1. Morgan Kaufmann Publishers Computer Organization and Assembly Language
September 12, 2018
CS 206 D
Computer Organization and Assembly Language
Chapter 1 — Computer Abstractions and Technology

2. Chapter 7 –Program Logic and Control
Lecture 5 and Lecture 6
Chapter 7 –Program Logic and Control

3. Chapter Outline
Short, near and far address
JMP Instruction
The CMP Instruction
Conditional Jump instruction
The Loop instruction
While Loop
REPEAT Loop

4. Short, Near and Far addresses
1- A short address, limited to a distance of -128 to 127 bytes
2- A near address, limited to a distance of -32,768 to 32,767 bytes
3- A far address, which may be within the same segment at a distance over 32K or in other segment
FAR
NEAR
SHORT
YES
JMP NO
JXXX(conditional jump)
LOOP
NlA
CALL

5. Unconditional Jumps - The JMP Instruction
The JMP (jump) instruction causes an unconditional transfer of
control (unconditional jump).
Syntax:
JMP destination
Example:
JMP L10
……..
L10: INC CX
SHORT/NEAR/FAR address
JMP
Backward and Forward jumps
L10:
…….
JMP L10; Backward
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
JMP L10; Forward

6. 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.

7. Flags Carry flag Overflow Parity flag Direction Interrupt enable
Auxiliary flag
Zero
Overflow
Direction
Interrupt enable
Trap
Sign
6 are status/ conditional flags
3 are control flag

8. Flag Register
Conditional flags:
They are set according to some results of arithmetic operation. You do not need to alter the value yourself.
Control flags:
Used to control some operations of the MPU. These flags are to be set by you in order to achieve some specific purposes.
Flag O D I T S Z A P C
Bit no. 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
CF (carry) Contains carry from leftmost bit following arithmetic, also contains last bit from a shift or rotate operation.
Example: ( )+( ) =
SF =1 ,CF=1 ,ZF=0 ,OF=0 ,PF=0

9. Flag Register
OF (overflow) The overflow flag is set to1 if the result of an arithmetic operation is too big for the microprocessor’s maximum word size, otherwise it is reset to 0
DF (direction) Indicates left or right for moving or comparing string data.
IF (interrupt) Indicates whether external interrupts are being processed or ignored.
TF (trap) Permits operation of the processor in single step mode.

10. Flag Register
SF (sign) Contains the resulting sign of an arithmetic operation (1=negative; 0=positive)
ZF (zero) Indicates when the result of arithmetic or a comparison is zero. (1= zero; 0= nonzero)
AF (auxiliary carry) Contains carry out of bit 3 into bit 4 for specialized arithmetic.
PF (parity) is set to 1 to indicate whether the result of the last operation contains either an even number of 1’s (even parity) or an odd number of 1’s (odd parity) depending on the microprocessor..

11. 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 instruction 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 condition is that the result of the previous operation is
not zero.

12. 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

13. 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

14. 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 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

15. 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
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, ; prepare to display
INT 21h ; display it

16. 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
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.
Pseudocode:
Read a character (into AL)
IF ('A' <= character) and (character <= 'Z')
THEN
display character
END_IF
AND condition
OR condition

17. 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:

18. 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

19. 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:

20. Loop Instruction Syntax:
The LOOP instruction can be used to implement a For loop.
Syntax:
The counter for the loop is the register CX, which is initialized to
loop_count.
Execution of the LOOP instruction causes CX to be decremented
automatically.
If (CX < > 0) control transfers to destination_label
else the next instruction after LOOP is done.
; initialize CX to loop_count
TOP:
; body of the loop
LOOP TOP
SHORT address
LOOP

21. FOR Loop Example: Write some code to display a row of 80 stars.
Solution:
Pseudocode:
FOR 80 times DO
display '*'
END_IF
; what if CX =0?
MOV CX, 80
MOV AH, 2
MOV DL, '*'
JCXZ SKIP ;jump if CX=0
TOP:
INT 21h
LOOP TOP
SKIP:
It can be coded as follows:
MOV CX, 80
MOV AH, 2
MOV DL, '*'
TOP:
INT 21h
LOOP TOP

22. WHILE Loop This loop depends on a condition. Pseudocode:
WHILE condition DO
statements
END_WHILE
initialize count to 0
read a character
WHILE
character <> carriage_return DO
count = count + 1
read character
Example: Write some code to count the number of characters in an input line.
WHILE_:
END_WHILE:
MOV DX, 0 ; DX counts characters
MOV AH, 1 ; prepare to read
INT 21h ; character in AL
CMP AL, 0Dh ; CR?
JE END_WHILE ; yes, exit
; not CR, increment count
INC DX
INT 21h ; read a character
JMP WHILE_ ; loop back

23. REPEAT Loop This loop depends on a condition. Pseudocode: REPEAT
Statements
UNTIL conditions
Example: write code to read characters until a blank is read
Read character
UNTIL character is blank
MOV AH,1
REAPEAT:
INT 21H
CMP AL,’ ‘
JNE REAPEAT