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PIC18FXX39

DS30485A-page 46

Preliminary

2002 Microchip Technology Inc.

4.10

Access Bank

The Access Bank is an architectural enhancement

which is very useful for C compiler code optimization.

The techniques used by the C compiler may also be

useful for programs written in assembly.

This data memory region can be used for:

Intermediate computational values

Local variables of subroutines

Faster context saving/switching of variables

Common variables

Faster evaluation/control of SFRs (no banking)

The Access Bank is comprised of the upper 128 bytes

in Bank 15 (SFRs) and the lower 128 bytes in Bank 0.

These two sections will be referred to as Access RAM

High and Access RAM Low, respectively. Figure 4-5

and Figure 4-6 indicate the Access RAM areas.

A bit in the instruction word specifies if the operation is

to occur in the bank specified by the BSR register, or in

the Access Bank. This bit is denoted by the ‘a’ bit (for

access bit).

When forced in the Access Bank (a = 0), the last

address in Access RAM Low is followed by the first

address in Access RAM High. Access RAM High maps

the Special Function registers, so that these registers

can be accessed without any software overhead. This is

useful for testing status flags and modifying control bits.

4.11

Bank Select Register (BSR)

The need for a large general purpose memory space

dictates a RAM banking scheme. The data memory is

partitioned into sixteen banks. When using direct

addressing, the BSR should be configured for the

desired bank.

BSR<3:0> holds the upper 4 bits of the 12-bit RAM

address. The BSR<7:4> bits will always read ‘0’s, and

writes will have no effect.

A MOVLB instruction has been provided in the

instruction set to assist in selecting banks.

If the currently selected bank is not implemented, any

read will return all ‘0’s and all writes are ignored. The

STATUS register bits will be set/cleared as appropriate

for the instruction performed.

Each Bank extends up to FFh (256 bytes). All data

memory is implemented as static RAM.

A MOVFF instruction ignores the BSR, since the 12-bit

addresses are embedded into the instruction word.

Section 4.12 provides a description of indirect address-

ing, which allows linear addressing of the entire RAM

space.

FIGURE 4-7:

DIRECT ADDRESSING

Note 1: For register file map detail, see Table 4-1.

2: The access bit of the instruction can be used to force an override of the selected bank (BSR<3:0>) to the

registers of the Access Bank.

3: The MOVFF instruction embeds the entire 12-bit address in the instruction.

Data

Memory(1)

Direct Addressing

Bank Select(2)

Location Select(3)

BSR<3:0>

From Opcode(3)

00h

01h

0Eh

0Fh

Bank 0

Bank 1

Bank 14 Bank 15

1FFh

100h

0FFh

000h

EFFh

E00h

FFFh

F00h

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