Subroutine Call and Return Instructions

Branch Type Instructions

These are of two types:

1. Subroutine CALL Instruction
2. JMP Instruction
   1. Unconditional JMP
   2. Conditional JMP

Subroutine Call Instructions: These instructions are used to calls a subroutine located at the specified address. There are two subroutine Call instructions, theses are ACAL and LCALL instruction

ACALL Instruction

Size                 : Two bytes

Requirement  : Subroutine that is called must be located in the same 2KByte block of program memory as the opcode following the ACALLinstruction.

Operation:

1. PC is first incemented twice to obtain the address of the following instruction
2. PC is saved on stack low-byte first and high byte at high address. SP increased by two.
3. Calcultes the subroutine address as:
4. by combining the 5 high-order bits of the incremented PC (for A15-A11), the 3 high-order bits of the ACALLinstruction opcode (for A10-A8), and the second byte of the instruction (for A7-A0)

Example:

LCALL

The LCALL instruction calls a subroutine located at the specified address. This instruction first adds 3 to the PC to generate the address of the next instruction. This result is pushed onto the stack low-byte first and the stack pointer is incremented by 2. The high-order and low-order bytes of the PC are loaded from the second and third bytes of the instruction respectively. Program execution is transferred to the subroutine at this address. No flags are affected by this instruction.

Example Subroutine Long Call

ORG 0

MAIN :            – –

LCALL subrtn_1

LCALL subrtn_2

LCALL subrtn_3

– –

HERE:      SJMP     HERE

;

subrt_1:          —

RET                 ; end of sub routine_1

subrtn_2:        —

RET                 ; end of sub routine_2

subrtn_3:        —

RET                 ; end of sub routine_3

END

RET

It is used to return from a subroutine

POPs  the saved address from stack and load in PC. Twice read, each time stack pointer decremented

Program execution resumes from the resulting address which is typically the instruction following an ACALL or LCALL instruction.