Flow Control Instructions 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? 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

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 CASE_C: ; code for case C goes here 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

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_: JCXZ EXIT_ ; If CX = 0, exit MOV DL, AL MOV AH, 2 INC AL LOOP TOP_ ; repeats for 10 times EXIT_: MOV AX, 4C00H

Examples of Flow Control Using Jump Instructions (7) WHILE Loops input Al while AL != ‘Q’ output AL end while START: MOV AH, 1 INT 21H CMP AL, 'Q' JE EXIT_ ; repeat while AL != 'Q' MOV DL, AL MOV AH, 2 JMP start EXIT_: MOV AX, 4C00H

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 = ‘[‘ (ascii code 5Bh) MOV AH, 2 MOV DL, 'A' TOP_: INT 21H INC DL CMP DL, '[' JNE top_ ; repeat until DL = '[' EXIT_: MOV AX, 4C00H

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

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

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 lesscase: MOV DL, ‘-’ display: MOV AH, 2 INT 21H

Example on JCXZ JCXZ – Jump if cx is zero, cx used as a counter Assume that CX contains some number and you want to Output this number of asterisks JCXZ OUT place: MOV AH, 2 MOV DL, ‘*’ INT 21H LOOP place OUT: …

Textbook Reading (Jones): Sections 5.1-5.4 Homework: Ex 5.1, p. 109, nos. 1-4 Ex 5.3, p. 115, no. 1