DSPIC30F datasheet(52/248 Pages) MICROCHIP

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dsPIC30F

DS70083G-page 50

Preliminary

 2004 Microchip Technology Inc.

4.4

Modulo Addressing

Modulo addressing is a method of providing an auto-

mated means to support circular data buffers using

hardware. The objective is to remove the need for soft-

ware 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

pointers into program space) and Y data spaces. Mod-

ulo addressing can operate on any W register pointer.

However, it is not advisable to use W14 or W15 for mod-

ulo 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 cer-

tain 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 buff-

ers 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 an ending address be specified and loaded into the

16-bit Modulo Buffer Address registers: XMODSRT,

XMODEND, YMODSRT, YMODEND (see Table 3-3).

If the length of an incrementing buffer is greater than

M= 2N-1, but not greater than M = 2N bytes, then the

last ‘N’ bits of the data buffer start address must be

zeros. There are no such restrictions on the end

address of an incrementing buffer. For example, if the

buffer size (modulus value) is chosen to be 100 bytes

(0x64), then the buffer start address for an increment-

ing buffer must contain 7 Least Significant zeros. Valid

start addresses may, therefore, be 0xXX00 and

0xXX80

, where ‘X’ is any hexadecimal value. Adding

the buffer length to this value and subtracting ‘1’ will

give the end address to be written into X/YMODEND.

For example, if the start address was chosen to be

0x2000

, then the X/YMODEND would be set to

(0x2000 + 0x0064 – 1) = 0x2063.

In the case of a decrementing buffer, the last ‘N’ bits of

the data buffer end address must be ones. There are

no such restrictions on the start address of a decre-

menting buffer. For example, if the buffer size (modulus

value) is chosen to be 100 bytes (0x64), then the buffer

end address for a decrementing buffer must contain

7 Least Significant ones. Valid end addresses may,

therefore, be 0xXXFF and 0xXX7F, where ‘X’ is any

hexadecimal value. Subtracting the buffer length from

this value and adding 1 will give the start address to be

written into X/YMODSRT. For example, if the end

address was chosen to be 0x207F, then the start

address would be (0x207F – 0x0064 + 1) = 0x201C,

which is the first physical address of the buffer.

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 pos-

sible length of the circular buffer is 32K words

(64 Kbytes).

A write operation to the MODCON register should not

be immediately followed by an indirect read operation

using any W register.

Note:

The start and end addresses are the first

and last byte addresses of the buffer (irre-

spective of whether it is a word or byte

buffer, or an increasing or decreasing

buffer). Moreover, the start address must

be even and the end address must be odd

(for both word and byte buffers).

Note:

‘Start address’ refers to the smallest

address boundary of the circular buffer.

The first access of the buffer may be at

any address within the modulus range

(see Section 4.4.4).

Note:

Y space modulo addressing EA calcula-

tions assume word sized data (LS bit of

every EA is always clear).

Note 1: Using a POP instruction to pop the con-

tents of the top-of-stack (TOS) location

into MODCON also constitutes a write to

MODCON.

Therefore,

the

instruction

immediately following such a POP cannot

be any instruction performing an indirect

read operation.

2: It should be noted that some instructions

perform

indirect

read

operation

implicitly. These are: POP, RETURN,

RETFIE, RETLW

and ULNK.

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