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docs: Document $macc

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Emil J. Tywoniak 2024-04-03 20:37:54 +02:00
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docs/source/yosys_internals/formats/cell_library.rst
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@ -619,6 +619,52 @@ Finite state machines
Add a brief description of the ``$fsm`` cell type.
Coarse arithmetics
~~~~~~~~~~~~~~~~~~~~~
The ``$macc`` cell type represents a multiply and accumulate block, for summing any number of negated and unnegated signals and arithmetic products of pairs of signals. Cell port A concatenates pairs of signals to be multiplied together. When the second signal in a pair is zero length, a constant 1 is used instead as the second factor. Cell port B concatenates 1-bit-wide signals to also be summed, such as "carry in" in adders.
The cell's ``CONFIG`` parameter determines the layout of cell port ``A``.
In the terms used for this cell, there's mixed meanings for the term "port". To disambiguate:
A cell port is for example the A input (it is constructed in C++ as ``cell->setPort(ID::A, ...))``
Multiplier ports are pairs of multiplier inputs ("factors").
If the second signal in such a pair is zero length, no multiplication is necessary, and the first signal is just added to the sum.
In this pseudocode, ``u(foo)`` means an unsigned int that's foo bits long.
The CONFIG parameter carries the following information:
.. code-block::
:force:
struct CONFIG {
u4 num_bits;
struct port_field {
bool is_signed;
bool is_subtract;
u(num_bits) factor1_len;
u(num_bits) factor2_len;
}[num_ports];
};
The A cell port carries the following information:
.. code-block::
:force:
struct A {
u(CONFIG.port_field[0].factor1_len) port0factor1;
u(CONFIG.port_field[0].factor2_len) port0factor2;
u(CONFIG.port_field[1].factor1_len) port1factor1;
u(CONFIG.port_field[1].factor2_len) port1factor2;
...
};
No factor1 may have a zero length.
A factor2 having a zero length implies factor2 is replaced with a constant 1.
Additionally, B is an array of 1-bit-wide unsigned integers to also be summed up.
Finally, we have:
.. code-block::
:force:
Y = port0factor1 * port0factor2 + port1factor1 * port1factor2 + ...
* B[0] + B[1] + ...
Specify rules
~~~~~~~~~~~~~
@ -1152,4 +1198,4 @@ file via ABC using the abc pass.
.. todo:: Add information about ``$lut`` and ``$sop`` cells.
.. todo:: Add information about ``$alu``, ``$macc``, ``$fa``, and ``$lcu`` cells.
.. todo:: Add information about ``$alu``, ``$fa``, and ``$lcu`` cells.