About an instructions of subroutine of Microcontroller 8051 which is used in the programming of 8051.

1. Subroutine in 8051
Microcontroller
By, Bhadresh Savani (140110120046)
Ronak Sejani (140110120047)
Aashiv Shah (140110120048)
Guided by, B. R. Hindocha

2. What is Subroutine?
Subroutine is a group of instructions Written separately from the main program to
perform a function that occurs repeatedly in the main program.
When you call a subroutine implementation of the current program is stopped, the
program counter PC is loaded with the memory location of the subroutine, running
up to the RET instruction (end of subroutine), where produce a return to the main
program resumes running.

3. Subroutines...
• Perform specific functions and are not operating on their own.
• Are always linked to a major program or other subroutines.
• Can be called many times as necessary as it reduces the code the program
to have the effect of code reuse.
• Allow the division of program blocks as perform the function of structure.
Providing greater visibility and understanding of it.

4. INSTRUCTIONS of Subroutine in
8051 Microcontroller
ACALL
LCALL
RET
RETI

5. ACALL INSTUCTION
Function: Absolute Call Within 2K Block
Syntax: ACALL code address
Description: ACALL unconditionally calls a subroutine at the indicated code
address. ACALL pushes the address of the instruction that follows ACALL onto
the stack, least-significant-byte first, most-significant-byte second. The Program
Counter is then updated so that program execution continues at the indicated
address.

6. The new value for the Program Counter is calculated by replacing the least-
significant-byte of the Program Counter with the second byte of the ACALL
instruction, and replacing bits 0-2 of the most-significant-byte of the Program
Counter with 3 bits that indicate the page. Bits 3-7 of the most-significant-byte of
the Program Counter remain unchanged.
Since only 11 bits of the Program Counter are affected by ACALL, calls may only
be made to routines located within the same 2k block as the first byte that follows
ACALL.

7. Instructions OpCode Bytes Cycles Flags
ACALL page0 0x11 2 2 None
ACALL page1 0x31 2 2 None
ACALL page2 0x51 2 2 None
ACALL page3 0x71 2 2 None
ACALL page4 0x91 2 2 None
ACALL page5 0xB1 2 2 None
ACALL page6 0xD1 2 2 None
ACALL page7 0xF1 2 2 None

8. LCALL INSTRUCTION
Function: Long Call
Syntax: LCALL code addr
Description: LCALL calls a program subroutine. LCALL increments the program
counter by 3 (to point to the instruction following LCALL) and pushes that value
onto the stack (low byte first, high byte second). The Program Counter is then set
to the 16-bit value which follows the LCALL opcode, causing program execution to
continue at that address.

9. Instructions OpCode Bytes Cycles Flags
LCALL 0x12 3 2 None

10. Difference B/W ACALL and LCALL
ACALL stands for absolute call while LCALL stands for long call.
ACALL allows you to jump to a subroutine within the same 2K page while
LCALL allows you to jump to a subroutine anywhere in the 64K code space.
The Advantage of ACALL over LCALL is that it is a 2-byte instruction while
LCALL is a 3-byte instruction.

11. RET INSTRUCTION
Function: Return From Subroutine
Syntax: RET
Description: RET is used to return from a subroutine previously called by LCALL
or ACALL. Program execution continues at the address that is calculated by
popping the topmost 2 bytes off the stack. The most-significant-byte is popped
off the stack first, followed by the least-significant-byte.

12. Instructions OpCode Bytes Cycles Flags
RET 0x22 1 2 None

13. RETI instruction
Function: Return from Interrupt
Syntax: RETI
Description:The RETI instruction is used to end an interrupt service routine. This
instruction pops the high-order and low-order bytes of the PC (and decrements
the stack pointer by 2) and restores the interrupt logic to accept additional
interrupts. No other registers are affected by this instruction.

14. The RETI instruction does not restore the PSW to its value before the interrupt.
The interrupt service routine must save and restore the PSW.
Execution returns to the instruction immediately after the point at which the
interrupt was detected. If another interrupt was pending when the RETI
instruction is executed, one instruction at the return address is executed before
the pending interrupt is processed.

15. Instructions OpCode Bytes Cycles Flags
RETI 0x32 1 2 None

16. Difference B/W RET and RETI
RETI releases the interrupts of the same level of priority than the interrupt that has
caused the branch, and RET doesn't.
RET stands for Return from Stack.
RETI stands for Return from Stack of Interrupt.

17. ADVANTAGE OF SUBROUTINE
Minimize the content of the sentences in the main program.
Dividing a large programming task among various programmers, or various
stages of a project.
Improving Traceability.
Make the program easier and it takes up less space in the ROM.

18. Disadvantages of SUBROUTINE
Invoking a subroutine (versus using inline code) imposes some computational
overhead in the call mechanism.
The subroutine typically requires standard housekeeping code – both at entry
to, and exit from, the function.

20. Resources
1. http://www.alciro.org/alciro/microcontroladores-8051_24/subrutina-subprograma_357_en.htm
2. https://en.wikipedia.org/wiki/Subroutine
3. http://www.edsim51.com/8051Notes/8051/subroutines.html
4. http://www.hobbyprojects.com/8051_tutorial/acall_absolute_call.html

21. Thank you!!