Datasheet

dsPIC33EPXXXGM3XX/6XX/7XX
DS70000689D-page 98  2013-2014 Microchip Technology Inc.
4.5 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
incrementing buffers) or end address (for decrementing
buffers), based upon the direction of the 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).
4.5.1 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).
4.5.2 W ADDRESS REGISTER SELECTION
The Modulo and Bit-Reversed Addressing Control
register bits, MODCON<15:0>, contain enable flags as
well as a W register field to specify the W Address regis-
ters. The XWM and YWM fields select the registers that
operate with Modulo Addressing:
• If XWM = 1111, X RAGU and X WAGU Modulo
Addressing is disabled
• If YWM = 1111, 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 ‘1111’ and the XMODEN bit is set
(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
‘1111’ and the YMODEN bit is set (MODCON<14>).
FIGURE 4-14: MODULO ADDRESSING OPERATION EXAMPLE
Note: Y space Modulo Addressing EA calcula-
tions assume word-sized data (LSb of
every EA is always clear).
0x1100
0x1163
Start Addr = 0x1100
End Addr = 0x1163
Length = 0x0032 words
Byte
Address
MOV #0x1100, W0
MOV W0, XMODSRT ;set modulo start address
MOV #0x1163, W0
MOV W0, MODEND ;set modulo end address
MOV #0x8001, W0
MOV W0, MODCON ;enable W1, X AGU for modulo
MOV #0x0000, W0 ;W0 holds buffer fill value
MOV #0x1110, W1 ;point W1 to buffer
DO AGAIN, #0x31 ;fill the 50 buffer locations
MOV W0, [W1++] ;fill the next location
AGAIN: INC W0, W0 ;increment the fill value