Assembly Language Lecture 5

Assembly Language Fundamentals of Assembly language Conditional Processing Motaz K. Saad Spring 2007 Motaz K. Saad, Dept. of CS

Overview Boolean and Comparison Instructions Conditional Jumps Conditional Loop Instructions Conditional Structures Application: Finite-State Machines Decision Directives Motaz K. Saad, Dept. of CS

Status Flags - Review The Zero flag is set when the result of an operation equals zero. The Carry flag is set when an instruction generates a result that is too large for the destination operand. The Sign flag is set if the destination operand is negative, and it is clear if the destination operand is positive. The Overflow flag is set when an instruction generates an invalid signed result. Motaz K. Saad, Dept. of CS

AND Instruction Performs a Boolean AND operation between each pair of matching bits in two operands Syntax: AND destination, source (same operand types as MOV) AND Motaz K. Saad, Dept. of CS

OR Instruction Performs a Boolean OR operation between each pair of matching bits in two operands Syntax: OR destination, source OR Motaz K. Saad, Dept. of CS

XOR Instruction Performs a Boolean exclusive-OR operation between each pair of matching bits in two operands Syntax: XOR destination, source XOR XOR is a useful way to toggle (invert) the bits in an operand. Motaz K. Saad, Dept. of CS

NOT Instruction Performs a Boolean NOT operation on a single destination operand Syntax: NOT destination NOT Motaz K. Saad, Dept. of CS

TEST Instruction Performs a nondestructive AND operation between each pair of matching bits in two operands No operands are modified, but the Zero flag is affected. Example: jump to a label if either bit 0 or bit 1 in AL is set. test al,00000011b jnz ValueFound Example: jump to a label if neither bit 0 nor bit 1 in AL is set. test al,00000011b jz ValueNotFound Motaz K. Saad, Dept. of CS

CMP Instruction (1 of 3) Compares the destination operand to the source operand Nondestructive subtraction of source from destination (destination operand is not changed) Syntax: CMP destination, source Example: destination == source mov al,5 cmp al,5 ; Zero flag set Motaz K. Saad, Dept. of CS Example: destination < source mov al,4 cmp al,5 ; Carry flag set

CMP Instruction (2 of 3) Example: destination > source mov al,6 cmp al,5 ; ZF = 0, CF = 0 (both the Zero and Carry flags are clear) Motaz K. Saad, Dept. of CS

CMP Instruction (3 of 3) Example: destination > source mov al,5 cmp al,-2 ; Sign flag == Overflow flag The comparisons shown here are performed with signed integers. Motaz K. Saad, Dept. of CS Example: destination < source mov al,-1 cmp al,5 ; Sign flag != Overflow flag

Conditional Jumps Jumps Based On . . . Specific flags Equality Unsigned comparisons Signed Comparisons Applications Motaz K. Saad, Dept. of CS

J cond Instruction A conditional jump instruction branches to a label when specific register or flag conditions are met Examples: JB, JC jump to a label if the Carry flag is set JE, JZ jump to a label if the Zero flag is set JS jumps to a label if the Sign flag is set JNE, JNZ jump to a label if the Zero flag is clear JcxZ jumps to a label if cx equals 0 Motaz K. Saad, Dept. of CS

J cond Ranges Prior to the 386: jump must be within –128 to +127 bytes from current location counter IA-32 processors: 32-bit offset permits jump anywhere in memory Motaz K. Saad, Dept. of CS

Jumps Based on Specific Flags Motaz K. Saad, Dept. of CS

Jumps Based on Equality Motaz K. Saad, Dept. of CS

Jumps Based on Unsigned Comparisons Motaz K. Saad, Dept. of CS

Jumps Based on Signed Comparisons Motaz K. Saad, Dept. of CS

Conditional Structures Block-Structured IF Statements Compound Expressions with AND Compound Expressions with OR WHILE Loops Switch Selection Motaz K. Saad, Dept. of CS

Your turn . . . Implement the following pseudocode in assembly language. All values are unsigned: Cmp bx,cx ja next Mov ax,5 Mov dx,6 next: If( bx <= cx ) { ax = 5; dx = 6; } Motaz K. Saad, Dept. of CS

Block-Structured IF Statements Assembly language programmers can easily translate logical statements written in C++/Java into assembly language. For example: mov ax,op1 cmp ax,op2 jne L1 mov X,1 jmp L2 L1: mov X,2 L2: if( op1 == op2 ) X = 1; else X = 2; Motaz K. Saad, Dept. of CS

Your turn . . . Implement the following pseudocode in assembly language mov ax,var1 cmp ax,var2 jle L1 mov var3,6 mov var4,7 jmp L2 L1: mov var3,10 L2: if( var1 <= var2 ) var3 = 10; else { var3 = 6; var4 = 7; } Motaz K. Saad, Dept. of CS

Compound Expression with AND (1 of 3) When implementing the logical AND operator, consider that HLLs use short-circuit evaluation In the following example, if the first expression is false, the second expression is skipped: if (al > bl) AND (bl > cl) X = 1; Motaz K. Saad, Dept. of CS

Compound Expression with AND (2 of 3) cmp al,bl ; first expression... ja L1 jmp next L1: cmp bl,cl ; second expression... ja L2 jmp next L2: ; both are true mov X,1 ; set X to 1 next: if (al > bl) AND (bl > cl) X = 1; This is one possible implementation . . . Motaz K. Saad, Dept. of CS

Compound Expression with AND (3 of 3) cmp al,bl ; first expression... jbe next ; quit if false cmp bl,cl ; second expression... jbe next ; quit if false mov X,1 ; both are true next: if (al > bl) AND (bl > cl) X = 1; But the following implementation uses 29% less code by reversing the first relational operator. We allow the program to "fall through" to the second expression: Motaz K. Saad, Dept. of CS

Your turn . . . Implement the following pseudocode in assembly language. All values are unsigned: cmp bx,cx ja next cmp cx,dx jbe next mov ax,5 mov dx,6 next: if( bx <= cx && cx > dx ) { ax = 5; dx = 6; } Motaz K. Saad, Dept. of CS

Compound Expression with OR (1 of 2) When implementing the logical OR operator, consider that HLLs use short-circuit evaluation In the following example, if the first expression is true, the second expression is skipped: Motaz K. Saad, Dept. of CS if (al > bl) OR (bl > cl) X = 1;

Compound Expression with OR (1 of 2) cmp al,bl ; is AL > BL? ja L1 ; yes cmp bl,cl ; no: is BL > CL? jbe next ; no: skip next statement L1: mov X,1 ; set X to 1 next: if (al > bl) OR (bl > cl) X = 1; We can use "fall-through" logic to keep the code as short as possible: Motaz K. Saad, Dept. of CS

Switch Selection CMP BL, 30H ; compare input digit and 0 JL @NEGATIVE ; jump to label @NEGATIVE if digit<0 JZ @ZERO ; jump to label @ZERO if digit=0 JG @POSITIVE ; jump to label @POSITIVE if digit>0 @NEGATIVE: ; jump label MOV DL, 'N‘ JMP @DISPLAY ; jump to label @DISPLAY @ZERO: ; jump label MOV DL, 'Z‘ JMP @DISPLAY ; jump to label @DISPLAY @POSITIVE: ; jump label MOV DL, 'P‘ JMP @DISPLAY ; jump to label @DISPLAY @DISPLAY: ; jump label MOV AH, 2 ; print the character INT 21H Motaz K. Saad, Dept. of CS

WHILE Loops while( ax < bx) ax = ax + 1; A WHILE loop is really an IF statement followed by the body of the loop, followed by an unconditional jump to the top of the loop. Consider the following example: Motaz K. Saad, Dept. of CS top: cmp ax,bx ; check loop condition jae next ; false? exit loop inc ax ; body of loop jmp top ; repeat the loop next: This is a possible implementation:

Your turn . . . top: cmp bx,val1 ; check loop condition ja next ; false? exit loop add bx,5 ; body of loop dec val1 jmp top ; repeat the loop next: while( bx <= val1) { bx = bx + 5; val1 = val1 - 1 } Implement the following loop Motaz K. Saad, Dept. of CS

Decision Directive Using the .IF Directive Runtime Expressions Relational and Logical Operators MASM-Generated Code .REPEAT Directive .WHILE Directive Motaz K. Saad, Dept. of CS

Runtime Expressions .IF ax > bx mov dx,1 .ELSE mov dx,2 .ENDIF .IF, .ELSE, .ELSEIF, and .ENDIF can be used to evaluate runtime expressions and create block-structured IF statements. Examples: MASM generates "hidden" code for you, consisting of code labels, CMP and conditional jump instructions. .IF ax > bx && ax > cx mov dx,1 .ELSE mov dx,2 .ENDIF Motaz K. Saad, Dept. of CS

Relational and Logical Operators Motaz K. Saad, Dept. of CS

MASM-Generated Code mov ax,6 cmp ax,val1 jbe @C0001 mov result,1 @C0001: .data val1 DWORD 5 result DWORD ? .code mov ax,6 .IF ax > val1 mov result,1 .ENDIF Generated code: MASM automatically generates an unsigned jump (JBE) because val1 is unsigned. Motaz K. Saad, Dept. of CS

MASM-Generated Code mov ax,6 cmp ax,val1 jle @C0001 mov result,1 @C0001: .data val1 SDWORD 5 result SDWORD ? .code mov ax,6 .IF ax > val1 mov result,1 .ENDIF Generated code: MASM automatically generates a signed jump (JLE) because val1 is signed. Motaz K. Saad, Dept. of CS

MASM-Generated Code mov bx,5 mov ax,6 cmp ax,bx jbe @C0001 mov result,1 @C0001: .data result DWORD ? .code mov bx,5 mov ax,6 .IF ax > bx mov result,1 .ENDIF Generated code: MASM automatically generates an unsigned jump (JBE) when both operands are registers . . . Motaz K. Saad, Dept. of CS

MASM-Generated Code mov bx,5 mov ax,6 cmp ax,bx jle @C0001 mov result,1 @C0001: .data result SDWORD ? .code mov bx,5 mov ax,6 .IF SDWORD PTR ax > bx mov result,1 .ENDIF Generated code: . . . unless you prefix one of the register operands with the SDWORD PTR operator. Then a signed jump is generated. Motaz K. Saad, Dept. of CS

.REPEAT Directive ; Display integers 1 – 10: mov ax,0 .REPEAT inc ax call WriteDec call Crlf .UNTIL ax == 10 Executes the loop body before testing the loop condition associated with the .UNTIL directive. Example: Motaz K. Saad, Dept. of CS

.WHILE Directive ; Display integers 1 – 10: mov ax,0 .WHILE ax < 10 inc ax call WriteDec call Crlf .ENDW Tests the loop condition before executing the loop body The .ENDW directive marks the end of the loop. Example: Motaz K. Saad, Dept. of CS

Assembly Language Lecture 5

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Assembly Language Lecture 5