DSPIC33FJ64GP710A datasheet(69/362 Pages) MICROCHIP

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DS70593D-page 69

dsPIC33FJXXXGPX06A/X08A/X10A

after modification) must, therefore, be valid addresses

within X data space for W8 and W9 and Y data space

for W10 and W11.

In summary, the following addressing modes are

supported by the MAC class of instructions:

• Register Indirect

• Register Indirect Post-Modified by 2

• Register Indirect Post-Modified by 4

• Register Indirect Post-Modified by 6

• Register Indirect with Register Offset (Indexed)

4.3.5

OTHER INSTRUCTIONS

Besides the various addressing modes outlined above,

some instructions use literal constants of various sizes.

For example, BRA (branch) instructions use 16-bit signed

literals to specify the branch destination directly, whereas

the DISI instruction uses a 14-bit unsigned literal field. In

some instructions, such as ADD Acc, the source of an

operand or result is implied by the opcode itself. Certain

operations, such as NOP, do not have any operands.

4.4

Modulo Addressing

Modulo Addressing mode is a method of providing an

automated means to support circular data buffers using

hardware. The objective is to remove the need for

software to perform data address boundary checks

when executing tightly looped code, as is typical in

many DSP algorithms.

Modulo Addressing can operate in either data or program

space (since the data pointer mechanism is essentially

the same for both). One circular buffer can be supported

in each of the X (which also provides the pointers into

program space) and Y data spaces. Modulo Addressing

can operate on any W register pointer. However, it is not

advisable to use W14 or W15 for Modulo Addressing

since these two registers are used as the Stack Frame

Pointer and Stack Pointer, respectively.

In general, any particular circular buffer can only be

configured to operate in one direction as there are

certain restrictions on the buffer start address (for incre-

menting buffers), or end address (for decrementing

buffers), based upon the direction of the buffer.

The only exception to the usage restrictions is for

buffers which have a power-of-2 length. As these

buffers satisfy the start and end address criteria, they

may operate in a bidirectional mode (i.e., address

boundary checks will be performed on both the lower

and upper address boundaries).

4.4.1

START AND END ADDRESS

The Modulo Addressing scheme requires that a starting

and ending address be specified and loaded into the

16-bit Modulo Buffer Address registers: XMODSRT,

XMODEND,

YMODSRT

and

YMODEND

(see

Table 4-1).

Note:

Addressing mode is only available for W9

(in X space) and W11 (in Y space).

Note:

space

Modulo

Addressing

calculations assume word sized data (LSb

of every EA is always clear).

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