searchVal$ = 8_array$ = 12_count$ = 16_i$ = -4

_FindArray PROC NEAR; 29 : {

push ebpmov ebp, esppush ecx

; 30 : for(int i = 0; i < count; i++)mov DWORD PTR _i$[ebp], 0jmp SHORT $L174

$L175:mov eax, DWORD PTR _i$[ebp]add eax, 1mov DWORD PTR _i$[ebp], eax

optimization switch turned off (1 of 3)

52

Code Produced by C++ CompilerCode Produced by C++ Compiler

$L174:mov ecx, DWORD PTR _i$[ebp]cmp ecx, DWORD PTR _count$[ebp]jge SHORT $L176

; 31 : if( searchVal == array[i] )mov edx, DWORD PTR _i$[ebp]mov eax, DWORD PTR _array$[ebp]mov ecx, DWORD PTR _searchVal$[ebp]cmp ecx, DWORD PTR [eax+edx*4]jne SHORT $L177

; 32 : return true;mov al, 1jmp SHORT $L172

$L177:; 33 : ; 34 : return false;

jmp SHORT $L175

(2 of 3)

53

Code Produced by C++ CompilerCode Produced by C++ Compiler

$L176:xor al, al ; AL = 0

$L172:; 35 : }

mov esp, ebp ; restore stack pointerpop ebpret 0

_FindArray ENDP

(3 of 3)

54

Hand-Coded Assembly LanguageHand-Coded Assembly Language (1 of 2) (1 of 2)

true = 1false = 0

; Stack parameters:srchVal equ [ebp+08]arrayPtr equ [ebp+12]count equ [ebp+16]

.code_FindArray PROC near push ebp mov ebp,esp push edi

mov eax, srchVal ; search value mov ecx, count ; number of items mov edi, arrayPtr ; pointer to array

55

Hand-Coded Assembly LanguageHand-Coded Assembly Language (2 of 2) (2 of 2)

repne scasd ; do the search jz returnTrue ; ZF = 1 if found

returnFalse: mov al, false jmp short exit

returnTrue: mov al, true

exit: pop edi pop ebp ret _FindArray ENDP

56

• CSCE 380• Department of Computer Science

and Computer Engineering• Pacific Lutheran University• 4/23/2003