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Restructure rst

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Emil J. Tywoniak 2024-04-05 14:01:25 +02:00
parent 9510293a94
commit 43ef916f86
1 changed files with 22 additions and 29 deletions
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51
docs/source/yosys_internals/formats/cell_library.rst
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@ -622,22 +622,36 @@ 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 ``$macc`` cell type represents a generalized multiply and accumulate operation. The cell is purely combinational. It outputs the result of summing up a sequence of products and other injected summands.
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.
.. code-block::
Y = 0 +- a0factor1 * a0factor2 +- a1factor1 * a1factor2 +- ...
+ B[0] + B[1] + ...
The A port consists of concatenated pairs of multiplier inputs ("factors").
A zero length factor2 acts as a constant 1, turning factor1 into a simple summand.
In this pseudocode, ``u(foo)`` means an unsigned int that's foo bits long.
.. code-block::
struct A {
u(CONFIG.mul_info[0].factor1_len) a0factor1;
u(CONFIG.mul_info[0].factor2_len) a0factor2;
u(CONFIG.mul_info[1].factor1_len) a1factor1;
u(CONFIG.mul_info[1].factor2_len) a1factor2;
...
};
The cell's ``CONFIG`` parameter determines the layout of cell port ``A``.
The CONFIG parameter carries the following information:
.. code-block::
struct CONFIG {
u4 num_bits;
struct port_field {
struct mul_info {
bool is_signed;
bool is_subtract;
u(num_bits) factor1_len;
@ -645,28 +659,7 @@ The CONFIG parameter carries the following information:
}[num_ports];
};
The A cell port carries the following information:
.. code-block::
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::
Y = port0factor1 * port0factor2 + port1factor1 * port1factor2 + ...
+ B[0] + B[1] + ...
B is an array of concatenated 1-bit-wide unsigned integers to also be summed up.
Specify rules
~~~~~~~~~~~~~