Introduction To The Mac; Multiply-Accumulate Functionality Diagram - Motorola ColdFire MCF5281 User Manual

Introduction to the MAC

• A 48-bit accumulation data path to allow the use of a 40-bit product plus the addition
of 8 extension bits to increase the dynamic number range when implementing signal
processing algorithms.
The three areas of functionality are addressed in detail in following sections. The logic
required to support this functionality is contained in a MAC module, as shown in
Figure 3-1.
Figure 3-1. Multiply-Accumulate Functionality Diagram
3.2 Introduction to the MAC
The MAC is an extension of the basic multiplier found in most microprocessors. It is
typically implemented in hardware within an architecture and supports rapid execution of
signal processing algorithms in fewer cycles than comparable non-MAC architectures. For
example, small digital filters can tolerate some variance in an algorithm's execution time,
but larger, more complicated algorithms such as orthogonal transforms may have more
demanding speed requirements beyond the scope of any processor architecture and may
require full DSP implementation.
To strike a balance between speed, size, and functionality, the ColdFire MAC is optimized
for a small set of operations that involve multiplication and cumulative additions.
Specifically, the multiplier array is optimized for single-cycle pipelined operations with a
possible accumulation after product generation. This functionality is common in many
signal processing applications. The ColdFire core architecture also has been modified to
allow an operand to be fetched in parallel with a multiply, increasing overall performance
for certain DSP operations.
Consider a typical filtering operation where the filter is defined as in Figure 3-2.
3-2
Operand Y Operand X
X
Shift 0,1,-1
+ / -
Accumulator(s)
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