A modulo buffer is a buffer in which the data pointer loops back to the beginning of the buffer once the pointer address value exceeds a specified limit.
The following figure depicts a modulo buffer with the limit six. Increasing the pointer address value to 0x106 makes it point to the same data it would point to if its address value were 0x100.
The CodeWarrior C compiler for DSP56800E uses intrinsic functions to create and manipulate modulo buffers. Normally, a modulo operation, such as the % operator, requires a runtime function call to the arithmetic library. For normally timed critical DSP loops, this binary operation imposes a large execution-time overhead.
The CodeWarrior implementation, however, replaces the runtime call with an efficient implementation of circular-address modification, either by using hardware resources or by manipulating the address mathematically.
Processors such as the DSP56800E have on-chip hardware support for modulo buffers. Modulo control registers work with the DSP pointer update addressing modes to access a range of addresses instead of a continuous, linear address space. But hardware support imposes strict requirements on buffer address alignment, pointer register resources, and limited modulo addressing instructions. For example, R0 and R1 are the only registers available for modulo buffers.
Accordingly, the CodeWarrior C compiler uses a well-defined set of intrinsic APIs to implement modulo buffers.