Create a Xilinx compatible memory. For Synthesis the XPM memory generator is used by generating a xpm_memory_tdpram instantiation with appropriate parameters. For Simulation hardcaml multiport memories are used to model the Xilinx memory behaviour.
The interface to the RAM differs subtly from the core primitives - we use separate read and write enables, rather than a single enable and write signal. The mapping is as follows:
- write_enable = enable & write
- read_enable = enable
The main difference occurs when write_enable and read_enable are both high. In the No_change collision mode, the read will not occur.
The documentation explains what happens across ports on address collisions, and in some cases this leads to Xs on the output of one port of the other. Hardcaml cannot model this behaviour, so it is up to the designer to be 'very careful' in these cases. Please see the table at the top of the implementation file for more information.
Ultraram has it's own subtle behaviour on address collision and this is modelled in hardcaml by putting the ports in opposite read/write first modes.
Distributed RAM is set up to work the same as BlockRAM.
There is a verilog testbench in test_hdl which works with the xsim_modelling application to test the subtle address collision cases.