ModelSim SE User's Manual - Electrical and Computer Engineering

UM-156 5 - Verilog simulation
Sparse memory modeling
ModelSim SE User’s Manual
Sparse memories are a mechanism for allocating storage for memory elements only when
they are needed. You mark which memories should be treated as sparse, and ModelSim
dynamically allocates memory for the accessed addresses during simulation.
Sparse memories are more efficient in terms of memory consumption, but access times to
sparse memory elements during simulation are slower. Thus, sparse memory modeling
should be used only on memories whose active addresses are "few and far between."
There are two methods of enabling sparse memories:
“Manually” by inserting attributes or meta-comments in your code
Automatically by setting the SparseMemThreshhold (UM-526) variable in the
modelsim.ini file
Manually marking sparse memories
You can mark memories in your code as sparse using either the mti_sparse attribute or the
sparse meta-comment. For example:
(* mti_sparse *) reg mem [0:1023]; // Using attribute
reg /*sparse*/ [0:7] mem [0:1023]; // Using meta-comment
The meta-comment syntax is supported for compatibility with other simulators.
Automatically enabling sparse memories
Using the SparseMemThreshhold (UM-526) .ini variable, you can instruct ModelSim to
mark as sparse any memory that is a certain size. Consider this example:
If SparseMemThreshold = 2048 then
reg mem[0:2047]; // will be marked as sparse automatically
reg mem[0:2046]; // will not be marked as sparse
Combining automatic and manual modes
Because mti_sparse is a Verilog 2001 attribute that accepts values, you can enable
automatic sparse memory modeling but still control individual memories within your code.
Consider this example:
If SparseMemThreshold = 2048 then
reg mem[0:2047]; // will be marked as sparse automatically
reg mem[0:2046]; // will not be marked as sparse
However, you can override this automatic behavior using mti_sparse with a value:
(* mti_sparse = 0 *) reg mem[0:2047]; // will *not* be marked as sparse even
though SparseMemThreshold = 2048
(* mti_sparse = 1*) reg mem[0:2046]; // will be marked as sparse even though
SparseMemThreshold = 2048