Configuring Modulo Arithmetic; Table 4-9 Programming M01 For Modulo Arithmetic - Motorola DSP56800 Manual

Upper Boundary: $00A4
Lower Boundary: $0080
When modulo arithmetic is performed on the buffer pointer register, only the low-order k bits are
modified; the upper 16 - k bits are held constant, fixing the address range of the buffer. The algorithm used
to update the pointer register (R0 in this case) is as follows:
R0[15:k] = R0[15:k]
R0[k-1:0] = (R0[k-1:0] + offset) MOD (M01 + 1)
Note that this algorithm can result in some memory addresses being unavailable. If the size of the buffer is
not an even power of two, there will be a range of addresses between M and 2
example) that are not addressable. Section 4.3.2.7.3, "Memory Locations Not Available for Modulo
Buffers," addresses this issue in greater detail.
4.3.2.2

Configuring Modulo Arithmetic

As noted in Section 4.3.2.1, "Modulo Arithmetic Overview," modulo arithmetic is enabled by
programming the address modifier register, M01. This single register enables modulo arithmetic for both
the R0 and R1 registers, although in order for modulo arithmetic to be enabled for the R1 register it must
be enabled for the R0 register as well. When both pointers use modulo arithmetic, the sizes of both buffers
are the same. They can refer to the same or different buffers as desired.
The possible configurations of the M01 register are given in Table 4-9.
16-Bit M01
Register Contents
$0000
$0001
$0002
Memory
$00B0
(Unavailable
Addresses)
$009F
Circular
Buffer
Figure 4-16. Circular Buffer with Size M=37
Table 4-9. Programming M01 for Modulo Arithmetic
Address Arithmetic
Performed
(Reserved)
Modulo 2
Modulo 3
Address Generation Unit
AGU Address Arithmetic
Lower Bound + Size - 1 = Upper Bound
Initial R0 Pointer Value
Lower Bound Relative to R0
k
-1 (37 and 63 in our
Pointer Registers
Affected
—
R0 pointer only
R0 pointer only
4-27