ST20-C1 datasheet(67/205 Pages) STMICROELECTRONICS

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5 Multiply accumulate

Memory access

A Q15 stored in memory may be loaded to Areg with a

lsxinc instruction which will

automatically sign-extend the value. Similarly a Q15 may be written to memory with a

ssinc instruction (which will discard the top 16 bits).

Saturation

A well-formed Q15 value is sign extended to the width of the field, and so bits 15 to 31

inclusive will all be identical. Conversely, if the bits from 15 to 31 inclusive are not all

identical, then the value is not well-formed. It has either

overflowed (if b31=0, so

positive) or

underflo wed (if bit31=1, so negative).

A Q15 value in Areg may be saturated with the sequence:

ldc 00007FFF;

order;

ldc FFFF8000;

order;

rev;

5.7.6

Q31 format

Q31 format is a 32-bit signed fractional value in the range -1

≤ n < 1. The value is

stored in two’s-complement form with a sign bit (bit 31), an implied binary point

between bits 31 and 30, and 31 significant fractional bits (bit 30 to bit 0).

A Q31 value in a 32-bit field is organiz ed as shown in Figure 5.9.

Figure 5.9 Q31 data format

The minimum and maximum representable values are as shown in Table 5.11.

Hex Value

Decimal Value

Minimum

FFFF8000

-1

Maximum

00007FFF

+0.9999

Table 5.10 Q15 limiting values

2-1

-(20)2-2

2-4

2-3

2-5

2-30

2-29

2-31

binary point

sign bit

Bit position

Most significant

Least significant

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