Assembly Language Lecture 5 & 6 (Flow Control Instructions)

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

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

3. 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

4. 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

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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

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

14. 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 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 TOP MOV AX, BX Flow Control Instructions 16

15. 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

16. 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 High-Level Language Structures 7

17. 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 High-Level Language Structures 8

18. 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. High-Level Language Structures 14

19. 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 High-Level Language Structures 15

20. 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

21. 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 High-Level Language Structures 17

22. 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: High-Level Language Structures 18

23. 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