ST20-C1 STMICROELECTRONICS [STMicroelectronics], ST20-C1 Datasheet - Page 65

• Manufacturer Part Number: ST20-C1
• Description: Instruction Set Reference Manual
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

5.7
This section gives details of the data formats used by the smacloop and biquad
instructions.
A signed fractional number of N bits is characterized as x.y, where x+y=N. This means
the number is made up from x bits before the binary point, an implied binary point, and
y fractional bits. Some examples are listed in Table 5.7.
5.7.1
A value n in x.y format has a range -2
For example, a value in the format 1.15 has range -1
range -2
5.7.2
The characteristic of the product of two fractional values is given by:
5.7.3
Table 5.8 shows the data formats for multiply-accumulate operations supported by the
ST20-C1.
Note that a Q15 value may be (optimally) stored in a 16-bit field, or a wider (>16-bit)
field with redundant sign bits. The two storage methods are described below.
5.7.4
Q15 format is a 16-bit signed fractional value in the range -1
stored in two’s-complement form with a sign bit (bit 15), an implied binary point
between bits 15 and 14, and 15 significant fractional bits (bit 14 to bit 0).
a.b * c.d = a+c.b+d
Signed 16-bit fractional
Signed 16-bit fractional
Signed 16-bit fractional
Data formats
Range
Multiplication
Supported formats
Q15 in 16 bits
n < 2.
Description
Table 5.8 Supported multiply accumulate data formats
Total bits
32
16
16
Table 5.7 Example data formats
-1
-1
-2
Range
n < 1
n < 2
n < 1
x-1
Name
Q14
Q15
Q31
n
Format
2
x-1
1.31
1.15
2.14
.
14 significant fr actional bits
15 significant fr actional bits
31 significant fr actional bits
n < 1, and the format 2.14 has
Short name
Q31
Q15
Q14
5 Multiply accumulate
Format
n < 1. The value is
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