Introduction to Assembly Language Programming



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COE 205 Lab Manual Lab 6: Conditional Processing – Page

Lab 6: Conditional Processing

Contents

  1. Unconditional Jump

  2. The Compare Instruction

  3. Conditional Jump Instructions

  4. Finding the Maximum of Three Integers

  5. Implementing High-Level Control Structures

  6. Conditional Loop Instructions

  7. Linear Search of an Integer Array

  8. Indirect Jump and the Switch Statement

6.1 Unconditional Jump

The unconditional jump instruction (jmp) unconditionally transfers control to the instruction located at the target address. The general format is:



jmp target

There are two ways to specify the target address of the jmp instruction: direct and indirect. The most common one is the direct jump. Indirect jumps will be discussed in a later section.

In direct jumps, the target instruction address is specified directly as part of the instruction. As a programmer, you only specify the target address by using a label and let the assembler figure out the jump value of the target instruction. In the following example, jmp L1 and jmp L2 are direct jumps. The first jump jmp L1 is a forward jump, while jmp L2 is a backward jump because the target instruction precedes the jump instruction.
L2:

. . .

jmp L1

. . .

L1:

. . .

jmp L2

6.1.1 Relative Displacement

The address that is specified in a jump instruction is not the absolute address of the target instruction. Rather, a relative displacement is stored in the jump instruction. The relative displacement is the number of bytes between the target instruction and the instruction following the jump instruction. Recall that EIP register points at the next instruction to be executed. Therefore, after fetching a jump instruction, EIP is advanced to point at the next instruction. When the processor executes the jump, it simply performs the following action:



EIP = EIP + relative-displacement

The relative displacement is a signed number stored inside the jump instruction itself. If the number is positive then it is a forward jump. Otherwise, it is a backward jump.

In an indirect jump, the target address is specified indirectly through a register or memory. We will defer their discussion to Section 6.6.

6.2 The Compare Instruction

The compare (CMP) instruction performs an implied subtraction of a source operand from a destination operand. Neither operand is modified. It has the following format:



CMP destination, source

The Overflow, Carry, Sign, and Zero flags are updated as if the subtract instruction has been performed. The main purpose of the compare instruction is to update the flags so that a subsequent conditional jump instruction can test them.



6.2.1 Lab Work: Demonstrating the Compare Instruction

The following program demonstrates the compare instruction and the affected flags.


TITLE Demonstrating the Compare Instruction (cmp.asm)
.686

.MODEL flat, stdcall

.STACK

INCLUDE Irvine32.inc
.data

var1 SDWORD -3056
.code

main PROC

mov eax, 0f7893478h

mov ebx, 1234F678h

cmp al, bl

cmp ax, bx

cmp eax, ebx

cmp eax, var1

exit

main ENDP

END main

Carry the execution of the compare instructions manually by doing subtraction by hand. Guess the values of the flags and write them in the specified boxes.





6.2.2 Lab Work: Assemble and Link cmp.asm

6.2.3 Lab Work: Trace the Execution of Program cmp.exe

Run the 32-bit Windows Debugger, either from the Tools menu in the ConTEXT editor, or by typing: windbg –QY –G cmp.exe at the command prompt. Open the source file cmp.asm from the File menu if it is not already opened. Watch and customize the registers to have the of cf sf zf flags and the eax ebx ax bx al and bl registers on top of the list.

Place the cursor at the beginning of the main procedure and press F7. Now step through the program by pressing F10 and watch the changes in the flags and registers. Observe that the compare instruction does not modify any operand. It only affects the flags. Check the flag answers that you wrote. Make the necessary corrections and understand your mistakes.

6.3 Conditional Jump Instructions

Conditional jump instructions can be divided into four groups:



  • Jumps based on the value of a single arithmetic flag

  • Jumps based on the value of CX or ECX

  • Jumps based on comparisons of signed operands

  • Jumps based on comparisons of unsigned operands

The following is a list of jumps based on the Zero, Carry, Overflow, Sign, and Parity flags.

Mnemonic

Description

Flags

JZ, JE

Jump if Zero, Jump if Equal

ZF = 1

JNZ, JNE

Jump if Not Zero, Jump if Not Equal

ZF = 0

JC

Jump if Carry

CF = 1

JNC

Jump if No Carry

CF = 0

JO

Jump if Overflow

OF = 1

JNO

Jump if No Overflow

OF = 0

JS

Jump if Signed (Negative)

SF = 1

JNS

Jump if Not Signed (Positive or Zero)

SF = 0

JP, JPE

Jump if Parity, Jump if Parity is Even

PF = 1

JNP, JPO

Jump if Not Parity, Jump if Parity is Odd

PF = 0

The following table shows the jumps based on the value of CX and ECX:

Mnemonic

Description

JCXZ

Jump if CX = 0

JECXZ

Jump if ECX = 0

The following table shows a list of signed jumps based on comparisons of signed operands:

Mnemonic

Description

Condition Tested

JG, JNLE

Jump if Greater, Jump if Not Less or Equal

ZF = 0 and SF = OF

JGE, JNL

Jump if Greater or Equal, Jump if Not Less

SF = OF

JL, JNGE

Jump if Less, Jump if Not Greater or Equal

SF ≠ OF

JLE, JNG

Jump if Less or Equal, Jump if Not Greater

ZF = 1 or SF ≠ OF

The following shows a list of unsigned jumps based on comparisons of unsigned operands:

Mnemonic

Description

Condition Tested

JA, JNBE

Jump if Above, Jump if Not Below or Equal

ZF = 0 and CF = 0

JAE, JNB

Jump if Above or Equal, Jump if Not Below

CF = 0

JB, JNAE

Jump if Below, Jump if Not Above or Equal

CF = 1

JBE, JNA

Jump if Below or Equal, Jump if Not Above

ZF = 1 or CF = 1


6.4 Lab Work: Finding the Maximum of Three Integers
TITLE Finding the Maximum of 3 Integers (max.asm)

.686

.MODEL flat, stdcall

.STACK

INCLUDE Irvine32.inc
.data

var1 DWORD -30 ; Equal to FFFFFFE2 (hex)

var2 DWORD 12

var3 DWORD 7

max1 BYTE "Maximum Signed Integer = ",0

max2 BYTE "Maximum Unsigned Integer = ",0
.code

main PROC

; Finding Signed Maximum

mov eax, var1

cmp eax, var2

jge L1

mov eax, var2

L1:

cmp eax, var3

jge L2

mov eax, var3

L2:

lea edx, max1

call WriteString

call WriteInt

call Crlf
; Finding Unsigned Maximum

mov eax, var1

cmp eax, var2

jae L3

mov eax, var2

L3:

cmp eax, var3

jae L4

mov eax, var3

L4:

lea edx, max2

call WriteString

call WriteHex

call Crlf
exit

main ENDP

END main

Analyze the above program and find its output:



frame1

6.4.1 Lab Work: Assemble and Link max.asm

6.4.2 Lab Work: Trace Program max.exe

Run the 32-bit Windows Debugger. Open the source file max.asm from the File menu if it is not already opened. Add a watch to variables var1, var2, var3, and register eax. You can view a register in the Watch window by typing @ before the register name as shown below.



Place the cursor at the beginning of the main procedure and press F7. Now step through the program by pressing F10 and watch the changes in the eax register. Observe when conditional branches are taken (or not taken). Check the console output that you wrote, make the correction, and try to understand your mistakes.



6.5 Implementing High-Level Control Structures

High-level programming languages provide a number of control structures that are used for selection and iteration. Typically, an IF statement is used for selection and a WHILE loop is used for conditional iteration.



6.5.1 Implementing an IF Statement

An IF statement has a Boolean expression followed a list of statements that is performed when the expression is true, and an optional ELSE part when the expression is false.



Examples on IF

Assembly-Language Translation


// One-Way Selection

if (a > b) { . . . } // if part // One-way selection

mov eax, a

cmp eax, b

jle end_if

. . . ; if part

end_if:


// Two-Way Selection

if (a <= b)

{ . . . } // if part

else

{ . . . } // else part

mov eax, a

cmp eax, b

jg else_part

. . . ; if part

jmp end_if

else_part:

. . . ; else part

end_if:



// Short-Circuit AND

if (a > b && a == c) { . . . } // One-way selection

mov eax, a

cmp eax, b

jle end_if

cmp eax, c

jne end_if

. . . ; if part

end_if:


// Short-Circuit OR

if (a > b || a == c ) { . . . }

mov eax, a

cmp eax, b

jg if_part

cmp eax, c

jne end_if

if_part:

. . . ; if part

end_if:

6.5.2 Implementing a WHILE Statement

A WHILE statement has a Boolean expression followed a list of statements that is performed repeatedly as long as the Boolean expression is true. The following table shows two translations for the WHILE statement.



WHILE Statement

Assembly-Language Translation



// First Translation

while (a > b) { . . . }


start_while:

mov eax, a

cmp eax, b

jle end_while

. . . ; while body

jmp start_while

end_while:



// Second Translation

while (a > b) { . . . } // One-way selection

jmp bool_expr

while_body:

. . . ; while body

bool_expr:

mov eax, a

cmp eax, b

jgt while_body

end_while:

In the first translation, the Boolean expression is placed before the loop body, whereas in the second translation, it is placed after the loop body. Both translations are correct, but the second translation is slightly better than the first one. The first translation has one forward conditional jump and one backward unconditional jump, which are executed for each loop iteration, while the second translation has only one conditional backward jump. The forward unconditional jump at the beginning of the second translation is done once.

6.5.3 Lab Work: Translating Nested Control Structures

Translate the following high-level control structure into assembly-language code:




while (a <= b) {

a++;

if (b == c)

a = a + b

else {

b = b - a

c--;

}

}




6.6 Conditional Loop Instructions

LOOP is a non-conditional instruction that uses the ECX register to maintain a repetition count. Register ECX is decremented and the loop repeats until ECX becomes zero.

LOOPZ (Loop if Zero) is a conditional loop instruction that permits a loop to continue while the Zero flag is set and the unsigned value of ECX is greater than zero. This is what happens when the LOOPZ instruction is executed:

ECX = ECX – 1

if (ECX > 0 and ZF == 1) jump to target instruction

LOOPE (Loop if Equal) instruction is equivalent to LOOPZ.

LOOPNZ (Loop if Not Zero) instruction is the counterpart of the LOOPZ instruction. The loop continues while the Zero flag is clear and the unsigned value of ECX is greater than zero. This is the action of the LOOPNZ instruction:

ECX = ECX – 1

if (ECX > 0 and ZF == 0) jump to target instruction

LOOPNE (Loop if Not Equal) instruction is equivalent to LOOPNZ.

These instructions are summarized in the following table:



Instruction

Action

LOOP target

ECX = ECX – 1

If (ECX > 0) jump to target

LOOPZ target

LOOPE target

ECX = ECX – 1

If (ECX > 0 and ZF == 1) jump to target

LOOPNZ target

LOOPNE target

ECX = ECX – 1

If (ECX > 0 and ZF == 0) jump to target

6.7 Lab Work: Linear Search of an Integer Array

The following program demonstrates the use of the LOOPNZ instruction.


TITLE Linear Search (search.asm)
.686

.MODEL flat, stdcall

.STACK
INCLUDE Irvine32.inc
.data

; First element is at index 0

intArray SDWORD 18,20,35,-12,66,4,-7,100,15

item SDWORD -12

FoundStr BYTE " is found at index ", 0

NotFoundStr BYTE " is not found", 0
.code

main PROC

mov ecx, LENGTHOF intArray ; loop counter

mov eax, item ; item to search

mov esi, -1 ; index to intArray
L1:

inc esi ; increment index before search

cmp intArray[4*esi], eax ; compare array element with item

loopnz L1 ; loop as long as item not found

jne notFound ; item not found
found:

call WriteInt ; write item

mov edx, OFFSET FoundStr

call WriteString ; " is found at index "

mov eax, esi

call WriteDec ; Write index

jmp quit
notFound:

call WriteInt ; write item

mov edx, OFFSET NotFoundStr

call WriteString ; " is not found"
quit:

call Crlf

exit

main ENDP

END main

Study the above program and write the Console Output in the specified box.



frame2

6.7.1 Lab Work: Assemble, Link, and Run search.exe

Check your answer in the above program and make the necessary corrections.

Modify the item value in the above program from -12 to 100. Write below the console output. Reassemble, link, and run the modified program and check your answer.

frame3

Repeat the above process but with item equal to -10. Write the Console Output in the box shown below.



frame4

6.8 Indirect Jump and the Switch Statement

So far, we have used direct jump instructions, where the target address is specified in the jump instruction itself. In an indirect jump, the target address is specified indirectly through memory. The syntax of the indirect jump is as follows for the 32-bit FLAT memory model:



jmp mem32 ; mem32 is a 32-bit memory location

6.8.1 Lab Work: Implementing a Switch Statement

Indirect jumps can be used to implement multiway conditional statements, such as a switch statement in a high-level language. As an example, consider the following code:

T
switch (ch) {

case '0':

exit();

case '1':

value++;

break;

case '2':

value--;

break;

case '3':

value += 5;

break;

case '4':

value -= 5;

break;

}
o have an efficient translation of the switch statement, we need to build a jump table of pointers. The input character is converted into an index into the jump table and the indirect jump instruction is used to jump into the correct case to execute. The following assembly-language program is a translation to the above statement. The switch statement is executed repeatedly until the user enters 0, at which point the program terminates.

The program uses a jump table, called table. This table is declared inside and is local to the main procedure. This table is an array of labels (case0, case1, …, case4). Each label is translated into a 32-bit address, which is the address of the instruction that comes after the label. The first jump instruction in the main procedure is used to bypass the table and to start execution at the break label, where value is displayed and the user is prompted to enter a digit between 0 and 4. The input character is checked. If it is out of range (<'0' or >'4'), it is ignored and a new character is read. Otherwise, the input character is echoed on the screen, converted from a character into a number, and then used to index the jump table in the indirect jump. This indirect jump transfers control to the corresponding case label.


TITLE Demonstrating Indirect Jump (IndirectJump.asm)
; This program shows the implementation of a switch statement

; A jump table and indirect jump are used
.686

.MODEL flat, stdcall

.STACK
INCLUDE Irvine32.inc
.data

value SDWORD 0

valuestr BYTE "Value = ",0

prompt BYTE "Enter Selection [Quit=0,Inc=1,Dec=2,Add5=3,Sub5=4]: ",0
.code

main PROC

; Start at break to bypass the jump table

jmp break
; Jump table is an array of labels (instruction addresses)

table DWORD case0, case1, case2, case3, case4
; Implementing a Switch Statement

case0:

exit

case1:

inc value

jmp break

case2:

dec value

jmp break

case3:

add value, 5

jmp break

case4:

sub value, 5

jmp break
break:

; Display value

mov edx, OFFSET valuestr

call WriteString

mov eax, value

call WriteInt

call Crlf
; Prompt for the user to enter his selection

mov edx, OFFSET prompt

call WriteString
; Read input character and check its value

readch:

mov eax,0 ; clear eax before reading

call ReadChar

cmp al, '0'

jb out_of_range ; character < '0'

cmp al, '4'

ja out_of_range ; character > '4'

call WriteChar ; echo character

call Crlf

sub al, 30h ; convert char into number

jmp table[4*eax] ; Indirect jump using table
; Out of range: ignore input and read again

out_of_range:

jmp readch

main ENDP

END main

Determine the Console Output when the user input sequence is 1, 3, 2, 0.



frame6

6.8.2 Lab Work: Assemble, Link, and Run IndirectJump.exe

Check your answer for the above program and make the necessary corrections.



Review Questions

1. Which conditional jump instructions are based on unsigned comparisons?

2. Which conditional jump instruction is based on the contents of the ECX register?

3. (Yes/No) Are the JA and JNBE instructions equivalent?

4. (Yes/No) Will the following code jump to the Target label?

mov ax, -42

cmp ax, 26

ja Target

5. Write instructions that jump to label L1 when the unsigned integer in DX is less than or equal to the unsigned integer in CX.

6. (Yes/No) The LOOPE instruction jumps to a label if and only if the zero flag is clear.

7. Implement the following statements in assembly language, for signed integers:



if (ebx > ecx) X = 1;

if (edx <= ecx && ecx <= ebx) X = 1; else X = -1;

while (ebx > ecx || ebx < edx) X++;

Programming Exercises

1. Write a program that uses a loop to input signed 32-bit integers from the user and computes and displays their minimum and maximum values. The program terminates when the user enters an invalid input.

2. Using the following table as a guide, write a program that asks the user to enter an integer test score between 0 and 100. The program should display the appropriate letter grade. The program should display an error message if the test score is <0 or >100.


90 to 100

85 to 89

80 to 84

75 to 79

70 to 74

65 to 70

60 to 64

55 to 59

0 to 54

A+

A

B+

B

C+

C

D+

D

F

3. Write a program that reads a single character ‘0’ to ‘9’, ‘A’ to ‘F’, or ‘a’ to ‘f’, and then converts it and displays it as a number between 0 and 15. Use the ReadChar procedure to read the character. Do not use the ReadHex procedure. Display an error message if the input character is invalid.

4. Write a program that reads a string of up to 50 characters and stores it in an array. It should then convert each lowercase letter to uppercase, leaving every other character unchanged. The program should output the modified string.

5. Write a program that reads a string of up to 50 characters and stores it in an array. It then asks the user to input a single character and matches this character against all occurrences in the string. Each matched character in the string should be converted into a blank.

6. Write a program that inputs an unsigned integer sum > 1, and then computes and displays the smallest integer n, such that 1 + 2 +…+ n > sum. An error message should be displayed if the input sum ≤ 1.



Prepared by Dr. Muhamed Mudawar © KFUPM –Revised August 2006



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