Conditional Jumps

1. Conditional Jumps • Jconddestination • Condrefers to a flag condition • Destination should be a local label • Destination must be within –128 to +127 bytes from the current location • Lots of different conditional jump instructions • Four types: • Specific flag comparisons • Equality between operands, or the value of ECX • unsigned comparisons • signed comparisons

2. Conditional Structures • There are not any high-level logic structures in the IA-32 Instruction set. • But any logic structure can be implemented using a combination of comparisons and jumps.

3. Implementing any high-level logic structure • Two steps are needed to execute • An operation such as CMP, AND, or SUB • To modify the flags • A conditional jump instruction tests the flags and causes a branch to a new address. • CMP al, 0 ;sets the zero flag if al = 0 jz L1 ;jump to L1 if ZF = 1 • AND dl, 1011 0000b jnz L2 ;jump to L2 if ZF = 0

4. Jumps Based on Specific Flag Values(10)

5. Jumps Based on Equality(4)

6. Jumps Based on Unsigned Comparisons(8)

7. Jumps Based on Signed Comparisons(8)

8. Conditional Jump Applications

9. In Class Problems • Will the following code jump to the label named Target? mov ax, 8109h cmp ax, 26h jg Target ??? • jg is a signed conditional jump (8109h < 26h) so no jump

10. In Class Problems • Will the following code jump to the label named Target? mov ax, -30 cmp ax, -50 jg Target ??? • jg → signed conditional jump (-30 > -50) so jump is taken

11. In Class Problems • Will the following code jump to the label named Target? mov ax, -42 cmp ax, 26 ja Target ??? • ja → unsigned conditional jump -42 = D6h, 26 = 1Ah D6 > 1A so jump is taken

12. In Class Problems • Write instructions that jump to label L1 when the unsigned integer in DX is less than or equal to the integer in CX. • cmp dx, cx jbe L1

13. In Class Problems • Write instructions that jump to label L2 when the signed integer in AX is greater than the integer in CX. • cmp ax, cx jg L2

14. In Class Problems • Write instructions that clear bits 0 and 1 in AL. If the destination operand is equal to zero, jump to label L3. Otherwise, jump to label L4. • AND AL, 0FCh JZ L3 JMP L4

15. Scanning an Array • A common task is to search for specific values in an array. • When the first match is found, it is common to either display its value or return a pointer to its location. • Example • ArryScan.asm looks for the first nonzero value in an array of 16-bit integers. If it finds one, it displays the value; otherwise, it displays a message stating that a value could not be found.

16. Application: String Encryption • XOR can be used twice to provide simple data encryption. • XOR a message string (plain text) with a string (key) to encrypt. • XOR the encrypted message (cipher text) with a string to decrypt and produce the original string • Symmetric encryption • A process where the same key is used for both encryption and decryption. • Example Encrypt.asm

17. Conditional Loop Instructions • LOOPZ – Loop if zero flag is set and the unsigned value of ECX is greated that zero. • The destination label must be between –128 and +127 bytes from the location of the next instruction. • LOOPZ destination

18. Conditional Loop Instructions • LOOPE – loop if equal shares the same circuitry as LOOPZ. • ECX = ECX –1 • If ECX > 0 and ZF =1, jump to destination

19. Conditional Loop Instructions • LOOPNZ – loop if not zero • LOOPNE – loop if not equal to • ECX = ECX –1 • If ECX > 0 and ZF =0, jump to destination • Example • Loopnz.asm scans each number in an array until a positive number is found (signed number declaration is used).

20. If ( op1 == op2) { X = 1; Y = 2; } Mov eax, op1 Cmp eax, op2 Je L1 Jmp L2 L1: Mov X, 1 Mov Y, 2 L2: Block-Structured IF Statements

21. If ( op1 == op2) { X = 1; Y = 2; } Else { X = 4; Y = 5; } Mov eax, op1 Cmp eax, op2 Je L1 Jmp L2 L1: Mov X, 1 Mov X, 2 Jmp L3 L2: Mov, X, 4 mov X, 5 L3: IF, Else Statements

22. If (al > bl) AND (bl > cl) { x = 1; } Cmp al, bl jbe next cmp bl, cl jbe next mov X, 1 Next: Logical AND OperatorFinding alternate ways to implement expressions may reduce the amount of code needed. Using JBE instead of JA:

23. If (al > bl) OR (bl > cl) { x = 1; } Cmp al, bl ja L1 cmp bl, cl jbe next L1: mov X, 1 Next: Logical OR OperatorThere are several ways to implement expressions

24. While (val1 < val2) { val1++; val2--; } mov eax, val1 While: cmp eax, val2 jnl endwhile inc eax dec val2 jmp while Endwhile: mov val1, eax While Loop

25. Table-Driven Selections • A way of using a table lookup to replace a multiway selection structure. • Create a table containing lookup values and the offsets of labels or procedures, and use a loop to search the table. • Link to ProcTble.asm

26. Part of a Table .data CaseTable BYTE ‘1’ ;lookup value DWORD Process_1 BYTE ‘2’ DWORD Process_2 ….. Lookup value ‘1’ 00000120 ‘2’ 00000130 ‘3’ 00000140 Address of process_2

27. Application: Finite State Machines • We studied FSMs in Digital • A specific instance of a more general type of structure called a directed graph (or digraph).

28. Validating an input string

29. Validating a Signed Integer

30. Translation of FSMs to Assembly Language Code • Each state in the diagram is represented in the program by a label. • The following actions are performed at each label: • A call to an input procedure reads the next character from input. • If the state is a terminal state, check to see if the user has pressed the Enter key to end the input • One or more compare instructions check for each possible transition leading away from the state. Each comparison is followed by a conditional jump instruction.