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doc: Constant

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Sebastien Bourdeauducq 2015-09-17 11:05:57 +08:00
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@ -13,13 +13,17 @@ FHDL is made of several elements, which are briefly explained below. They all ca
Expressions
***********
Integers and booleans
=====================
Constants
=========
Python integers and booleans can appear in FHDL expressions to represent constant values in a circuit. ``True`` and ``False`` are interpreted as 1 and 0, respectively.
The ``Constant`` object represents a constant, HDL-literal integer. It behaves like specifying integers and booleans but also supports slicing and can have a bit width or signedness different from what is implied by the value it represents.
``True`` and ``False`` are interpreted as 1 and 0, respectively.
Negative integers are explicitly supported. As with MyHDL [countin]_, arithmetic operations return the natural results.
To lighten the syntax, assignments and operators automatically wrap Python integers and booleans into ``Constant``. Additionally, ``Constant`` is aliased to ``C``. The following are valid Migen statements: ``a.eq(0)``, ``a.eq(a + 1)``, ``a.eq(C(42)[0:1])``.
.. [countin] http://www.jandecaluwe.com/hdldesign/counting.html
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