branch ins 8051

BRANCH INSTRUCTIONS CALL, 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 1 st byte and the lower 8-bits of the address form the 2 nd 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 32H and 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 instruction LCALL 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 1 st 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_TBL JMP @A + DPTR LOOK_TBL: AJMP LOC0 AJMP LOC1 AJMP 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,LOC1 DJNZ 30H,LOC2 DJNZ 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 port CLR P1.2 NOP NOP NOP NOP SETB P1.2

branch ins 8051

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branch ins 8051