DSPIC33EP64GS708 datasheet(57/484 Pages) MICROCHIP

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4.7

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 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).

4.7.1

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 4-2).

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).

4.7.2

W ADDRESS REGISTER SELECTION

The Modulo and Bit-Reversed Addressing Control

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 (XWM) register, to

which Modulo Addressing is to be applied, is stored in

MODCON<3:0> (see Table 4-2). 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 (YWM) register, 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 (MODCON<14>) is set.

FIGURE 4-10:

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

AGAIN, #0x31

;fill the 50 buffer locations

MOV

W0, [W1++]

;fill the next location

AGAIN: INC W0, W0

;increment the fill value

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