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Ics312 set 8 Flow Control Instructions
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tarix | 07.11.2018 | ölçüsü | 112,5 Kb. | | #78726 |
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ICS312 SET 8
Flow Control Instructions
Jump Instructions (1) The unconditional jump instruction, branches to a new address in the program code. JMP NEXT ; jumps to address of "NEXT" Conditional jump instructions test the condition of one or two FLAGS in the flags register, then "jump" to a new location in the program based on the result of the test. Example: TOP: SUB AX, BX ;AX = AX - BX JC EXIT: ;IF AX < BX then jump to EXIT EXIT: MOV AX, 4C00H INT 21H
Jump Instructions (2) CMP Instruction The CMP instruction sets the flags the same as if a subtraction had been performed, but the destination operand is NOT changed. Example (simulating the LOOP instruction): MOV CX, 5 TOP_2: CMP CX, 0 ; CX = 0 ? JE EXIT_: ; IF CX = 0, then EXIT_ SUB AX, BX ; AX = AX - BX DEC CX JMP TOP_2 ;else, repeat until CX = 0 EXIT_: MOV AX, 4C00H INT 21H
Signed versus Unsigned Conditional Jumps Note:In this course we will only make use of signed conditional jumps. Unsigned Conditional Jumps: JA and JB Signed Conditional Jumps: JG or JL Example 1. Will the JG instruction in the following code result in a jump? MOV AL, 80h MOV BL, 7FH CMP AL, BL JG EXIT_ ... Example 2. Will the JA instruction in the following code result in a jump? MOV AL, 80h MOV BL, 7FH CMP AL, BL JA EXIT_ ...
Examples of Flow Control Using Jump Instructions (1)
Examples of Flow Control Using Jump Instructions (2)
Examples of Flow Control Using Jump Instructions (3)
Examples of Flow Control Using Jump Instructions (4)
Examples of Flow Control Using Jump Instructions (5)
Examples of Flow Control Using Jump Instructions (6)
Examples of Flow Control Using Jump Instructions (7)
Examples of Flow Control Using Jump Instructions (8)
Used to implement "for loops" more conveniently than above. Decrements CX register and jumps to specified label as long as CX != 0 after being decremented. Example: Input a number, and display that number of asterisks .CODE START: MOV DL, '*" MOV AH, 1 INT 21H SUB AL, 30H MOVSX CX, AL ; sign extend into CX register MOV AH, 2 TOP_: LOOP TOP_ ; repeats CX times EXIT_: MOV AX, 4C00H INT 21H
MOVSX Instruction
JMP Instructions For signed arithmetic J[N] {G | L | Z } [E] eg: JG, JGE, JNZ, JLE For unsigned arithmetic (not used in this course) J[N] {A | B | Z } [E] eg: JA, JAE, JNZ, JBE
Example on JE
Example on JGE
Example on JE & JL
Example on JCXZ
Textbook Reading (Jones):
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