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ICS312 SET 8

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

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ICS312 SET 8

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  1. ICS312 SET 8 Flow Control Instructions

  2. 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

  3. 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

  4. 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

  5. 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_ ...

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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.

  16. 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

  17. 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, 0 REPEAT: MOV AH, 1 INT 21H CMP AL, 0DH JE OUT INC BL JMP REPEAT OUT: …

  18. 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: …

  19. 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

  20. Example on JCXZ JCXZ – 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: …

  21. 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.

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