P12C508 datasheet(16/84 Pages) MICROCHIP

16 / 84 page

PIC12C5XX

DS40139A-page 16

Advanced Information

4.5

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 PC Latch (PCL) is

mapped to PC<7:0>. Bit 5 of the STATUS register

provides page information to bit 9 of the PC (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 -

PIC12C508/C509

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 pro-

gram 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’

4.5.1

EFFECTS OF RESET

The Program Counter 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 XX, 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.

4.6

Stack

PIC12C5XX devices have a 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 program

counter value, incremented by one, into stack level 1.

If more than two sequential CALL’s are executed, only

the most recent two return addresses are stored.

A RETLW instruction will

pop the contents of stack level

1 into the program counter and then copy stack level 2

contents into level 1. If more than two sequential

RETLW

’s are executed, the stack will be filled with the

address previously stored in 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.

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