Transcript IF-THEN - CS 321

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
IF-THEN
• Pseudocode:
IF condition is true true or false
THEN
execute true-branch statements
END_IF
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
CMP AX, 0
JNL END_IF
; then
NEG AX
END_IF:
High-Level Language Structures
; AX < 0
; no, exit
; yes, change sign
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
JG/JNLE
JGE/JNL
JL/JNGE
JLE/JNG
Description
jump if grater than
jump if not less than or equal
jump if grater than or equal
jump if not less than
jump if less than
jump if not greater than or equal
jump if less than or equal
jump if not grater than
Flow Control Instructions
Condition for Jumps
ZF = 0 & SF = OF
SF = OF
SF <> OF
ZF = 1 or SF <> OF
7
Conditional Jumps
• Unsigned Jumps: used for unsigned interpretations.
Symbol
JA/JNBE
JAE/JNB
JB/JNAE
JBE/JNA
Description
jump if above
jump if not below or equal
jump if above or equal
jump if not below
jump if below
jump if not above or equal
jump if below or equal
jump if not above
Flow Control Instructions
Condition for Jumps
CF = 0 & ZF = 0
CF = 0
CF = 1
CF = 1 or ZF = 1
8
Conditional Jumps
• Single Flag Jumps: operates on settings of individual flags.
Symbol
JE/JZ
JNE/JNZ
JC
JNC
JO
JNO
JS
JNS
JP/JPE
JNP/JPO
Description
jump if equal/ jump if equal to 0
jump if not equal/ jump if not 0
jump if carry
jump if no carry
jump if overflow
jump if no overflow
jump if sign negative
jump if nonnegative sign
jump if parity even
jump if parity odd
Flow Control Instructions
Condition for Jumps
ZF = 1
ZF = 0
CF = 1
CF = 0
OF = 1
OF = 0
SF = 1
SF = 0
PF = 1
PF = 0
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
Flow Control Instructions
a label in the same segment as the JMP itself
15
Unconditional Jumps - The JMP Instruction
• JMP can be used to get around the range restriction of a conditional
jump.
• Example:
TOP:
TOP:
; body of the loop
DEC CX
JNZ TOP
MOV AX, BX
Flow Control Instructions
If the loop body
; body of the loop
contains so many
DEC CX
instructions that
JNZ BOTTOM
label top is out of
JMP EXIT
range for JNZ
BOTTOM:
JMP TOP
EXIT:
MOV AX, BX
16
IF-THEN-ELSE
• Pseudocode:
IF condition is true
THEN
execute true-branch statements
ELSE
execute false-branch statements
END_IF
False
False-branch statements
High-Level Language Structures
condition
True
True-branch statements
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
JNBE ELSE_
; then
MOV DL, AL
JMP DISPLAY
ELSE_:
MOV DL, BL
DISPLAY:
MOV AH, 2
INT 21h
High-Level Language Structures
; AL <= BL?
; no, display char in BL
; AL <= BL
; move char to be displayed
; go to display
; BL < AL
; prepare to display
; display it
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
AND condition
OR condition
where condition_1 and condition_2 are either true or false.
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
MOV AH,1
; prepare to read
INT 21h
; char in AL
; if ('A' <= char) and (char <='Z')
CMP AL, 'A'
; char >= 'A'?
JNGE END_IF
; no, exit
CMP AL, 'Z'
; char <= 'Z'?
JNLE END_IF
; no, exit
; then display char
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
MOV AH,1
INT
21h
; prepare to read
; char in AL
CMP
JE
CMP
JE
JMP
AL, 'y'
THEN
AL, 'Y'
THEN
ELSE_
; char = 'y'?
; yes, go to display it
; char = 'Y'?
; yes, go to display it
; no, terminate
MOV
MOV
INT
JMP
DL, AL
AH, 2
21h
END_IF
; get char
; prepare to display
; display char
; and exit
; if (char = 'y') or (char = 'Y')
THEN:
ELSE_:
MOV AH, 4Ch
INT 21h
END_IF:
High-Level Language Structures
; DOS exit
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