The document discusses control transfer instructions in assembly language programming. It describes unconditional jumps that transfer control without regard for conditions, and conditional jumps that transfer control based on a truth value set in flags registers by a comparison instruction. Some examples of conditional jump instructions are JE for equality, JNE for inequality, JL/JNGE for less than, JLE/JNG for less than or equal, and others. The document shows how conditional jumps can be used to implement if statements by comparing values and branching to a label if the condition is true.

1. Group Activity
Sample: scanf(“%c”,&x);
mov eax, 3
mov ebx, 0
mov ecx, x
int 80h

2. Group Activity
1. printf(“%c”,x);
2. sum = x + y;
3. prod = x * y;
(assume x and y are byte-size variables)

3. Group Activity
1. printf(“%c”,x);
mov eax, 4
mov ebx, 1
mov ecx, x
mov edx, 1
int 80h

4. Group Activity
2. sum = x + y;
mov bl, [x]
add bl, [y]
mov [sum], bl

5. Group Activity
3. prod = x * y;
mov al, [x]
mul byte[y]
mov [prod], ax

6. Structured Assembly Language
Programming Techniques
Control Transfer Instructions

7. Objectives
At the end of the discussion, the students
should be able to:
 Implement selection statements in
assembly
 Describe how unconditional jumps and
conditional statements work

8. Control Transfer Instructions
 allows program control to transfer to
specified label
 Unconditional or Conditional
 Unconditional
– executed without regards to any situation
or condition in the program
– transfer of control goes from one code to
another by force
jmp label – unconditional jump

9. Control Transfer Instructions
mov al, 5 next:
add [num1],al
mov eax, 4
jmp next
mov ebx, 1
mov ecx, num2
mov eax, 4 mov edx, 1
mov ebx, 1 int 80h
mov ecx, num1
mov edx, 1
int 80h
(1) (2)

10. Control Transfer Instructions
 Conditional
– a jump carried out on the basis of a
truth value
– the information on which such
decisions are based is contained in
the flags registers

11. Boolean Expressions
 evaluates to True or False
 compares two values
 cmp source1, source2
 Source1 may be a register or memory
 Source2 may be a register, memory or
immediate
 Operands cannot be both memory.
 Operands must be of the same size.

12. Conditional Jumps
 usually placed after a cmp instruction
conditional_jump label
 JE – branches if source1 == source2
 JNE – branches if source1 ≠ source2

13. Conditional Jumps
 Signed Conditional Jump
– JL or JNGE
 branches if source1 < source2
– JLE or JNG
 branches if source1 ≤ source2
– JG or JNLE
 branches if source1 > source2
– JGE or JNL
 branches if source1 ≥ source2

14. Conditional Jumps
 Unsigned Conditional Jumps
– JB or JNAE
 branches if source1 < source2
– JBE or JNA
 branches if source1 ≤ source2
– JA or JNBE
 branches if source1 > source2
– JAE or JNB
 branches if source1 ≥ source2

15. Control Structure: IF Statement
if (boolean) cmp AX, CX
{statements;} jg if_statement
jmp next_statement
if(AX>CX){
BX = DX + 2; if_statement:
} add DX, 2
mov BX, DX
next_statement:
...