PIC18F23K20 datasheet(87/456 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS41303G-page 87

PIC18F2XK20/4XK20

5.5.3

MAPPING THE ACCESS BANK IN

INDEXED LITERAL OFFSET MODE

The use of Indexed Literal Offset Addressing mode

effectively changes how the first 96 locations of Access

RAM (00h to 5Fh) are mapped. Rather than containing

just the contents of the bottom section of Bank 0, this

mode maps the contents from a user defined “window”

that can be located anywhere in the data memory

space. The value of FSR2 establishes the lower bound-

ary of the addresses mapped into the window, while the

upper boundary is defined by FSR2 plus 95 (5Fh).

Addresses in the Access RAM above 5Fh are mapped

as previously described (see Section 5.3.2 “Access

Bank”). An example of Access Bank remapping in this

addressing mode is shown in Figure 5-12.

Remapping of the Access Bank applies only to opera-

tions using the Indexed Literal Offset mode. Operations

that use the BSR (Access RAM bit is ‘1’) will continue

to use direct addressing as before.

5.6

PIC18 Instruction Execution and

the Extended Instruction Set

Enabling the extended instruction set adds eight

additional commands to the existing PIC18 instruction

set. These instructions are executed as described in

Section 24.2 “Extended Instruction Set”.

FIGURE 5-12:

REMAPPING THE ACCESS BANK WITH INDEXED LITERAL OFFSET

ADDRESSING

Data Memory

000h

100h

200h

F60h

F00h

FFFh

Bank 1

Bank 15

Bank 2

through

Bank 14

SFRs

ADDWF f, d, a

FSR2H:FSR2L = 120h

Locations in the region

from

the

FSR2

pointer

(120h) to the pointer plus

05Fh (17Fh) are mapped

the

bottom

the

Access RAM (000h-05Fh).

Special File Registers at

F60h through FFFh are

mapped to 60h through

FFh, as usual.

Bank 0 addresses below

5Fh can still be addressed

by using the BSR.

Access Bank

00h

60h

FFh

SFRs

Bank 1 “Window”

Bank 0

Window

Example Situation:

120h

17Fh

5Fh

Bank 1

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