Branch Type Instructions
These are of two types:
- Subroutine CALL Instruction
- JMP Instruction
- Unconditional JMP
- 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:
- PC is first incemented twice to obtain the address of the following instruction
- PC is saved on stack low-byte first and high byte at high address. SP increased by two.
- Calcultes the subroutine address as:
- 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 |
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) |
none |