Motorola DSP56000 manual Modulo Modifier Example, 2.2.2 Mn=$8001 to $BFFF

ADDRESSING

EXAMPLE: MOVE X0,X:(R2)+N

LET:

M2

N2

R2

+

R2

0..010 00000 k=5

.....00	0010100
.....00	0001111
00	1001011

(90)

N2

(75)

(69)

MODULUS=21

OFFSET=15

POINTER=75

(84)

XD BUS

21

X0 (64)

Figure 4-13 Modulo Modifier Example

The MOVE instruction in Figure 4-13takes the contents of the X0 register and moves it to a location in the X memory pointed to by (R2), and then (R2) is updated modulo 21. The new value of R2 is not 90 (75+ 15), which would be the case if linear arithmetic had been used, but rather is 69 since modulo arithmetic was used.

4.4.2.2.2Mn=$8001 to $BFFF

In this range, the modulo (M) equals (Mn+1)-$8000, where Mn is the value in the modi- fier register (see Table 4-2). This range firmly restricts the address register to the same buffer, causing the address register to wrap around within the buffer. This multiple wrap- around addressing feature reduces argument overhead and is useful for decimation, interpolation, and waveform generation.

The address modification is performed modulo M, where M may be any power of 2 in the range from 21 to 214. Modulo M arithmetic causes the address register value to remain within an address range of size M defined by a lower and upper address boundary. The value M-1 is stored in the modifier register Mn least significant 14 bits while the two most significant bits are set to ‘10’. The lower boundary (base address) value must have zeroes in the k LSBs, where 2k = M, and therefore must be a multiple of 2k. The upper boundary is the lower boundary plus the modulo size minus one (base address plus M-1).

MOTOROLA ADDRESS GENERATION UNIT 4 - 21