Microchip Technology MA330031-2 Data Sheet
dsPIC33EPXXXGP50X, dsPIC33EPXXXMC20X/50X AND PIC24EPXXXGP/MC20X
DS70000657H-page 114
2011-2013 Microchip Technology Inc.
4.6
Modulo Addressing
(dsPIC33EPXXXMC20X/50X and
dsPIC33EPXXXGP50X Devices
Only)
(dsPIC33EPXXXMC20X/50X and
dsPIC33EPXXXGP50X Devices
Only)
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 Pro-
gram 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 point-
ers into Program Space) and Y Data Spaces. Modulo
Addressing can operate on any W Register Pointer. How-
ever, 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).
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 Pro-
gram 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 point-
ers into Program Space) and Y Data Spaces. Modulo
Addressing can operate on any W Register Pointer. How-
ever, 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).
4.6.1
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
starting and ending address be specified, and loaded
into the 16-bit Modulo Buffer Address registers:
XMODSRT, XMODEND, YMODSRT and YMODEND
(see
The length of a circular buffer is not directly specified. It
is determined by the difference between the corre-
sponding start and end addresses. The maximum
possible length of the circular buffer is 32K words
(64 Kbytes).
is determined by the difference between the corre-
sponding start and end addresses. The maximum
possible length of the circular buffer is 32K words
(64 Kbytes).
4.6.2
W ADDRESS REGISTER
SELECTION
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 the registers that
operate with Modulo Addressing:
• If XWM = 1111, X RAGU and X WAGU Modulo
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 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
which Modulo Addressing is to be applied, is stored in
MODCON<3:0> (see
). 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 at MODCON<14>.
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 at MODCON<14>.
FIGURE 4-20:
MODULO ADDRESSING OPERATION EXAMPLE
Note:
Y space Modulo Addressing EA calcula-
tions assume word-sized data (LSb of
every EA is always clear).
tions assume word-sized data (LSb of
every EA is always clear).
0x1100
0x1163
Start Addr = 0x1100
End Addr = 0x1163
Length = 50 words
End Addr = 0x1163
Length = 50 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