MCF5282CVM66 Datasheet, PDF (92/766 Pages) Freescale Semiconductor, Inc – MCF5282 and MCF5216 ColdFire® Microcontroller User’s Manual

Enhanced Multiply-Accumulate Unit (EMAC)
As with change or use stalls between accumulators and general-purpose registers, introducing intervening
instructions that do not reference the busy register can reduce or eliminate sequence-related store-MAC
instruction stalls. A major benefit of the EMAC is the addition of three accumulators to minimize stalls
caused by exchanges between accumulator(s) and general-purpose registers.
3.3.4 Data Representation
MACSR[S/U,F/I] selects one of the following three modes, where each mode defines a unique operand
type:
1. Two’s complement signed integer: In this format, an N-bit operand value lies in the range -2^(N-1)
< operand < 2^(N-1) - 1. The binary point is right of the lsb.
2. Unsigned integer: In this format, an N-bit operand value lies in the range 0 < operand < 2^N - 1. The
binary point is right of the lsb.
3. Two’s complement, signed fractional: In an N-bit number, the first bit is the sign bit. The remaining
bits signify the first N-1 bits after the binary point. Given an N-bit number, a_N-1a_N-2a_N-3... a₂a₁a₀,
its value is given by the equation in Equation 3-3.
N–2
∑ ^{value = – (1 ⋅ aN – 1) + 2–(i + 1 – N) ⋅ ai}
Eqn. 3-3
i=0
This format can represent numbers in the range -1 < operand < 1 - 2^(N-1).
For words and longwords, the largest negative number that can be represented is -1, whose internal
representation is 0x8000 and 0x8000_0000, respectively. The largest positive word is 0x7FFF or (1 - 2^-15);
the most positive longword is 0x7FFF_FFFF or (1 - 2^-31).
3.3.5 MAC Opcodes
MAC opcodes are described in the ColdFire Programmer’s Reference Manual.
Remember the following:
• Unless otherwise noted, the value of MACSR[N,Z] is based on the result of the final operation that
involves the product and the accumulator.
• The overflow (V) flag is managed differently. It is set if the complete product cannot be represented
as a 40-bit value (this applies to 32 × 32 integer operations only) or if the combination of the
product with an accumulator cannot be represented in the given number of bits. The EMAC design
includes an additional product/accumulation overflow bit for each accumulator that are treated as
sticky indicators and are used to calculate the V bit on each MAC or MSAC instruction. See
Section 3.2.1, “MAC Status Register (MACSR)”.
• For the MAC design, the assembler syntax of the MAC (multiply and add to accumulator) and
MSAC (multiply and subtract from accumulator) instructions does not include a reference to the
single accumulator. For the EMAC, assemblers support this syntax and no explicit reference to an
accumulator is interpreted as a reference to ACC0. Assemblers also support syntaxes where the
destination accumulator is explicitly defined.
3-14
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3
Freescale Semiconductor