User`s guide

MPLAB

IDE User’s Guide

• The Extended Microcontroller mode allows access to both internal and external

program memories as a single block. The device can access its entire on-chip

program memory; above this, the device accesses external program memory up

to the program counter accessible limit (see Section 5.2.4 “Program Counter”).

Execution automatically switches between the two memories, as required.

In all modes, the device has complete access to data RAM. For more information,

consult the device data sheet section “Memory Organization”.

5.2.2 PIC18F6XXX/8XXX Program Memory Modes

Certain PIC18F6XXX/8XXX devices are capable of operating in any one of several

Program Memory modes, using combinations of on-chip and external program

memory. Please see the data sheet for your device to determine if it supports external

memory.

For supported devices, available Program Memory modes are as follows:

• The Microprocessor mode permits access only to external program memory; the

contents of the on-chip Flash memory are ignored. The program counter permits

access to a linear program memory space and defines the amount of accessible

program memory (see Section 5.2.4 “Program Counter”).

• The Microprocessor with Boot Block mode accesses on-chip Flash memory

from address 000000h to the end of the boot block. Above this, external program

memory is accessed all the way up to the program counter accessible limit (see

Section 5.2.4 “Program Counter”). Program execution automatically switches

between the two memories, as required.

• The Microcontroller mode accesses only on-chip Flash memory. Attempts to

read above the physical limit of the on-chip Flash causes a read of all ‘0’s (a NOP

instruction).

• The Extended Microcontroller mode allows access to both internal and external

program memories as a single block. The device can access its entire on-chip

Flash memory; above this, the device accesses external program memory up to

the program counter accessible limit (see Section 5.2.4 “Program Counter”). As

with Boot Block mode, execution automatically switches between the two

memories, as required.

In all modes, the device has complete access to data RAM and EEPROM. For more

information, consult the device data sheet section “Memory Organization”.

5.2.3 PIC18C601/801 ROMless Devices

For PIC18C601/801 devices, all program memory address space is external. The

on-chip program counter permits access to a linear program memory space and

defines the amount of accessible program memory (see Section 5.2.4 “Program

Counter”).

There is a provision for configuring the last 512 bytes of general purpose user RAM as

program memory, called “Boot RAM”. See the device data sheet for more information.

5.2.4 Program Counter

The size of the program counter will determine how much program memory can be

accessed, e.g., a 21-bit program counter permits access to a 2-Mbyte (1-Mword)

program memory space (on-chip, off-chip or a combination of both types of program

memory).