Assembly Language for Intel

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Assembly Language for Intel

Assembly Language for Intel - - Based Based Computers, 4

Computers, 4

thth

Edition Edition

Chapter 5: Procedures

(c) Pearson Education, 2002. All rights reserved.

Kip R. Irvine

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Chapter Overview Chapter Overview

• Linking to an External Library

• The Book's Link Library

• Stack Operations

• Defining and Using Procedures

• Program Design Using Procedures

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Link Library Overview Link Library Overview

• A file containing procedures that have been compiled into machine code

• constructed from one or more OBJ files

• To build a library, . . .

• start with one or more ASM source files

• assemble each into an OBJ file

• create an empty library file (extension .LIB)

• add the OBJ file(s) to the library file, using the

Microsoft LIB utility

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Calling a Library Procedure Calling a Library Procedure

INCLUDE Irvine32.inc .code

mov eax,1234h ; input argument call WriteHex ; show hex number

call Crlf ; end of line

• Call a library procedure using the CALL instruction. Some procedures require input arguments. The INCLUDE directive copies in the procedure prototypes (declarations).

• The following example displays "1234" on the console:

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Library Procedures

Library Procedures – – Overview [1/7] Overview [1/7]

Clrscr - Clears the console and locates the cursor at the upper left corner.

• This is typically done at the beginning and ending of a program.

Crlf - Writes an end of line sequence to standard output.

• To advance the cursor to the beginning of the next line of standard output. (0Dh and 0Ah)

Delay - Pauses the program execution for a specified n millisecond interval.

• Set EAX to the desired interval, in millisecond

mov eax,1000 ; 1 second

call Delay

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Library Procedures

Library Procedures – – Overview [2/7] Overview [2/7]

DumpMem - Writes a block of memory to standard output in hexadecimal.

• Pass the starting address of memory in ESI, the number of units in ECX, and the unit size in EBX

.data

array DWORD 1,2,3,4,5,6,7,8,9,0Ah,0Bh .code

main PROC

mov esi,OFFSET array ; starting OFFSET mov ecx,LENGTHOF array ; number of units mov ebx,TYPE array ; doubleword format call DumpMem

00000001 00000002 00000003 00000004 00000005 00000006 00000007 00000008 00000009 0000000A 0000000B

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Library Procedures

Library Procedures – – Overview [3/7] Overview [3/7]

DumpRegs - Displays the EAX, EBX, ECX, EDX, ESI, EDI, EBP, ESP, EFLAGS, and EIP registers in hexadecimal. Also displays the Carry, Sign, Zero, and Overflow flags.

• Be useful for program debugging

GetCommandtail - Copies the program’s command-line arguments (called the command tail) into an array of bytes.

• It permits the user of a program to pass information on the command line.

GetMseconds - Returns the number of milliseconds that have elapsed since midnight.

• To measure the time between events (the return value is in EAX)

.data

cmdTail BYTE 129 DUP (0) .code

mov edx,OFFSET cmdTail call GetCommandtail

Encrypt file1.txt file2.txt

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Library Procedures

Library Procedures – – Overview [4/7] Overview [4/7]

Gotoxy - Locates cursor at row (0~24) and column (0~79) on the console.

Random32 - Generates a 32-bit

pseudorandom integer in the range 0 to FFFFFFFFh.

Randomize - Seeds the random number generator.

RandomRange - Generates a pseudorandom integer within a specified range.

mov dh,10 ; row 10 mov dl,20 ; column 20 call Gotoxy ; locate cursor

.data

randVal DWORD ? .code

call Random32 mov randVal,eax

call Randomize mov exc,10

L1: call Random32

; use or display random value in EAX here Loop L1

.data

randVal DWORD ? .code

mov eax,5000

call RandomRange mov randVal,eax

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Library Procedures

Library Procedures – – Overview [5/7] Overview [5/7]

ReadChar - Reads a single character from standard input.

ReadHex - Reads a 32-bit hexadecimal integer from

standard input, terminated by the Enter key.

ReadInt - Reads a 32-bit signed decimal integer from standard input, terminated by the Enter key.

ReadString - Reads a string from standard input, terminated by the Enter key.

.data

char BYTE ?

.code

call ReadChar mov char,al

.data

hexVal DWORD ? .code

call ReadHex mov hexVal,eax

.data

intVal SWORD ? .code

call ReadInt mov intVal,eax

.data

buffer BYTE 50 DUP(0) ; holds the char.

byteCount DWORD ? ; holds counter .code

mov edx, OFFSET buffer ; point to the buffer mov ecx,(SIZEOF buffer) -1 ; specify max char.

call ReadString ; input the string

mov byteCount,eax ; no. of char.

41 42 43 44 45 46 47 00 ABCDEFG

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Library Procedures

Library Procedures – – Overview [6/7] Overview [6/7]

SetTextColor - Sets the foreground and background colors of all subsequent text output to the console.

• EAX

WaitMsg - Displays message, waits for Enter key to be pressed.

WriteBin - Writes an unsigned 32-bit integer to standard output in ASCII binary format.

• EAX

WriteChar - Writes a single character to standard output.

• AL

white=15 lightCyan=11

lightGray=7 cyan=3

yellow=14 lightGreeb=10

brown=6 green=2

lightMagenta=13 lightBlue=9

magenta=5 blue=1

lightRed=12 gray=8

red=4 black=0

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Library Procedures

Library Procedures – – Overview [7/7] Overview [7/7]

WriteDec - Writes an unsigned 32-bit integer to standard output in decimal format.

• EAX

WriteHex - Writes an unsigned 32-bit integer to standard output in hexadecimal format.

• EAX

WriteInt - Writes a signed 32-bit integer to standard output in decimal format.

• EAX

WriteString - Writes a null-terminated string to standard output.

.data

prompt BYTE “Enter your name: ”,0 .code

mov edx,OFFSET prompt call WriteString

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Example 1 Example 1

.code

call Clrscr mov eax,500 call Delay call DumpRegs

Clear the screen, delay the program for 500 milliseconds, and dump the registers and flags.

EAX=00000613 EBX=00000000 ECX=000000FF EDX=00000000 ESI=00000000 EDI=00000100 EBP=0000091E ESP=000000F6 EIP=00401026 EFL=00000286 CF=0 SF=1 ZF=0 OF=0

Sample output:

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Example 2 Example 2

.data

str1 BYTE "Assembly language is easy!",0 .code

mov edx,OFFSET str1 call WriteString call Crlf

Display a null-terminated string and move the cursor to the

beginning of the next screen line.

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Example 3 Example 3

IntVal = 35 ; constant

.code

mov eax,IntVal

call WriteBin ; display binary call Crlf

call WriteDec ; display decimal call Crlf

call WriteHex ; display hexadecimal call Crlf

Display the same unsigned integer in binary, decimal, and hexadecimal. Each number is displayed on a separate line.

0000 0000 0000 0000 0000 0000 0010 0011 35

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Sample output:

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Example 4 Example 4

.data

fileName BYTE 80 DUP(0) .code

mov edx,OFFSET fileName

mov ecx,SIZEOF fileName – 1 call ReadString

Input a string from the user. EDX points to the string and ECX specifies the maximum number of characters the user is

permitted to enter.

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Example 5 Example 5

.code

mov ecx,10 ; loop counter L1: mov eax,100 ; ceiling value

call RandomRange ; generate random int call WriteInt ; display signed int

call Crlf ; goto next display line

loop L1 ; repeat loop

Generate and display ten pseudorandom signed integers in the

range 0 – 99. Each integer is passed to WriteInt in EAX and

displayed on a separate line.

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Example 6 Example 6

.data

str1 BYTE "Color output is easy!",0 .code

mov eax,yellow + (blue * 16) call SetTextColor

mov edx,OFFSET str1 call WriteString

call Crlf

Display a null-terminated string with yellow characters on a blue background.

The background color must be multiplied by 16 before you add it to the foreground color.

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Stack Operations Stack Operations

• Runtime Stack

• PUSH Operation

• POP Operation

• PUSH and POP Instructions

• Using PUSH and POP

• Example: Reversing a String

• Related Instructions

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Runtime Stack Runtime Stack

• Managed by the CPU, using two registers

• SS (stack segment)

• ESP (stack pointer)

00000006 00001000 ESP

Offset

00000FF8 00000FF4 00000FF0 00000FFC

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PUSH Operation [1/2]

PUSH Operation [1/2]

• A 32-bit push operation decrements the stack pointer by 4 and copies a value into the location pointed to by the stack pointer.

00000006 00000006

ESP 00001000

00000FFC 00000FF8 00000FF4 00000FF0

000000A5 ESP

00001000

BEFORE

00000FFC 00000FF8 00000FF4 00000FF0

AFTER

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PUSH Operation [2/2]

PUSH Operation [2/2]

• This is the same stack, after pushing two more integers:

00000006

ESP 00001000

Offset

00000FFC 00000FF8 00000FF4 00000FF0

000000A5 00000001 00000002

The stack grows downward. The area below ESP is always

available (unless the stack has overflowed).

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POP Operation POP Operation

• Copies value at stack[ESP] into a register or variable.

Adds n to ESP, where n is either 2 or 4.

• depends on the attribute of the operand receiving the data

BEFORE AFTER

00000006 000000A5 00000001

00000002 ESP

00000006 000000A5

00000001 ESP 00001000

00000FFC 00000FF8 00000FF4 00000FF0

00001000 00000FFC 00000FF8 00000FF4 00000FF0

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PUSH and POP Instructions PUSH and POP Instructions

• PUSH syntax:

• PUSH r/m16

• PUSH r/m32

• PUSH imm32

• POP syntax:

• POP r/m16

• POP r/m32

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Using PUSH and POP Using PUSH and POP

push esi ; push registers

push ecx push ebx

mov esi,OFFSET dwordVal ; starting OFFSET mov ecx,LENGTHOF dwordVal ; number of units

mov ebx,TYPE dwordVal ; size of a doubleword call DumpMem ; display memory

pop ebx ; opposite order

pop ecx pop esi

Save and restore registers when they contain important values.

Note that the PUSH and POP instructions are in the opposite

order:

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Example: Reversing a String Example: Reversing a String

• Use a loop with indexed addressing

• Push each character on the stack

• Start at the beginning of the string, pop the stack in reverse order, insert each character into the string

• Source code

• Q: Why must each character be put in EAX before it is pushed?

Because only word (16-bit) or doubleword (32-bit) values

can be pushed on the stack.

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Your turn . . . Your turn . . .

• Using the String Reverse program as a starting point,

• Modify the program so the user can input a string of

up to 50 characters, and then displays the characters

in reverse order.

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Related Instructions Related Instructions

• PUSHFD and POPFD

• push and pop the EFLAGS register

• PUSHAD pushes the 32-bit general-purpose registers on the stack

• order: EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI

• POPAD pops the same registers off the stack in reverse order

• PUSHA and POPA do the same for 16-bit registers

.data

saveFlags DWORD ? .code

pushfd ; push flags on stack pop saveFlags ; copy into a variable

push saveFlags ; push saved flag values popfd ; copy into a variable

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Your Turn . . . Your Turn . . .

• Write a program that does the following:

• Assigns integer values to EAX, EBX, ECX, EDX, ESI, and EDI

• Uses PUSHAD to push the general-purpose registers on the stack

• Using a loop, the program pops each integer from the

stack and displays it on the screen

Figure

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References

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