Branch Instruction:
- The branch instruction is used to transfer the control of the program to a new address. This branch can be Conditional or Unconditional. Once the branch is taken the execution of instructions will take place from this new address called branch address. Usually the Conditional branch is taken after the ADD, SUB, INC, DEC and the instructions used to implement the LOOP. When these types of instructions are executed, the CS and IP registers get loaded with new values of CS and IP corresponding to the location where a branch is required. The branch instruction can be one of the following types.
- Unconditional Jump instructions: JMP, CALL, RET, INT N, INTO, IRET, LOOP,
- Conditional Jump instructions: JA/JNBE, JAE/JNB, JB/JNAE, JC, JNC, JE /JZ, JNE /JNZ, JO, JNO, JP/JPE, JG/JNG, JNP/JPO, JG/JNLE, JA /JNL, JL/JNGE, JLE/JNG, JS, JNS
Unconditional Jump instructions
| CALL : Unconditional call | call a subroutine (procedure) from the main program. Address of procedure may be Specified Directly or indirectly. Two types are: Near Call: within +/- 32k DISP. Far Call : anywhere outside the segment In both case CS and IP Change to new address |
| RET : Return from procedure | On execution, the previously stored content of IP and CS Along with flags are retrieved into the CS, IP and Flag registers from the stack and execution of the main programs continues further |
| INT N : Interrupt Type N | When executed, the type byte N is multiplied by 4 and the contents of the IP and CS of the interrupt service routine will be taken from memory Block in 0000 segment. |
| INTO : Interrupt in overflow | This instruction is executed when the overflow flag is set, This is equivalent to a Type 4 interrupt instruction. |
| JMP : Conditional jump | unconditionally transfers the control of execution to the Specified address using an 8 bit ot 16 bit Displacement |
| IRET : Return from stack | When its is executed the values of IP , CS and flags are retrieved from the stack to continue the execution of the main program. |
| LOOP : LOOP Unconditional | This instruction executes the part of the program from the Label or address Specified in the instruction up to the loop instruction CX number of times . At each iteration , CX is decremented automatically and JUMP OF NOT ZERO structure. |
Conditional Jump instructions:
- The following table gives a list of instructions that will take the program to a Specified location if a particular condition is matched.
The Conditional types of instruction test a certain condition such as the flags (CF, PF, SF, ZF, OF, ), on testing for equality/non-equality, above or less etc. and then jump to a new Specified address. The execution the program then starts from the new address. The Different Conditional instructions are given below.
| Instructions | Description | |
| JA/JNBE label | Jump if above /jump if not below nor equal | |
| JAE/JNB label | Jump if above or equal /jump if not below | |
| JB/JNAE label | Jump if below/jump if not above nor equal | |
| JC label | Jump on carry | |
| JE /JZ label | Jump if equal/jump if zero flag is set | |
| JNC label | Jump if no carry | |
| JNE/JNZ label | Jump if not equal/jump if zero (ZF is not set) | |
| JO label | Jump if overflow (OF is set) | |
| JNO label | Jump if no overflow (OF =0) | |
| JP/JPE label | Jump if PF = 1/jump if parity if is even | |
| JNP /JPO label | Jump if not parity PF = 0/jump if parity if is odd | |
| JG/JNLE label | Jump if greater/jump id | |
| JA /JNL label | Jump if above / jump if not less than | |
| JL/JNGE label | Jump if less than /jump if not greater nor equal | |
| JLE/JNG label | Jump if less than and equal to / jump if not greater | |
| JS label | Jump if -ve (SF = 1) | |
| JNS label | Jump if not -ve (SF=0) |
Conditional LOOP instructions:
| Format | Purpose | Example |
| LOOPZ / LOOPE label | Loop through a sequence of instructions from label while ZF = 1 and CX # 0. The loop exits if CX becomes zero or the quantities being compared become unequAL. | LEA BX, ARRAY DEC BX MOV CX, 12 L1: INC BX CMP [BX], 0FFh LOOPE L1 |
| LOOPNZ / LOOPNE Label | Loop through a sequence of instructions from label while ZF = 0 and CX is not equal to zero. Looping is done as long as count is not equal to zero, and the element of the array is not equal to a certain value you are looking for. | |
| JCXZ label | Jump to a Specified address if CX is zero |
Examples:
Write an ALP for 8086 to convert 8-bit binary number stored in memory into ASCII and store the converted ASCII data back in memory
| Using assemble such as MASM | Using KIT |
| Source variable X
Destination variable : Y |
Assume source packed data at location : 2500:1000
Converted Binary to be stored at 2500: 1001 and 1002 |
| Data SEGMENT
X db 10101010B Y db 2 dup(?) Data ENDS Code SEGMENT ASSUME CS: code, DS: data MOV ax, data MOV ds, ax MOV SI, offset Y MOV AL, X MOV BL, AL AND AL, 0F0h MOV CL, 04 ROR AL, CL CALL ASCII MOV [SI], AL INC SI MOV AL, BL ANL AL, 0F0h CALL ASCII MOV [SI], AL JMP L1 ASCII: CMP AL, 0Ah JC L2 ADD AL, 07h L2: ADD AL, 30h RET L1: MOV AH, 4Ch INT 21h Code ENDS END
|
MOV ds, ax MOV AL, [1000] MOV BL, AL AND AL, 0F0h MOV CL, 04 ROR AL, CL CALL ASCII MOV [1001], AL MOV AL, BL ANL AL, 0F0h CALL ASCII MOV [102], AL JMP L1 ASCII: CMP AL, 0Ah JC L2 ADD AL, 07h L2: ADD AL, 30h RET L1: HLT
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