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

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

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

3. 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

4. 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

5. Unconditional Jumps - The JMP Instruction • Backward and Forward jumps • Backward: • L10: • ……. • JMP L10 • Forward: • JMP L10 • ……. • L10:

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. 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.

8. 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

9. 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

10. 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

11. continue 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

12. IF-THEN-ELSE It can be coded as follows: ; if AL <= BL ; then 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

13. AND condition OR condition 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.

14. continue 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

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:

16. continue 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

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'? JE THEN ; yes, go to display it 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:

18. Loop Instruction • 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.

19. Loop Instruction • Using the instruction LOOP, a FOR loop can be implemented as • follows: • ; initialize CX to loop_count • TOP: • ; body of the loop • LOOP TOP

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

21. WHILE Loop • This loop depends on a condition. • Pseudocode: • WHILE condition DO • statements • END_WHILE

22. continue WHILE Loop • Example: Write some code to count the number of characters in an • input line. • Solution: • Pseudocode: • initialize count to 0 • read a character • WHILE character <> carriage_return DO • count = count + 1 • read character • END_WHILE

23. WHILE Loop It can be coded as follows: 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 INC DX ; not CR, increment count INT 21h ; read a character JMP WHILE_ ; loop back

24. REPEAT Loop • This loop depends on a condition. • Pseudocode: • REPEAT • Statements • UNTIL conditions

25. REPEAT Loop • Example: write code to read characters until a blank is read • Pseudocode: • REPEAT • Read character • UNTIL character is blank The code is: MOV AH,1 REAPEAT: INT 21H CMP AL,’ ‘ JNE REAPEAT

26. WHILE Versus REPEAT • Use of a WHILE loop or a REPEAT loop is a matter of personal • preference. • A WHILE loop can be bypasses if the terminating condition is • initially false. (a REPEAT loop must be done at least once) • The code for a REPEAT loop is likely to be a little shorter because • there is only one jump. (WHILE loops has two jumps)