MCP19124 datasheet(70/236 Pages) MICROCHIP

70 / 236 page

MCP19124/5

DS20005619A-page 70

 2016 Microchip Technology Inc.

10.1.1

READING PROGRAM MEMORY

AS DATA

There are two methods of accessing constants in

program 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.

10.1.1.1

RETLW Instruction

The RETLW instruction can be used to provide access

to the tables of constants. The recommended way to

create such tables is shown in Example 10-1.

EXAMPLE 10-1:

RETLW INSTRUCTION

The BRW instruction makes this type of table very

simple to implement. If your code must remain portable

with previous generations of microcontrollers, then the

BRW

instruction is not available, so the older table-read

method must be used.

10.2

Data Memory Organization

The data memory (refer to Figure 10-1) 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, 120h-16Fh in

Bank 2 and 1A0h-1EFh in Bank 3 are General Purpose

Registers, implemented as static RAM. All other RAM

is unimplemented and returns ‘0’ when read. The

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

select bits.

EXAMPLE 10-2:

BANK SELECT

To move values from one register to another register,

the value must pass through the W register. This

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

instruction cycles are required.

The entire data memory can be accessed either

directly or indirectly. Direct addressing may require the

use of the RP<1:0> bits. Indirect addressing requires

the use of the FSR. Indirect addressing uses the

Indirect Register Pointer (IRP) bit in the STATUS

Bank2/Bank3 areas of data memory.

10.2.1

GENERAL PURPOSE REGISTER

FILE

The register file is organized as 64 x 8 in the

MCP19124/5. Each register is accessed, either directly

or indirectly, through the FSR (refer to Section 10.5

“Indirect Addressing, INDF and FSR Registers”).

constants

BRW

;Add Index in W to

;program counter to

;select data

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

RP1

RP0

 Bank 0 is selected

 Bank 1 is selected

 Bank 2 is selected

 Bank 3 is selected

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