Freescale Semiconductor StarCore SC140 - Manual (Page 78)

2-46 SC140 DSP Core Reference Manual
Address Generation Unit
register Rn has one Mj register assigned to it by encoding in the MCTL. The lower boundary value of the
buffer resides in the Bn register, and the upper boundary is calculated as Bn+Mj-1. Mj must be smaller
than 2
31
- 1 (Mj < 2
31
- 1).
The modulo addressing definition, using a base register (Bn) and a modulo register (Mj), enables the
programmer to locate the modulo buffer at any address. The buffer start address is only required to be
aligned to the access width.
The address pointer Rn is not required to start at the lower address boundary, nor to end on the upper
address boundary. Rn can initially point anywhere (aligned to its access width) within the defined modulo
address range, Bn
≤ Rn < B+Mj. Assuming the (Rn)+ indirect addressing mode, if the address register
pointer increments past the upper boundary of the buffer (base address + Mj-1), it wraps around through
the base address (lower boundary). Alternatively, assuming the (Rn)- indirect addressing mode, if the
address decrements past the lower boundary (base address), it wraps around through the base address +
Mj-1 (upper boundary).
The following constraints apply:
1. For proper modulo addressing, if an offset Ni is used in the address calculation, the 32-bit
absolute effective value |Ni| must be less than or equal to Mj, where “effective” means the
programmed Ni is multiplied by the access width. For example, move.w (r0)+n0,d0 translates
to the restriction 2*n0
≤ Μj, and move.l (r0)+,d0 translates to 4 ≤ Mj. If effective Ni > Mj, the
result of the address calculation is undefined. Multiple wrap-around modulo addressing
supports the situation of effective Ni greater than Mj.
2. Mj must be aligned to the access width used. For example, if the buffer is used with a
MOVE.2L instruction, Mj must be aligned to 8 (be a multiple of 8). If the modulus is less
than the access width, the data accessed as well as the address calculations are undefined.
3. When Bn is used as a base address register, the use of R
n+8
as a pointer is illegal since this
is the same physical register.
Modulo addressing is illustrated in Figure 2-15. Addresses will be kept within the eleven addresses shown.
For the instruction,
move.w (r0+$000e),d0 , the access will be made from $26 (38), if the base address
is $20, the modulus is $c, and r0 is $24. The operation is 36+14=50=38 in modulus 12, base address 32
(50–44 + 32 = 38).
Figure 2-15. Modulo Addressing Example
$002c = B + M – 1
32
36
38
44
M = 12
$0020 = B