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ICS312 SET 8. Flow Control Instructions. Flow Control Instructions. Flow control, or branching, instructions for jumps, loops and conditional jumps allow us to control the execution of program statements and to build high-level programming - PowerPoint PPT Presentation
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ICS312 SET 8
Flow Control Instructions
Flow Control Instructions
Flow control, or branching, instructions for jumps, loops and conditional jumps allow us to control the execution of program statements and to build high-level programming structures such as if-else statements, case statements, and loops in assembler programs
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 JMP TOP ;else, repeat until AX < BX 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
Note: the above code simulates the effect of the LOOP instruction
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)
•IF-THEN-ELSE Structures
START: MOV AH, 1 INT 21H ; input AL CMP AL, 'Q' ; if AL = 'Q' JE EXIT ; then, branch to EXIT MOV AH, 2 ; else output AL MOV DL, AL INT 21H JMP START ; repeat
EXIT: MOV AX, 4C00H INT 21H
Examples of Flow Control Using Jump Instructions (2)
•IF-THEN-ELSE If AL ≤ BL, display BL else display AL
CMP AL, BL ; AL ≤ BL? JG ELSE_ ; no, go to "else" MOV DL, BL ; display BL JMP DISPLAY
ELSE_: ; else, AL > BL MOV DL, AL ; display AL
DISPLAY: ; display result MOV AH, 2 INT 21H
Examples of Flow Control Using Jump Instructions (3)
•CASE Structure MOV AH, 1 INT 21H CMP AL, 'A' JE CASE_A CMP AL, 'B' JE CASE_B CMP AL, 'C' JE CASE_C CMP AL, 'D' JE CASE_D
CASE_A: ; code for case A goes here JMP NEXT_ CASE_B: ; code for case B goes here JMP NEXT_ CASE_C: ; code for case C goes here JMP NEXT_ CASE_D: ; code for case D goes here NEXT_: ; code continues here
Examples of Flow Control Using Jump Instructions (4)
•AND Conditions: Is AL >= 30H and <= 39H? •I.e. is it the code for a decimal digit?
MOV AH, 1 INT 21H CMP AL, 30H ; AL >= 30h? JL ERROR ; no, error CMP AL, 39H ; AL <= 39h? JG ERROR ; no, error
... ; continue: AL >= 30h and AL <= 39h
ERROR: ... ; code for handling an error
Examples of Flow Control Using Jump Instructions (5)
•OR Conditions Input until the character is Q or q.
START: MOV AH, 1 INT 21H CMP AL, 'Q' JE EXIT_ ; if AL = 'Q', then exit CMP AL, 'q' JNE START ; if AL != 'Q' and AL != 'q', repeat input EXIT_: ; if AL = 'Q' or AL = 'q' MOV AX, 4COOH INT 21H
Examples of Flow Control Using Jump Instructions (6)
•FOR Loops Input the value of AL. For I = 1 to 10 output this value+I.
START: MOV AH, 1 INT 21H MOV CX, 10 TOP_: MOV DL, AL MOV AH, 2 INT 21H INC AL LOOP TOP_ ; repeats for 10 times EXIT_: MOV AX, 4C00H INT 21H
Examples of Flow Control Using Jump Instructions (7)
•WHILE Loops input Al while AL != ‘Q’ output AL input Al end while
START: MOV AH, 1 INT 21H CMP AL, 'Q' JE EXIT_ ; repeat while AL != 'Q' MOV DL, AL MOV AH, 2 INT 21H JMP start EXIT_: MOV AX, 4C00H INT 21H
Examples of Flow Control Using Jump Instructions (8)
•REPEAT-UNTIL Loops Set Dl to ‘A’ TOP: output DL add 1 to DL repeat top until DL = ‘[‘ ‘[‘ has ascii code 5Bh, 1 larger than ascii code for ‘Z’
MOV AH, 2 MOV DL, 'A' TOP_: INT 21H INC DL CMP DL, '[' JNE top_ ; repeat until DL = '[' EXIT_: MOV AX, 4C00H INT 21H
Loop Instruction 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
.586 ; required for MOVSX or MOVZX instructions .CODE START: MOV DL, '*" MOV AH, 1 INT 21H SUB AL, 30H MOVSX CX, AL ; sign extend into CX register MOV AH, 2 TOP_: INT 21H ; display asterisks LOOP TOP_ ; repeats CX times EXIT_: MOV AX, 4C00H INT 21H
MOVSX Instruction
Move with sign extension. Copies a byte or word from a source operand to a register, and sign extends into the
upper half of the destination. This is used to copy an 8-bit
or 16-bit operand into a larger destination.
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
Count the number of characters entered which terminate in a CR (0DH), don’t count CR. Use BL to store that number.
MOV BL, 0REPEAT: MOV AH, 1 INT 21H CMP AL, 0DH JE OUT INC BL JMP REPEATOUT: …
Example on JGE
To put the absolute value of AX into AX.
CMP AX, 0 JGE OUT ; jump if greater than or equal NEG AX
OUT: …
Example on JE & JL
If AX < 0, output – ; If AX = 0, output 0; If AX > 0, output +
CMP AX, 0 JE zerocase JL lesscase MOV DL, ‘+’ JMP display zerocase: MOV DL, 0 JMP display lesscase: MOV DL, ‘-’ display: MOV AH, 2 INT 21H
Example on JCXZJCXZ – Jump if cx is zero, cx used as a counter
Assume that CX contains the no. of asterisks to print. One needs to include the case where CX is 0. JCXZ OUT place: MOV AH, 2 MOV DL, ‘*’ INT 21H LOOP place
OUT: …
Textbook Reading (Jones):
Sections 5.1-5.4
Ex 5.1, p. 109, nos. 1a,b,c; 2; 3a,b; 4a,b. Ex 5.3, p. 115, no. 1.