Datasheet

 2011-2012 Microchip Technology Inc. DS70652E-page 75
dsPIC33FJ16(GP/MC)101/102 AND dsPIC33FJ32(GP/MC)101/102/104
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 be config-
ured to operate in only 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 circular buffer.
The only exception to the usage restrictions is for
buffers that have a power-of-two length. As these
buffers satisfy the start and end address criteria, they
can operate in a bidirectional mode (that is, address
boundary checks are 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 ).
The length of a circular buffer is not directly specified. It
is determined by the difference between the
corresponding start and end addresses. The maximum
possible length of the circular buffer is 32K words
(64 Kbytes).
4.4.2 W ADDRESS REGISTER
SELECTION
• The Modulo and Bit-Reversed Addressing Control
register, MODCON<15:0>, contains enable flags
as well as a W register field to specify the W
Address registers. The XWM and YWM fields
select which registers will operate with Modulo
Addressing.
• If XWM = 15, X RAGU and X WAGU Modulo
Addressing is disabled.
• If YWM = 15, Y AGU Modulo Addressing is
disabled.
The X Address Space Pointer W register (XWM), to
which Modulo Addressing is to be applied, is stored in
MODCON<3:0> (see Table 4-1). Modulo Addressing is
enabled for X data space when XWM is set to any value
other than ‘15’ and the XMODEN bit is set at
MODCON<15>.
The Y Address Space Pointer W register (YWM) to
which Modulo Addressing is to be applied is stored in
MODCON<7:4>. Modulo Addressing is enabled for Y
data space when YWM is set to any value other than
‘15’ and the YMODEN bit is set at MODCON<14>.
Note: Y space Modulo Addressing EA calcula-
tions assume word-sized data (LSb of
every EA is always clear).