Freescale Semiconductor MCF5480 - Manual (Page 152)

MCF548x Reference Manual, Rev. 3
4-4 Freescale Semiconductor
Figure 4-5. Signed and Unsigned Integer Alignment
Thus, the 48-bit accumulator definition is a function of the EMAC operating mode. Given that each 48-bit
accumulator is the concatenation of 16-bit accumulator extension register (ACCextn) contents and 32-bit
ACCn contents, the specific definitions are as follows:
if MACSR[6:5] == 00/* signed integer mode */
Complete Accumulator[47:0] = {ACCextn[15:0], ACCn[31:0]}
if MACSR[6:5] == -1/* signed fractional mode */
Complete Accumulator [47:0] = {ACCextn[15:8], ACCn[31:0], ACCextn[7:0]}
if MACSR[6:5] == 10/* unsigned integer mode */
Complete Accumulator[47:0] = {ACCextn[15:0], ACCn[31:0]}
The four accumulators are represented as an array, ACCn, where n selects the register.
Although the multiplier array is implemented in a four-stage pipeline, all arithmetic MAC instructions
have an effective issue rate of 1 cycle, regardless of input operand size or type.
All arithmetic operations use register-based input operands, and summed values are stored internally in an
accumulator. Thus, an additional move instruction is needed to store data in a general-purpose register.
One new feature found in EMAC instructions is the ability to choose the upper or lower word of a register
as a 16-bit input operand. This is useful in filtering operations if one data register is loaded with the input
data and another is loaded with the coefficient. Two 16-bit multiply accumulates can be performed without
fetching additional operands between instructions by alternating the word choice during the calculations.
The EMAC has four accumulator registers versus the MAC’s one accumulator. The additional registers
improve the performance of some algorithms by minimizing pipeline stalls needed to store an accumulator
value back to general-purpose registers. Many algorithms require multiple calculations on a given data set.
By applying different accumulators to these calculations, it is often possible to store one accumulator
without any stalls while performing operations involving a different destination accumulator.
X
OperandY
OperandX
Product
Extended Product
Accumulator
32
32
32
32
32
8
8
8
24
8
8
+
Extension Byte Upper [7:0]
Extension Byte Lower [7:0]
Accumulator [31:0]