Comparing and Branching ifs and loops part A. JMP instruction Consider the forever loop: for ( ; ; ) { … } How can we accomplish this in Assembler?

Comparing and Branching

ifs and loopspart A

JMP instruction

Consider the forever loop:

for ( ; ; ) {…

}

How can we accomplish this in Assembler?

JMP instruction

• jump/branch unconditionally (always)• Transfers program control to a different point

in the instruction stream without recording return information.

• The destination (target) operand specifies the address of the instruction being jumped to.

• This operand can be an immediate value, a general-purpose register, or a memory location.

JMP instruction

JMP instruction


for ( ; ; ) {…

}

How can we accomplish this in Assembler?

JMP instruction


for ( ; ; ) {…

}

lp:…jmp lp

Comparing and branching

Consider a few ifs in Java:

if (x == 1) {…

}

if (x < 5) {…

} else {…

}

if (x == 1) {…

} else if (x >= 1000) {…

} else if (x >= 100) {…

} else {…

}We need to develop a technique to accomplish this in Assembler.

CMP instruction

• Compares the first source operand with the second source operand and sets the status flags in the EFLAGS register according to the results.

• The comparison is performed by subtracting the second operand from the first operand and then setting the status flags in the same manner as the SUB instruction.

• Operation:– temp ← SRC1 − SignExtend(SRC2);– ModifyStatusFlags; (* Modify status flags in the same manner as the

SUB instruction*)

• Flags Affected:– The CF, OF, SF, ZF, AF, and PF flags are set according to the result.

CMP instruction

Jcc instructions (jump/branch conditionally)

• Checks the state of one or more of the status flags in the EFLAGS register (CF, OF, PF, SF, and ZF) and, if the flags are in the specified state (condition), performs a jump to the target instruction specified by the destination operand.


• A condition code (cc) is associated with each instruction to indicate the condition being tested for. If the condition is not satisfied, the jump is not performed and execution continues with the instruction following the Jcc instruction.


• Notes:

– The terms “less” and “greater” are used for comparisons of signed integers.

– The terms “above” and “below” are used for unsigned integers.


Note: je and jz are exactly the same but are provided for readability.

Note: JGE and JNL are exactly the same but are provided for readability.

Most useful Jcc’s

• JE / JZ• JG• JGE• JL• JLE• JNE / JNZ



if (x == 1) {…

}

if (x < 5) {…

} else {…

}

if (x == 1) {…

} else if (x >= 1000) {…

} else if (x >= 100) {…

} else {…

}So how can we code each of these in assembler?



if (x == 1) {…

}



if (x == 1) {…

}

cmp x, 1jne more…

more:



if (x == 1) {…

}

cmp x, 1jne more…

more:Jumps (takes the branch) only when ZF=0.



if (x == 1) {…

}

cmp x, 1jne more…

more:Jumps (takes the branch) only when ZF=0.

jnz also takes the branch when ZF=0.



if (x == 1) {…

}

cmp x, 1jne more…

more:

Avoid this:

cmp x, 1je doItjne more

doIt:…

more:



if (x < 5) {…

} else {…

}



if (x < 5) {…

} else {…

}

One possible solution:

cmp x, 5jnl else1…jmp end1

else1:…

end1:



if (x < 5) {…

} else {…

}

Another possible solution:

cmp x, 5jge else1

;same!…jmp end1

else1:…

end1:



if (x == 1) {…

} else if (x >= 1000) {…

} else if (x >= 100) {…

} else {…

}



if (x == 1) {…

} else if (x >= 1000) {…

} else if (x >= 100) {…

} else {…

}

cmp x, 1jne elif1…jmp done

elif1:cmp x, 1000jl elif2 ; or jnge…jmp done

elif2:cmp x, 100jl el1 ;or jnge…jmp done

el1:…

done:



if (x > 2 && x <= 5) {…

} else {…

}



if (x > 2 && x <= 5) {…

} else {…

}

cmp x, 2jle elcmp x, 5jg el…jmp done

el:…

done:



if (x < 5 || y > 2) {…

} else {…

}



if (x < 5 || y > 2) {…

} else {…

}

cmp x, 5jl yescmp y, 2jle no

yes:…jmp done

no:…

done:



if (x != y) {…

} else {…

}

(Hint: We don’t have cmp m32, m32!)



if (x != y) {…

} else {…

}

mov eax, xcmp eax, yje el…jmp done

el:…

done:

Avoid double jumps!if (x>10) {

i = 12;} else {

i = 9;}correct:

cmp x, 10jle nomov i, 12jmp done

no:mov i, 9

done:

wrong:cmp x, 10jg yesjmp no

yes:mov i, 12jmp done

no:mov i, 9jmp done

done:

ADVANCED TOPIC: DISJUNCTIONDanger, Will Robinson!

Disjunction

• Java (and other languages as well) support a variety of similar boolean operators:

if (a && b) …

if (a & b) …

if (a || b) … Are they exactly the same, orare they different?

if (a | b)…

Disjunction

• http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.23

– “The && operator is like & (§15.22.2), but evaluates its right-hand operand only if the value of its left-hand operand is true.”

– “The || operator is like | (§15.22.2), but evaluates its right-hand operand only if the value of its left-hand operand is false.”

http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.23

http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.23

Disjunction

1. How can we demonstrate that this is true in Java?

2. How can we implement this in Assembler?

Disjunction

public static void main ( String[] s ) {if ( true || false ) {

System.out.println( "main: in first if" );}

if ( true | false ) {System.out.println( "main: in second if" );

}}

These are the cases we wish to test. But we need more.

Disjunctionprivate static boolean T ( ) {

System.out.println( "T()" );return true;

}private static boolean F ( ) {

System.out.println( "F()" );return false;

}public static void main ( String[] s ) {

if ( true || false ) {System.out.println( "main: in first if" );}

if ( true | false ) {System.out.println( "main: in second if" );}

}

How can these functions help?






if ( T() || F() ) {System.out.println( "main: in first if" );}

if ( T() | F() ) {System.out.println( "main: in second if" );}

}

How can these functions help?

DISASSEMBLY, DISJUNCTION, AND THE JVM

Disjunction

• Let’s look at some JVM (Java Virtual Machine) code.

• javap (see http://download.oracle.com/javase/1,5.0/docs/tooldocs/windows/javap.html) run on a .class file will disassemble it for us into JVM code.

javap –c test

• The JVM spec can be found here: http://java.sun.com/docs/books/jvms/.

http://download.oracle.com/javase/1,5.0/docs/tooldocs/windows/javap.html

http://java.sun.com/docs/books/jvms/

Disjunction

public static void main(java.lang.String[]); Code: 0: invokestatic #6; //Method T:()Z 3: ifne 12 //br if true 6: invokestatic #7; //Method F:()Z 9: ifeq 20 //br if false

12: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream; 15: ldc #8; //String main: in first if 17: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

20: invokestatic #6; //Method T:()Z 23: invokestatic #7; //Method F:()Z 26: ior 27: ifeq 38 //br if false

30: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream; 33: ldc #9; //String main: in second if 35: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

38: return






if ( T() || F() ) {System.out.println( "main: in first if" );

}

if ( T() | F() ) {System.out.println( "main: in second if" );

}}

public static void main(java.lang.String[]); Code: 0: invokestatic #6; //Method T:()Z 3: ifne 12 6: invokestatic #7; //Method F:()Z 9: ifeq 20

12: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream; 15: ldc #8; //String main: in first if 17: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

20: invokestatic #6; //Method T:()Z 23: invokestatic #7; //Method F:()Z 26: ior 27: ifeq 38

30: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream; 33: ldc #9; //String main: in second if 35: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

38: return

NEXT TOPIC: LOOPS

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Comparing and Branching ifs and loops part A. JMP instruction Consider the forever loop: for ( ; ; ) { … } How can we accomplish this in Assembler?