(Flow Control Instructions) Assembly Language Lecture 5 & 6 (Flow Control Instructions)

Lecture Outline Branching structures: IF-THEN IF-THEN-ELSE Unconditional Jumps Flow Control Instructions 1

Branching Structures In high-level languages, branching structures enable a program to take different paths, depending on conditions. Branching structures: IF-THEN IF-THEN-ELSE CASE High-Level Language Structures 3

Introduction Jump instructions can be used to implement branches and loops. Because the jumps are so primitive, it is difficult to code an algorithm with them without some guidelines. High-level language structures could be used as a guideline when coding in assembly language. High-Level Language Structures 2

True-branch statements IF-THEN Pseudocode: IF condition is true THEN execute true-branch statements END_IF true or false False condition True True-branch statements High-Level Language Structures 4

IF-THEN Example: replace the number in AX by its absolute value. Solution: Pseudocode: IF AX < 0 THEN replace -AX by AX END_IF It can be coded as follows: ; if AX < 0 ; then CMP AX, 0 ; AX < 0 JNL END_IF ; no, exit NEG AX ; yes, change sign END_IF: High-Level Language Structures 5

The CMP Instruction The jump condition is often provided by the CMP (compare) instruction Syntax: CMP destination, source Compares by computing destination contents minus source contents. The result is not stored, but the flags are affected. Destination may not be a constant. CMP is just like SUB, except that destination is not changed. Flow Control Instructions 10

Conditional Jumps Syntax Jxxx destination_label Example JNZ PRINT_LOOP If the condition for the jump is true, the next instruction to be executed is the one at destinaltion_label (PRINT_LOOP), which may precede or follow the jump instruction itself. If the condition is false, the instruction immediately following the jump is done next. For JNZ, the cindition is that the result of the previous operation is not zero. Flow Control Instructions 4

Conditional Jumps Signed Jumps: used for signed interpretations. Symbol Description Condition for Jumps JG/JNLE jump if grater than ZF = 0 & SF = OF jump if not less than or equal JGE/JNL jump if grater than or equal SF = OF jump if not less than JL/JNGE jump if less than SF <> OF jump if not greater than or equal JLE/JNG jump if less than or equal ZF = 1 or SF <> OF jump if not grater than Flow Control Instructions 7

Conditional Jumps Unsigned Jumps: used for unsigned interpretations. Symbol Description Condition for Jumps JA/JNBE jump if above CF = 0 & ZF = 0 jump if not below or equal JAE/JNB jump if above or equal CF = 0 jump if not below JB/JNAE jump if below CF = 1 jump if not above or equal JBE/JNA jump if below or equal CF = 1 or ZF = 1 jump if not above Flow Control Instructions 8

Conditional Jumps Single Flag Jumps: operates on settings of individual flags. Symbol Description Condition for Jumps JE/JZ jump if equal/ jump if equal to 0 ZF = 1 JNE/JNZ jump if not equal/ jump if not 0 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 sign negative SF = 1 JNS jump if nonnegative sign SF = 0 JP/JPE jump if parity even PF = 1 JNP/JPO jump if parity odd PF = 0 Flow Control Instructions 9

How the CPU Implements a Conditional Jump The CPU looks at the FLAGS register. If the conditions for the jump are: True: the CPU adjusts the IP to point to the destination_label, so that the instruction at this label will be done next. False: the IP is not altered; this means that the next instruction in line will be done. Flow Control Instructions 6

Unconditional Jumps - The JMP Instruction The JMP (jump) instruction causes an unconditional transfer of control (unconditional jump). Syntax: JMP destination a label in the same segment as the JMP itself Flow Control Instructions 15

Unconditional Jumps - The JMP Instruction JMP can be used to get around the range restriction of a conditional jump. Example: TOP: ; body of the loop DEC CX JNZ BOTTOM JMP EXIT BOTTOM: JMP TOP EXIT: MOV AX, BX If the loop body contains so many instructions that label top is out of range for JNZ TOP: ; body of the loop DEC CX JNZ TOP MOV AX, BX Flow Control Instructions 16

IF-THEN-ELSE Pseudocode: IF condition is true THEN execute true-branch statements ELSE execute false-branch statements END_IF False condition True False-branch statements True-branch statements High-Level Language Structures 6

IF-THEN-ELSE Example: Suppose AL and BL contain extended ASCII characters. Display the one that comes first in the character sequence. Solution: Pseudocode: IF AL <= BL THEN display the character in AL ELSE display the character in BL END_IF continue High-Level Language Structures 7

IF-THEN-ELSE It can be coded as follows: ; if AL <= BL CMP AL, BL ; AL <= BL? JNBE ELSE_ ; no, display char in BL ; AL <= BL MOV DL, AL ; move char to be displayed JMP DISPLAY ; go to display ELSE_: ; BL < AL MOV DL, BL DISPLAY: MOV AH, 2 ; prepare to display INT 21h ; display it High-Level Language Structures 8

Branches with compound Conditions Sometimes the branching condition in an IF or CASE takes the form: condition_1 AND condition_2 or condition_1 OR condition_2 where condition_1 and condition_2 are either true or false. AND condition OR condition High-Level Language Structures 14

AND Condition An AND condition is true if and only if all conditions are true. Example: Read a character, and if it’s an uppercase letter, display it. Solution: Pseudocode: Read a character (into AL) IF ('A' <= character) and (character <= 'Z') THEN display character END_IF continue High-Level Language Structures 15

AND Condition It can be coded as follows: ; read a character ; if ('A' <= char) and (char <='Z') ; then display char MOV AH,1 ; prepare to read INT 21h ; char in AL CMP AL, 'A' ; char >= 'A'? JNGE END_IF ; no, exit CMP AL, 'Z' ; char <= 'Z'? JNLE END_IF ; no, exit MOV DL, AL ; get char MOV AH, 2 ; prepare to display INT 21h ; display char END_IF: High-Level Language Structures 16

OR Condition An OR condition is true if at least one of the conditions is true. Example: Read a character. If it’s 'y' or 'Y', display it; otherwise, terminate the program. Solution: Pseudocode: Read a character (into AL) IF (character = 'y') or (character = 'Y') THEN display character ELSE terminate the program END_IF continue High-Level Language Structures 17

OR Condition It can be coded as follows: ; read a character ; if (char = 'y') or (char = 'Y') MOV AH,1 ; prepare to read INT 21h ; char in AL CMP AL, 'y' ; char = 'y'? JE THEN ; yes, go to display it CMP AL, 'Y' ; char = 'Y'? JMP ELSE_ ; no, terminate THEN: MOV DL, AL ; get char MOV AH, 2 ; prepare to display INT 21h ; display char JMP END_IF ; and exit ELSE_: MOV AH, 4Ch INT 21h ; DOS exit END_IF: High-Level Language Structures 18

Working with Characters In working with the standard ASCII character set, either signed or unsigned jumps may be used. Why? Because the sign bit of a byte containing a character code is always zero. Flow Control Instructions 13