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2009 Microchip Technology Inc.

DS41236E-page 27

PIC12F508/509/16F505

4.7

Program Counter

As a program instruction is executed, the Program

Counter (PC) will contain the address of the next

program instruction to be executed. The PC value is

increased by one every instruction cycle, unless an

instruction changes the PC.

For a GOTO instruction, bits 8:0 of the PC are provided

by the GOTO instruction word. The Program Counter

(PCL) is mapped to PC<7:0>. Bit 5 of the STATUS

(Figure 4-6).

For a CALL instruction, or any instruction where the

PCL is the destination, bits 7:0 of the PC again are

provided by the instruction word. However, PC<8>

does not come from the instruction word, but is always

cleared (Figure 4-6).

Instructions where the PCL is the destination, or modify

PCL instructions, include MOVWF PC, ADDWF PC and

BSF PC,5.

FIGURE 4-6:

LOADING OF PC

BRANCH INSTRUCTIONS

4.7.1

EFFECTS OF RESET

The PC is set upon a Reset, which means that the PC

addresses the last location in the last page (i.e., the

oscillator

calibration

instruction).

After

executing

MOVLW

, the PC will roll over to location 00h and

begin executing user code.

The STATUS register page preselect bits are cleared

upon a Reset, which means that page 0 is pre-selected.

Therefore, upon a Reset, a

GOTO

instruction will

automatically cause the program to jump to page 0 until

the value of the page bits is altered.

4.8

Stack

The PIC12F508/509/16F505 devices have a 2-deep,

12-bit wide hardware PUSH/POP stack.

A CALL instruction will PUSH the current value of Stack

1 into Stack 2 and then PUSH the current PC value,

incremented by one, into Stack Level 1. If more than two

sequential CALLs are executed, only the most recent two

return addresses are stored.

A RETLW instruction will POP the contents of Stack

Level 1 into the PC and then copy Stack Level 2

contents into Stack Level 1. If more than two sequential

RETLW

s are executed, the stack will be filled with the

address previously stored in Stack Level 2. Note that

the W register will be loaded with the literal value

specified in the instruction. This is particularly useful for

the implementation of data look-up tables within the

program memory.

Note:

Because PC<8> is cleared in the CALL

instruction or any modify PCL instruction,

all subroutine calls or computed jumps are

limited to the first 256 locations of any

program memory page (512 words long).

PA0

Status

PCL

Instruction Word

GOTO

Instruction

CALL

or Modify PCL Instruction

PA0

Status

PCL

Instruction Word

Reset to ‘0’

Note 1: There are no Status bits to indicate stack

overflows or stack underflow conditions.

2: There are no instruction mnemonics

called PUSH or POP. These are actions

that occur from the execution of the CALL

and RETLW instructions.

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