BRANCH INSTRUCTIONSCALL, JMP, RET

Branching instructions

• Program branching instructions are used to

control the flow of actions in a program

• Some instructions provide decision making capabilities and transfer control to other parts of the program.– e.g. conditional and unconditional

branches

CALL ACALL & LCALL• The 8051 provides 2 forms for the

CALL instruction:– Absolute Call – ACALL

• Uses an 11-bit address• The subroutine must be within the same

2K page.

– Long Call – LCALL• Uses a 16-bit address• The subroutine can be anywhere.

– Both forms push the 16-bit address of PC on the stack and update the stack pointer.

Absolute Call – ACALL addr11

• This instruction unconditionally calls a subroutine indicated by the address

• 2 byte instruction: The upper 3-bits of the address combine with the 5-bit opcode to form the 1st byte and the lower 8-bits of the address form the 2nd byte

Eg. ACALL LOC_SUB• If SP=07H initially • label “LOC_SUB” is at memory 0567H,• then executing instruction at 0230H (PC),– SP=09H, internal RAM locations 08H and

09H will contain 32Hand 02H respectively and PC=0567H

LONG CALL - LCALL addr16

• It is a Long call, the subroutine may therefore begin anywhere in the full 64 kB program memory address space

• 3 byte instructionLCALL LOC_SUB

• Initially, SP=07H • label “LOC_SUB” is at memory 4100H• Executing the instruction at 0230H

( PC),– SP=09H, internal RAM locations 08H

and 09H contain 33H and 02H respectively and PC=4100H

RETURN RET & RETI• The 8051 provides 2 forms for the

return instruction:– Return from subroutine – RET• Pop the return address from the stack

and continue execution there.– Return from ISR – RETI• Pop the return address from the stack.• Restore the interrupt logic to accept

additional interrupts at the same priority level as the one just processed.• Continue execution at the address

retrieved from the stack.• The PSW is not automatically restored.

JUMP SJMP • The 8051 provides four different types of

unconditional jump instructions:• Short Jump – SJMP addr

• Uses an 8-bit signed offset relative to the 1st byte of the next instruction.• the range of destination allowed is from -128 to+127 bytes from the instruction

SJMP RELSRT• If the label RELSRT is at program memory

location 0120H and the SJMP instruction is located at address 0100H ( PC) ,after executing the instruction, PC=0120H

JUMP LJMP• Long Jump – LJMP

• Uses a 16-bit address.• 3 byte instruction capable of referencing any location in the entire 64K of program memory.

LJMP FAR_ADR• If the label FAR_ADR is at program memory

location 3456H

– the LJMP instruction at location 0120H (PC)

– After instruction, it loads the PC with 3456H

JUMP AJMP– Absolute Jump – AJMP• Uses an 11-bit address.• 2 byte instruction• The 11-bit address is substituted for

the lower 11-bits of the PC to calculate the 16-bit address of the target.• The location referenced must be within

the 2K Byte

AJMP NEAR• If the label NEAR is at program

memory location 0120H, the AJMP instruction at location 0234H (PC) loads the PC with 0120H

Indirect Jump• This instruction adds the 8-bit unsigned

value of the ACC to the 16-bit data pointer and the resulting sum is returned to the PC

• Neither ACC nor DPTR is altered• No flags are affected

MOV DPTR, #LOOK_TBLJMP @A + DPTR

LOOK_TBL: AJMP LOC0AJMP LOC1AJMP LOC2

• If the ACC=02H, execution jumps to LOC2• AJMP is a two byte instruction

CONDITIONAL JUMP• The 8051 supports different

conditional jump instructions.– ALL conditional jump instructions

use an 8-bit address.– Jump on Zero – JZ / JNZ• Jump if the A == 0 / A != 0

– The check is done at the time of the instruction execution.

– Jump on Carry – JC / JNC• Jump if the C flag is set / cleared.

CONDITIONAL JUMP

– Jump on Bit – JB / JNB• Jump if the specified bit is set / cleared.• Any addressable bit can be specified.

– Jump if the Bit is set then Clear the bit – JBC• Jump if the specified bit is set.• Then clear the bit.

Compare and Jump if Not Equal – CJNE

– Compare the magnitude of the two operands and jump if they are not equal.• The values are considered to be

unsigned.• The Carry flag is set / cleared

appropriately.• CJNE A, direct, rel• CJNE A, #data, rel• CJNE Rn, #data, rel• CJNE @Ri, #data, rel

Decrement and Jump if Not Zero – DJNZ

– Decrement the first operand by 1 and jump to the location identified by the second operand if the resulting value is not zero.

DJNZ 20H,LOC1DJNZ 30H,LOC2DJNZ 40H,LOC3

• If internal RAM locations 20H, 30H and 40H contain the values 01H, 5FH and 16H respectively,

• the above instruction sequence will cause a jump to the instruction at LOC2, with the values 00H, 5EH, and 15H in the 3 RAM locations

NOP

• This is the no operation instruction• The instruction takes one machine

cycle operation time• Hence it is useful to time the ON/OFF

bit of an output portCLR P1.2

NOPNOPNOPNOP

SETB P1.2