OPTION_REG datasheet(68/222 Pages) MICROCHIP

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MCP19111

DS22331A-page 68

 2013 Microchip Technology Inc.

11.1.1

READING PROGRAM MEMORY AS

DATA

There are two methods of accessing constants in pro-

gram memory. The first method is to use tables of

RETLW

instructions. The second method is to set a

Files Select Register (FSR) to point to the program

memory.

11.1.1.1

RETLW

Instruction

The RETLW instruction can be used to provide access

to tables of constants. The recommended way to create

such a table is shown in Example 11-1.

EXAMPLE 11-1:

RETLW

INSTRUCTION

11.1.1.2

Indirect Read with Files Select

FSR)

The program memory can be accessed as data by

setting bit 7 of the FSRxH register and reading the

matching INDFx register. The MOVIW instruction will

place the lower 8 bits of the addressed word in the

W register. Writes to the program memory cannot be

performed via the INDF registers. Instructions that

access the program memory via the FSR require one

extra instruction cycle to complete. Example 11-2

demonstrates accessing the program memory via an

FSR.

The HIGH directive will set bit<7> if a label points to a

location in program memory.

EXAMPLE 11-2:

ACCESSING PROGRAM

MEMORY VIA FSR

11.2

Data Memory Organization

The data memory (see Figure 11-1) is partitioned into

four banks, which contain the General Purpose

Registers (GPR) and the Special Function Registers

(SFR). The Special Function Registers are located in

the first 32 locations of each bank. Register locations

20h-7Fh in Bank 0, A0h-EFh in Bank 1 and 120h-16Fh

Bank 2

are

General

Purpose

Registers,

implemented as static RAM. All other RAM is

unimplemented and returns ‘0’

when read. The

RP<1:0> bits of the STATUS register are the bank

select bits.

To move values from one register to another register,

the value must pass throught the W register. This

means that for all register-to-register moves, two

instruction cycles are required.

The

STATUS

shown

contains:

• the arithmetic status of the ALU

• the Reset status

• the bank select bits for data memory (RAM)

The STATUS register can be the destination for any

instruction, like any other register. If the STATUS

the Z, DC or C bits, then the write to these three bits is

disabled. These bits are set or cleared according to the

device logic. Furthermore, the TO and PD bits are not

writable. Therefore, the result of an instruction with the

STATUS register as destination may be different than

intended.

constants

RETLW DATA0

;Index0 data

RETLW DATA1

;Index1 data

RETLW DATA2

RETLW DATA3

my_function

;… LOTS OF CODE…

MOVLW

DATA_INDEX

call constants

;… THE CONSTANT IS IN W

constants

RETLW DATA0

;Index0 data

RETLW DATA1

;Index1 data

RETLW DATA2

RETLW DATA3

my_function

;… LOTS OF CODE…

MOVLW

LOW constants

MOVWF

FSR1L

MOVLW

HIGH constants

MOVWF

FSR1H

MOVIW 0[FSR1]

;THE PROGRAM MEMORY IS IN W

RP1

RP0

-> Bank 0 is selected

-> Bank 1 is selected

-> Bank 2 is selected

-> Bank 3 is selected

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