Datasheet

 2009-2012 Microchip Technology Inc. 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
Tabl e 4 -1).
Note: Register Indirect with Register Offset
Addressing mode is only available for W9
(in X space) and W11 (in Y space).
Note: Y space Modulo Addressing EA
calculations assume word sized data (LSb
of every EA is always clear).