PIC18F2420/2520/4420/4520
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.
5.5.3
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 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 5.3.2 “Access Bank”). An example of Access
Bank remapping in this addressing mode is shown in
Figure 5-10.
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 5-10:
REMAPPING THE ACCESS BANK WITH INDEXED LITERAL OFFSET ADDRESSING
Example Situation:
000h
ADDWF f, d, a
Bank 0
05Fh
07Fh
FSR2H:FSR2L = 120h
Locations in the region
from the FSR2 Pointer
(120h) to the pointer plus
05Fh (17Fh) are mapped
to the bottom of the
Access RAM (000h-05Fh).
Bank 0
100h
120h
17Fh
Bank 1
Window
00h
5Fh
Bank 1
Bank 1 “Window”
Bank 0
200h
Locations in Bank 0 from
060h to 07Fh are mapped,
as usual, to the middle half
of the Access Bank.
7Fh
80h
Bank 2
through
Bank 14
SFRs
Special Function Registers
at F80h through FFFh are
mapped to 80h through
FFh, as usual.
FFh
Access Bank
F00h
Bank 0 addresses below
5Fh can still be addressed
by using the BSR.
Bank 15
SFRs
F80h
FFFh
Data Memory
DS39631E-page 72
© 2008 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
