Motorola ColdFire MCF5281 - Manual (Page 92)

Enhanced Multiply-Accumulate Unit (EMAC)
3-14 Freescale Semiconductor
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-1
a
N-2
a
N-3
... a
2
a
1
a
0
,
its value is given by the equation in Equation 3-3 .
Eqn. 3-3
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.
value 1 a
N 1 –
⋅ ( )
– 2
i 1 N – + ( ) –
ai
⋅
i 0 =
N 2 –
∑
+ =
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3