memory_bram - map memories to block rams

    memory_bram -rules <rule_file> [selection]

This pass converts the multi-port $mem memory cells into block ram instances.
The given rules file describes the available resources and how they should be
used.

The rules file contains a set of block ram description and a sequence of match
rules. A block ram description looks like this:

    bram RAMB1024X32     # name of BRAM cell
      init 1             # set to '1' if BRAM can be initialized
      abits 10           # number of address bits
      dbits 32           # number of data bits
      groups 2           # number of port groups
      ports  1 1         # number of ports in each group
      wrmode 1 0         # set to '1' if this groups is write ports
      enable 4 1         # number of enable bits
      transp 0 2         # transparent (for read ports)
      clocks 1 2         # clock configuration
      clkpol 2 2         # clock polarity configuration
    endbram

For the option 'transp' the value 0 means non-transparent, 1 means transparent
and a value greater than 1 means configurable. All groups with the same
value greater than 1 share the same configuration bit.

For the option 'clocks' the value 0 means non-clocked, and a value greater
than 0 means clocked. All groups with the same value share the same clock
signal.

For the option 'clkpol' the value 0 means negative edge, 1 means positive edge
and a value greater than 1 means configurable. All groups with the same value
greater than 1 share the same configuration bit.

Using the same bram name in different bram blocks will create different variants
of the bram. Verilog configuration parameters for the bram are created as needed.

It is also possible to create variants by repeating statements in the bram block
and appending '@<label>' to the individual statements.

A match rule looks like this:

    match RAMB1024X32
      max waste 16384    # only use this bram if <= 16k ram bits are unused
      min efficiency 80  # only use this bram if efficiency is at least 80%
    endmatch

It is possible to match against the following values with min/max rules:

    words  ........  number of words in memory in design
    abits  ........  number of address bits on memory in design
    dbits  ........  number of data bits on memory in design
    wports  .......  number of write ports on memory in design
    rports  .......  number of read ports on memory in design
    ports  ........  number of ports on memory in design
    bits  .........  number of bits in memory in design
    dups ..........  number of duplications for more read ports

    awaste  .......  number of unused address slots for this match
    dwaste  .......  number of unused data bits for this match
    bwaste  .......  number of unused bram bits for this match
    waste  ........  total number of unused bram bits (bwaste*dups)
    efficiency  ...  total percentage of used and non-duplicated bits

    acells  .......  number of cells in 'address-direction'
    dcells  .......  number of cells in 'data-direction'
    cells  ........  total number of cells (acells*dcells*dups)

The interface for the created bram instances is derived from the bram
description. Use 'techmap' to convert the created bram instances into
instances of the actual bram cells of your target architecture.

A match containing the command 'or_next_if_better' is only used if it
has a higher efficiency than the next match (and the one after that if
the next also has 'or_next_if_better' set, and so forth).

A match containing the command 'make_transp' will add external circuitry
to simulate 'transparent read', if necessary.

A match containing the command 'make_outreg' will add external flip-flops
to implement synchronous read ports, if necessary.

A match containing the command 'shuffle_enable A' will re-organize
the data bits to accommodate the enable pattern of port A.