PIC18F4550EMLSQTP datasheet(79/430 Pages) MICROCHIP

79 / 430 page

Preliminary

DS39632D-page 77

PIC18F2455/2550/4455/4550

5.6.3

MAPPING THE ACCESS BANK IN

INDEXED LITERAL OFFSET MODE

The use of Indexed Literal Offset Addressing mode

effectively changes how the lower portion of Access

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

just the contents of the bottom half of Bank 0, this mode

maps the contents from Bank 0 and a user-defined

“window” that can be located anywhere in the data

memory space. The value of FSR2 establishes the

lower boundary 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 5.3.3 “Access Bank”). An example of Access

Bank remapping in this addressing mode is shown in

Figure 5-9.

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. Any indirect or

indexed operation that explicitly uses any of the indirect

file operands (including FSR2) will continue to operate

as standard Indirect Addressing. Any instruction that

uses the Access Bank, but includes a register address

of greater than 05Fh, will use Direct Addressing and

the normal Access Bank map.

5.6.4

BSR IN INDEXED LITERAL

OFFSET MODE

Although the Access Bank is remapped when the

extended instruction set is enabled, the operation of the

BSR remains unchanged. Direct Addressing, using the

BSR to select the data memory bank, operates in the

same manner as previously described.

FIGURE 5-9:

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 Function Regis-

ters at F60h through FFFh

are

mapped

60h

through FFh as usual.

Bank 0 addresses below

5Fh are not available in

this mode. They can still

be addressed by using the

BSR.

Access Bank

00h

60h

FFh

Bank 0

SFRs

Bank 1 “Window”

Window

Example Situation:

120h

17Fh

5Fh

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