Motorola ColdFire MCF5281 - Manual (Page 93)

Enhanced Multiply-Accumulate Unit (EMAC)
Freescale Semiconductor 3-15
• The optional 1-bit shift of the product is specified using the notation {<< | >>} SF, where <<1
indicates a left shift and >>1 indicates a right shift. The shift is performed before the product is
added to or subtracted from the accumulator. Without this operator, the product is not shifted. If the
EMAC is in fractional mode (MACSR[F/I] is set), SF is ignored and no shift is performed. Because
a product can overflow, the following guidelines are implemented:
— For unsigned word and longword operations, a zero is shifted into the product on right shifts.
— For signed, word operations, the sign bit is shifted into the product on right shifts unless the
product is zero. For signed, longword operations, the sign bit is shifted into the product unless
an overflow occurs or the product is zero, in which case a zero is shifted in.
— For all left shifts, a zero is inserted into the lsb position.
The following pseudocode explains basic MAC or MSAC instruction functionality. This example is
presented as a case statement covering the three basic operating modes with signed integers, unsigned
integers, and signed fractionals. Throughout this example, a comma-separated list in curly brackets, {},
indicates a concatenation operation.
switch (MACSR[6:5]) /* MACSR[S/U, F/I] */
{
case 0: /* signed integers */
if (MACSR.OMC == 0 || MACSR.PAVn == 0)
then {
MACSR.PAVn = 0
/* select the input operands */
if (sz == word)
then {if (U/Ly == 1)
then operandY[31:0] = {sign-extended Ry[31], Ry[31:16]}
else operandY[31:0] = {sign-extended Ry[15], Ry[15:0]}
if (U/Lx == 1)
then operandX[31:0] = {sign-extended Rx[31], Rx[31:16]}
else operandX[31:0] = {sign-extended Rx[15], Rx[15:0]}
}
else {operandY[31:0] = Ry[31:0]
operandX[31:0] = Rx[31:0]
}
/* perform the multiply */
product[63:0] = operandY[31:0] * operandX[31:0]
/* check for product overflow */
if ((product[63:39] != 0x0000_00_0) and and (product[63:39] != 0xffff_ff_1))
then { /* product overflow */
MACSR.PAVn = 1
MACSR.V = 1
if (inst == MSAC and and MACSR.OMC == 1)
then if (product[63] == 1)
then result[47:0] = 0x0000_7fff_ffff
else result[47:0] = 0xffff_8000_0000
else if (MACSR.OMC == 1)
then /* overflowed MAC,
saturationMode enabled */
if (product[63] == 1)
then result[47:0] = 0xffff_8000_0000
else result[47:0] = 0x0000_7fff_ffff
}
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3