PIC18F25852680 datasheet(95/482 Pages) MICROCHIP

95 / 482 page

Preliminary

DS39625C-page 93

PIC18F2585/2680/4585/4680

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 half of Access RAM

(00h to 7Fh) 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 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 5-9.

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 File Registers at

F60h through FFFh are

mapped to 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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