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dsPIC30F3010/3011

DS70141F-page 42

2010 Microchip Technology Inc.

4.2.3

MODULO ADDRESSING

APPLICABILITY

Modulo Addressing can be applied to the Effective

Address (EA) calculation associated with any W regis-

ter. It is important to realize that the address boundar-

ies check for addresses less than or greater than the

upper (for incrementing buffers) and lower (for decre-

menting buffers) boundary addresses (not just equal

to). Address changes may, therefore, jump beyond

boundaries and still be adjusted correctly.

4.3

Bit-Reversed Addressing

Bit-Reversed Addressing is intended to simplify data

re-ordering for radix-2 FFT algorithms. It is supported

by the X AGU for data writes only.

The modifier, which may be a constant value or register

contents, is regarded as having its bit order reversed.

The address source and destination are kept in normal

order. Thus, the only operand requiring reversal is the

modifier.

4.3.1

BIT-REVERSED ADDRESSING

IMPLEMENTATION

Bit-Reversed Addressing is enabled when:

BWM (W register selection) in the MODCON

not be accessed using Bit-Reversed

Addressing) and

the BREN bit is set in the XBREV register and

the addressing mode used is Register Indirect

with Pre-Increment or Post-Increment.

If the length of a bit-reversed buffer is M = 2N bytes,

then the last ‘N’ bits of the data buffer start address

must be zeros.

XB<14:0> is the bit-reversed address modifier or ‘pivot

point’ which is typically a constant. In the case of an

FFT computation, its value is equal to half of the FFT

data buffer size.

When enabled, Bit-Reversed Addressing will only be

executed for Register Indirect with Pre-Increment or

Post-Increment

Addressing

and

word-sized

data

writes. It will not function for any other addressing

mode or for byte-sized data, and normal addresses will

be generated instead. When Bit-Reversed Addressing

is active, the W Address Pointer will always be added

to the address modifier (XB) and the offset associated

with the Register Indirect Addressing mode will be

ignored.

addition,

word-sized

data

requirement, the LSb of the EA is ignored (and always

clear).

If Bit-Reversed Addressing has already been enabled

by setting the BREN (XBREV<15>) bit, then a write to

the XBREV register should not be immediately followed

by an indirect read operation using the W register that

has been designated as the Bit-Reversed Pointer.

FIGURE 4-2:

BIT-REVERSED ADDRESS EXAMPLE

Note:

The modulo corrected effective address is

written back to the register only when Pre-

Modify or Post-Modify Addressing mode is

used to compute the effective address.

When an address offset (e.g., [W7 + W2])

is used, Modulo Addressing correction is

performed, but the contents of the register

remains unchanged.

Note:

All bit-reversed EA calculations assume

word-sized data (LSb of every EA is

always clear). The XB value is scaled

accordingly to generate compatible (byte)

addresses.

Note:

Modulo Addressing and Bit-Reversed

Addressing

should

not

enabled

together. In the event that the user

attempts

this,

Bit-Reversed

Addressing will assume priority when

active for the X WAGU, and X WAGU

Modulo Addressing will be disabled. How-

ever, Modulo Addressing will continue to

function in the X RAGU.

b3 b2 b1

b2 b3 b4

Bit Locations Swapped Left-to-Right

Around Center of Binary Value

Bit-Reversed Address

XB = 0x0008 for a 16-word Bit-Reversed Buffer

b7 b6 b5 b1

b7 b6 b5 b4

b11 b10 b9 b8

b15 b14 b13 b12

Sequential Address

Pivot Point

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