Lecture 5 Program Logic and Control

Transcription

1 Lecture 5 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 SHORT NEAR FAR JMP YES YES YES JXXX(conditional jump) YES YES NO LOOP YES NO NO CALL NlA YES YES

4 Unconditional Jumps - The JMP Instruction The JMP (jump) instruction causes an unconditional transfer of control (unconditional jump). Syntax: JMP destination JMP SHORT/NEAR/FAR address 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 Example JNZ destination_label 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 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

12 IF-THEN-ELSE It can be coded as follows: ; if AL <= BL CMP AL, BL ; AL <= BL? JNBE ELSE_ ; no, display char in BL ; then ; 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 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.

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

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:

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

17 OR Condition It can be coded as follows: ; read a character MOV AH,1 ; prepare to read INT 21h ; char in AL ; if (char = 'y') or (char = 'Y') 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: LOOP SHORT address 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 In for loop the loop statements are repeated a known number of times.

20 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 ; what if CX =0? MOV CX, 80 MOV AH, 2 MOV DL, '*' JCXZ SKIP ;jump if CX=0 TOP: INT 21h LOOP TOP SKIP:

21 WHILE Loop This loop depends on a condition. The condition is checked at the top; if true the statements are executed; if false program goes into whatever follow. Pseudocode: WHILE condition DO statements END_WHILE

22 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 continue

23 WHILE Loop It can be coded as follows: 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 END_WHILE:

24 REPEAT Loop This loop depends on a condition. the statements are executed then the condition is checked. If true the loop terminates; if false, control branches to the top of the loop. 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: REAPEAT: MOV AH,1 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)