Assembly Language
Lecture 5 & 6(Flow Control Instructions)
Flow Control Instructions 1
Lecture Outline
•Branching structures:• IF-THEN• IF-THEN-ELSE
•Unconditional Jumps
3High-Level Language Structures
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
Introduction
2High-Level Language Structures
• 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.
4High-Level Language Structures
IF-THEN
• Pseudocode: IF condition is true THEN execute true-branch statements END_IF
true or false
condition
True-branch statements
TrueFalse
5High-Level Language Structures
IF-THEN
• Example: replace the number in AX by its absolute value.
• Solution: Pseudocode:
IF AX < 0 THEN
replace -AX by AXEND_IF
CMP AX, 0 ; AX < 0JNL END_IF ; no, exit
NEG AX ; yes, change signEND_IF:
It can be coded as follows: ; if AX < 0
; then
Flow Control Instructions 10
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.
4Flow Control Instructions
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.
7Flow Control Instructions
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
8Flow Control Instructions
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
9Flow Control Instructions
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
6Flow Control Instructions
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 15
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 16
Unconditional Jumps - The JMP Instruction
• JMP can be used to get around the range restriction of a conditional jump.
• Example:
TOP:; body of the loopDEC CXJNZ TOPMOV AX, BX
TOP:; body of the loopDEC CXJNZ BOTTOMJMP 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
6High-Level Language Structures
IF-THEN-ELSE
• Pseudocode: IF condition is true THEN execute true-branch statements ELSE
execute false-branch statements END_IF
condition
True-branch statements
TrueFalse
False-branch statements
7High-Level Language Structures
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 BLEND_IF
continue
8High-Level Language Structures
IF-THEN-ELSE
CMP AL, BL ; AL <= BL?JNBE ELSE_ ; no, display char in BL
; AL <= BLMOV DL, AL ; move char to be displayedJMP DISPLAY ; go to display
ELSE_: ; BL < ALMOV DL, BL
DISPLAY:MOV AH, 2 ; prepare to displayINT 21h ; display it
It can be coded as follows: ; if AL <= BL
; then
High-Level Language Structures 14
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 15
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 characterEND_IF
continue
High-Level Language Structures 16
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 readINT 21h ; char in AL
CMP AL, 'A' ; char >= 'A'?JNGE END_IF ; no, exitCMP AL, 'Z' ; char <= 'Z'?JNLE END_IF ; no, exit
MOV DL, AL ; get charMOV AH, 2 ; prepare to displayINT 21h ; display char
END_IF:
High-Level Language Structures 17
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 programEND_IF continue
High-Level Language Structures 18
OR Condition
It can be coded as follows: ; read a character
; if (char = 'y') or (char = 'Y')MOV AH,1 ; prepare to readINT 21h ; char in AL
CMP AL, 'y' ; char = 'y'?JE THEN ; yes, go to display itCMP AL, 'Y' ; char = 'Y'?JE THEN ; yes, go to display itJMP ELSE_ ; no, terminate
THEN:MOV DL, AL ; get charMOV AH, 2 ; prepare to displayINT 21h ; display charJMP END_IF ; and exit
ELSE_:MOV AH, 4ChINT 21h ; DOS exit
END_IF:
Flow Control Instructions 13
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.