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- Unswap shift/shiftx - Add brief overview to cell lib - Clarify $div cell B input - Clarify unary operators - What is $modfloor
91 lines
4.5 KiB
ReStructuredText
91 lines
4.5 KiB
ReStructuredText
.. role:: verilog(code)
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:language: Verilog
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Binary operators
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~~~~~~~~~~~~~~~~
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All binary RTL cells have two input ports ``A`` and ``B`` and one output port
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``Y``. They also have the following parameters:
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``A_SIGNED``
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Set to a non-zero value if the input ``A`` is signed and therefore should be
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sign-extended when needed.
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``A_WIDTH``
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The width of the input port ``A``.
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``B_SIGNED``
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Set to a non-zero value if the input ``B`` is signed and therefore should be
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sign-extended when needed.
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``B_WIDTH``
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The width of the input port ``B``.
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``Y_WIDTH``
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The width of the output port ``Y``.
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.. table:: Cell types for binary operators with their corresponding Verilog expressions.
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======================= =============== ======================= ===========
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Verilog Cell Type Verilog Cell Type
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======================= =============== ======================= ===========
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:verilog:`Y = A & B` `$and` :verilog:`Y = A ** B` `$pow`
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:verilog:`Y = A | B` `$or` :verilog:`Y = A < B` `$lt`
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:verilog:`Y = A ^ B` `$xor` :verilog:`Y = A <= B` `$le`
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:verilog:`Y = A ~^ B` `$xnor` :verilog:`Y = A == B` `$eq`
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:verilog:`Y = A << B` `$shl` :verilog:`Y = A != B` `$ne`
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:verilog:`Y = A >> B` `$shr` :verilog:`Y = A >= B` `$ge`
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:verilog:`Y = A <<< B` `$sshl` :verilog:`Y = A > B` `$gt`
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:verilog:`Y = A >>> B` `$sshr` :verilog:`Y = A + B` `$add`
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:verilog:`Y = A && B` `$logic_and` :verilog:`Y = A - B` `$sub`
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:verilog:`Y = A || B` `$logic_or` :verilog:`Y = A * B` `$mul`
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:verilog:`Y = A === B` `$eqx` :verilog:`Y = A / B` `$div`
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:verilog:`Y = A !== B` `$nex` :verilog:`Y = A % B` `$mod`
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``N/A`` `$shift` ``N/A`` `$divfloor`
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``N/A`` `$shiftx` ``N/A`` `$modfloor`
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======================= =============== ======================= ===========
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The `$shl` and `$shr` cells implement logical shifts, whereas the `$sshl` and
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`$sshr` cells implement arithmetic shifts. The `$shl` and `$sshl` cells
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implement the same operation. All four of these cells interpret the second
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operand as unsigned, and require ``B_SIGNED`` to be zero.
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Two additional shift operator cells are available that do not directly
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correspond to any operator in Verilog, `$shift` and `$shiftx`. The `$shift` cell
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performs a right logical shift if the second operand is positive (or unsigned),
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and a left logical shift if it is negative. The `$shiftx` cell performs the same
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operation as the `$shift` cell, but the vacated bit positions are filled with
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undef (x) bits, and corresponds to the Verilog indexed part-select expression.
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For the binary cells that output a logical value (`$logic_and`, `$logic_or`,
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`$eqx`, `$nex`, `$lt`, `$le`, `$eq`, `$ne`, `$ge`, `$gt`), when the ``Y_WIDTH``
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parameter is greater than 1, the output is zero-extended, and only the least
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significant bit varies.
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Division and modulo cells are available in two rounding modes. The original
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`$div` and `$mod` cells are based on truncating division, and correspond to the
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semantics of the verilog ``/`` and ``%`` operators. The `$divfloor` and
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`$modfloor` cells represent flooring division and flooring modulo, the latter of
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which corresponds to the ``%`` operator in Python. See the following table for a
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side-by-side comparison between the different semantics.
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.. table:: Comparison between different rounding modes for division and modulo cells.
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+-----------+--------+-----------+-----------+-----------+-----------+
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| Division | Result | Truncating | Flooring |
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+-----------+--------+-----------+-----------+-----------+-----------+
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| | | $div | $mod | $divfloor | $modfloor |
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+===========+========+===========+===========+===========+===========+
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| -10 / 3 | -3.3 | -3 | -1 | -4 | 2 |
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+-----------+--------+-----------+-----------+-----------+-----------+
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| 10 / -3 | -3.3 | -3 | 1 | -4 | -2 |
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+-----------+--------+-----------+-----------+-----------+-----------+
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| -10 / -3 | 3.3 | 3 | -1 | 3 | -1 |
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+-----------+--------+-----------+-----------+-----------+-----------+
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| 10 / 3 | 3.3 | 3 | 1 | 3 | 1 |
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+-----------+--------+-----------+-----------+-----------+-----------+
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.. autocellgroup:: binary
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:members:
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:source:
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:linenos:
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