ST20-C1 datasheet(63/205 Pages) STMICROELECTRONICS

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5 Multiply accumulate

5.6.3

Pre-scaling and rounding -

smacloop

The initial accumulator value must be loaded into the accumulator for the start of the

smacloop instruction. The initial accumulator is either in Q31 format (LongMode), or

Q15 format (

ShortMode) and is handled accordingly.

LongMode,

• left shift by 7 places

(right 1 + left 8)

• sign extend from bit 38 to the most significant bit

• set bit 6 = 1 (for rounding)

ShortMode,

• left shift by 23 places

(left 15 + left 8)

• sign extend from bit 38 to the most significant bit

• set bit 22 = 1 (for rounding)

Note that rounding is achieved by adding half of the least significant bit to the initial

value, which by associativity is equivalent to adding it to the final value.

5.6.4

Pre-scaling and rounding -

biquad

The biquad instruction starts with an empty accumulator. Since the result is always

Q15 (equivalent to

ShortMode), rounding is achieved by loading the accumulator with

222 (which is half of the least significant bit of the result).

5.6.5

Post-scaling and saturation -

smacloop

At the end of a

smacloop instruction the final accum ulator value is saturated and

scaled to the appropriate format, according to mac_mode.

If either the

overflow or underflow bits in the status register are set, then the final value

is set to the appropriate exceptional value from Table 5.6. Note that this does not

involve testing the accumulator value, which is considered to be invalid if either of

these status register bits are set.

Otherwise, when the status register reports no overflow or underflow, bits 38 to 47

inclusive of the accumulator are tested for mutual equality. If they are all the same, the

accumulator value is well-formed and post-scaling is applied to produce the final accu-

mulator value.

LongMode,

• arithmetic right shift by 7 places(left 1 + right 8)

• truncate to low 32 bits

ShortMode,

• arithmetic right shift by 23 places (right 15 + right 8)

• truncate to low 32 bits

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