The FixedPointBits Class

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class fixedpoint.FixedPointBits

FixedPointBits inherits from int and adds item access. You should not need to instantiate this, but this allows for the FixedPoint.bits attribute to be indexed, sliced, and mapped.

s
Type

bool

Value

True for signed, False for unsigned.

m
Type

int

Value

Number of integer bits.

n
Type

int

Value

Number of fractional bits.

__getitem__(key)

Note

This is the built-in square bracket [] operator.

Parameters

key (int or slice or str) – Bit index, slice, or mapping

Return type

int or str

Raises

  • KeyError – Unsupported mapping string

  • IndexError – Invalid slice step or index out of range

The square brackets allow access to one or more bits at a time. No matter the access scheme (indexing, slicing, or mapping, described below), the return value is always shifted to be no more than N bits, where N is the number of bits accessed. E.g., accessing 3 bits will return an integer in the range [0, 23), regardless of where the are located in the FixedPointBits.

Indexing

When key is an int, a single bit is accessed in FixedPoint.bits. Index 0 is the LSb and index \(m + n - 1\) is the MSb.

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Slicing

When key is a slice (either an explicit slice object, or generated by using one or more :s), one or more bits can be accessed. With bits as FixedPointBits and integers A, B, and C such that A > B:

  • bits[A:B:C] returns bits A down to B (inclusive) with index 0 being the LSb and \(m + n - 1\) being the MSb. C can be omitted, but must be -1 if specified.

  • bits[B:A:C] returns bits A up to B (inclusive) with index 0 being the MSb and \(m + n - 1\) being the LSb. C can be omitted, but must be 1 if specified.

  • bits[A:A:C] with C == -1 returns bit A within index 0 being the LSb and \(m + n - 1\) being the MSb.

  • bits[B:B:C] with C == 1 returns bit B within index 0 being the MSb and \(m + n - 1\) being the LSb.

Any slicing format not specified above treats the FixedPointBits as a binary digit str (indexed from 0 to \(m + n - 1\)).

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Mapping

Common bit slices are mapped to string keywords:

Key String

Bit Slice

Assumptions

'm'

integer bits only

FixedPointBits.m > 0

'int'

'n'

fractional bits only

FixedPointBits.n > 0

'frac'

's'

most significant bit
FixedPointBits.s
is True

'sign'

'msb'

–

'lsb'

least significant bit

If the mapping is accessed and the assumption(s) for that mapping are not satisfied, a KeyError is raised.

When the key string is UPPERCASED, the return value is a str of binary bits.

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