PIC16F5X datasheet(22/90 Pages) MICROCHIP

22 / 90 page

PIC16F5X

DS41213C-page 20

 2004 Microchip Technology Inc.

3.6

Stack

The PIC16F54 device has a 9-bit wide, two-level hard-

ware PUSH/POP stack. The PIC16F57 and PIC16F59

devices have an 11-bit wide, two-level 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.

For the RETLW instruction, the PC is loaded with the

Top-of-Stack (TOS) contents. All of the devices cov-

ered in this data sheet have a two-level stack. The

stack has the same bit width as the device PC, there-

fore, paging is not an issue when returning from a sub-

routine.

3.7

Indirect Data Addressing; INDF

and FSR Registers

The INDF register is not a physical register. Addressing

INDF actually addresses the register whose address is

contained in the FSR Register (FSR is a pointer). This

is indirect addressing.

EXAMPLE 3-1:

INDIRECT ADDRESSING

• Register file 08 contains the value 10h

• Register file 09 contains the value 0Ah

• Load the value 08 into the FSR register

• A read of the INDF register will return the value

of 10h

• Increment the value of the FSR register by one

(FSR = 09h)

• A read of the INDF register now will return the

value of 0Ah.

Reading INDF itself indirectly (FSR = 0) will produce

00h. Writing to the INDF register indirectly results in a

no-operation (although Status bits may be affected).

A simple program to clear RAM locations 10h-1Fh

using indirect addressing is shown in Example 3-2.

EXAMPLE 3-2:

HOW TO CLEAR RAM

USING INDIRECT

ADDRESSING

The FSR is either a 5-bit (PIC16F54), 7-bit (PIC16F57)

or 8-bit (PIC16F59) wide register. It is used in conjunc-

tion with the INDF register to indirectly address the data

memory area.

The FSR<4:0> bits are used to select data memory

addresses 00h to 1Fh.

Note:

The W register will be loaded with the

literal value specified in the instruction.

This

particularly

useful

for

the

implementation of data look-up tables

within the program memory.

PIC16F54: This does not use banking. FSR<7:5> bits

are unimplemented and read as ‘1’s.

PIC16F57: FSR<7> bit is unimplemented and read as

‘1’. FSR<6:5> are the bank select bits and are used to

select the bank to be addressed (00 = Bank 0,

= Bank 1, 10 = Bank 2, 11 = Bank 3).

PIC16F59: FSR<7:5> are the bank select bits and are

used to select the bank to be addressed

(000 = Bank 0, 001 = Bank 1, 010 = Bank 2,

011

= Bank 3, 100 = Bank 4, 101 = Bank 5,

110

= Bank 6, 111 = Bank 7).

Note:

A CLRF FSR instruction may not result in

an FSR value of 00h if there are

unimplemented bits present in the FSR.

MOVLW

H'10'

;initialize pointer

MOVWF

FSR

;to RAM

CLRF

INDF

;clear INDF Register

INCF

FSR,F

;inc pointer

BTFSC

FSR,4

;all done?

GOTO

;NO, clear next

CONTINUE

;YES, continue

PIC16F5X Datasheet(PDF) 22 Page - Microchip Technology

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