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)

Format	Opcode/ Encoding	2nd Byte of Op-Code	example / Operation	Flags affected
ACALL addr11	A10-810001	A7-0	ACALL label PC=PC+2 SP=SP+1 (SP)=PC7-0 SP=SP+1 (SP)=PC15-8 PC10-0=A10-0	P

 

Example:

Logical Address	Encoding	Label	ORG 0
0	74     55h		MOV A, #55h	A=55h
2	F5    80h	BACK:	MOV P1, A	P1=A
4	11     09h		ACALL DELAY	A10-0=09; so A10-8=000 encoding of ACALL= 00010001 Call location = A7-0=09
6	F4h		CPL A	Complement A
7	80    02h		SJMP Back	Jump within 256 location
9	7D    FFh	Delay:	MOV R5, #0FF	R5=FFh=256
B	D5   0B	Again:	DJNZ R5, Again	R5=R5-1; If R5<>0; PC=PC+offset i.e 0 PC=PC+0=PC Else PC=PC+2
D			RET
			END

 

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.

 

format	example	Operation	Opcode/ Encoding	Flags affected	Byte	Cycles
LCALL addr16	LCALL sub1	PC=PC+2 SP=SP+1 (SP)=PC[7-0] SP=SP+1 (SP)=PC[15-8] PC=addr16	00010010, A15-8, A7-0	none	3	2

 

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.

Format	Opcode/ Encoding	2nd byte	Example/ Operation	Flags affected
RET	00100010	—	RET PC15-8=(SP) SP=SP-1 PC7-0=(SP) SP=SP-1	none