1. Subroutines and the Stack
Source:

2. Acall & Lcall
ACALL allows you to jump to a subroutine within the same 2K page.
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.
Calling range
Instruction length
ACALL
(Absolute call)
Within the 2 K page.
2 bytes
LCALL
(Long call)
Anywhere in the 64K code space
3 bytes

3. JMP and CALL are not 8051 instructions.
In source program
By the assembler
CALL
JMP
CALL is replaced by ACALL.
JMP is replaced by either SJMP or AJMP.
LCALL
LJMP
LCALL and LJMP must be programmed explicitly.

4. Operation during execution of LCALL
LCALL add16 (long call to subroutine)
machine cod: aaaaaaaa aaaaaaaa (3-byte instruction)
A15 ~ A8
A7 ~ A0
(PC) <- (PC) ; PC points to the next instruction following the CALL (SP) <- (SP) + 1 ((SP)) <- (PC7-PC0) (SP) <- (SP) + 1 ((SP)) <- (PC15 - PC8) (PC) <- add15 - add0 ; Go to the called subroutine
On system reset, the SP is initialized with the value 07H. Therefore, the first item pushed onto the stack will be stored in location 08H.

5. RET instruction RET (Return from subroutine) Machine code: 0010 0010
(PC15 - PC8)< - ((SP)) (SP) <- (SP) - 1 (PC7 - PC0) <- ((SP)) (SP) <- (SP) - 1
The processor needs some way of knowing where to jump back to once execution of the subroutine is complete.

6. What is a stack?
A stack is a storage mechanism with the first-in-last-out (FILO) or last-in-first-out (LIFO) access scheme.
Source:

7. PUSH during CALL and POP during RET
Addreess
Instruction
Time slot PC SP
Stack
1233H
ORG 1233H
Main: DIV AB 1
1234H
07H
Empty
LCALL SUBR 2
1237H
5678H
09H
MUL AB 6
1238H
5677H
ORG 5677H
CLR A
SUBR: SETB C 3
5679H
CLR C 4
567AH
RET 5
567BH
CPL A

8. CALL to PUSH, RET to POP
After CALL but before executing the subroutine
After starting to execute the subroutine, but before RET
After RET
Program counter
Stack pointer
PC15 ~ PC08
PC07 ~ PC00
PC:
aaaaaaaa
bbbbbbbb
PC15 ~ PC08
PC07 ~ PC00
PC:
uuuuuuuu
vvvvvvvv
PC15 ~ PC08
PC07 ~ PC00
PC:
aaaaaaaa
bbbbbbbb
SP 
aaaaaaaa
bbbbbbbb
xxxxxxxx
PUSH
POP
SP 
xxxxxxxx
SP 
xxxxxxxx

9. Stack
Right before executing LCALL sub
While entering the subroutine

10. A program example (to show the advantage of the indirect address mode)
Example - Getting the Average of a Set of Numbers
Use the direct address mode
Use the indirect address mode
(to avoid having to add an instruction for each number to be added)
average:
MOV A, 30H ADD A, 31H ADD A, 32H ADD A, 33H ADD A, 34H MOV B, #05H DIV AB MOV 20H, A MOV 21H, B RET
; The main program
MOV R0, #30H ; initialize the subroutine by putting the start address into R0 MOV R1, #05H ; and by putting the size of the set into R1 CALL average
; The subroutine
MOV B, R ; copy the size of the set into the B register CLR A ; clear the ACC
loop:
ADD ; add to the ACC the data in the location pointed to by R0
INC R ; increment R0 so that it points to the next memory location DJNZ R1, loop ; decrement R1 and if it is still not zero jump back to loop DIV AB ; once all the numbers have been added together, divide them by the size of the set, which is stored in B RET ; return from subroutine

11. Saving the program status before executing the subroutine
;the main program using 0 ; assembler directive that indicates to the assembler which register bank is being used (in this case bank 0)
MOV R0, #30H; initialize the subroutine by putting the start address into R0 MOV R1, #05H; and by putting the size of the set into R1 CALL average
;the subroutine
average:
PUSH PSW PUSH AR0 PUSH AR MOV B, R1; copy the size of the set into the B register CLR A ; clear the ACC
loop:
ADD add to the ACC the data in the location pointed to by R0
INC R0; increment R0 so that it points to the next memory location DJNZ R1, loop; decrement R1 and if it is still not zero jump back to loop DIV AB; once all the numbers have been added together, divide them by the size of the set, which is stored in B POP R1 POP R0 POP PSW RET; return from subroutine

12. Initial state of the stack
On system reset, the SP is initialized with the value 07H. Therefore, the first item pushed onto the stack will be stored in location 08H.
Note: SP points to the top of the stack. 0C 0B 0A 09 08
Bottom of the stack
SP  07