Map Persistent Arrays and dsp.Delay to RAM

How To Enable RAM Mapping

In the HDL Workflow Advisor, select MATLAB to HDL Workflow > Code Generation > Optimizations tab.
Select the Map persistent array variables to RAMs option.
Set the RAM mapping threshold to the size (in bits) of the smallest persistent array, user-defined System object™ private property, or dsp.Delay that you want to map to RAM.

RAM Mapping Requirements for Persistent Arrays and System object Properties

The following table shows a summary of the RAM mapping behavior for persistent arrays and private properties of a user-defined System object.

Map Persistent Array Variables to RAMs Setting	Mapping Behavior
on	Map to RAM. For restrictions, see RAM Mapping Restrictions.
off	Map to registers in the generated HDL code.

RAM Mapping Restrictions

When you enable RAM mapping, a persistent array or user-defined System object private property maps to a block RAM when all of the following conditions are true:

Each read or write access is for a single element only. For example, submatrix access and array copies are not allowed.
Address computation logic is not read-dependent. For example, computation of a read or write address using the data read from the array is not allowed.
Persistent variables or user-defined System object private properties are initialized to 0 if they have a cyclic dependency. For example, if you have two persistent variables, A and B, you have a cyclic dependency if A depends on B, and B depends on A.
If an access is within a conditional statement, the conditional statement uses only simple logic expressions (&&, ||, ~) or relational operators. For example, in the following code, r1 does not map to RAM:
```
if (mod(i,2) > 0)
    a = r1(u);
else
    r1(i) = u;
end 
```
Rewrite complex conditions, such as conditions that call functions, by assigning them to temporary variables, and using the temporary variables in the conditional statement. For example, to map r1 to RAM, rewrite the previous code as follows:
```
temp = mod(i,2);
if (temp > 0)
    a = r1(u);
else
    r1(i) = u;
end 
```

The persistent array or user-defined System object private property value depends on external inputs.

For example, in the following code, bigarray does not map to RAM because it does not depend on u:

function z = foo(u)

persistent cnt bigarray
if isempty(cnt)
    cnt = fi(0,1,16,10,hdlfimath);
    bigarray = uint8(zeros(1024,1));
end
z = u + cnt;
idx = uint8(cnt);
temp = bigarray(idx+1);
cnt(:) = cnt + fi(1,1,16,0,hdlfimath) + temp;
bigarray(idx+1) = idx;

RAMSize is greater than or equal to the RAMMappingThreshold value. RAMSize is the product NumElements * WordLength * Complexity.
- NumElements is the number of elements in the array.
- WordLength is the number of bits that represent the data type of the array.
- Complexity is 2 for arrays with a complex base type; 1 otherwise.
Access to the persistent variable that you are mapping to RAM cannot be within a loop, such as a for loop, unless the loop is unrolled. For more information, see coder.unroll.
Access to the persistent variable that you are mapping to RAM cannot be within a nested conditional statement such as a nested if statement, or nested switch statement.

If any of the above conditions is false, the persistent array or user-defined System object private property maps to a register in the HDL code.

RAM Mapping Requirements for `dsp.Delay` System Objects

A summary of the mapping behavior for a dsp.Delay System object is in the following table.

Map Persistent Array Variables to RAMs Option Mapping Behavior

Map Persistent Array Variables to RAMs Option	Mapping Behavior
on	A `dsp.Delay` System object maps to a block RAM when all of the following conditions are true: `Length` property is greater than 4. `InitialConditions` property is 0. Delay input data type is one of the following: Real scalar with a non-floating-point data type. Complex scalar with real and imaginary parts that are non-floating-point. Vector where each element is either a non-floating-point real scalar or complex scalar. `RAMSize` is greater than or equal to the RAM Mapping Threshold value. `RAMSize` is the product `Length` * `InputWordLength`. `InputWordLength` is the number of bits that represent the input data type. If any of the conditions are false, the `dsp.Delay` System object maps to registers in the HDL code.
off	A `dsp.Delay` System object maps to registers in the generated HDL code.

A dsp.Delay System object maps to a block RAM when all of the following conditions are true:

Length property is greater than 4.
InitialConditions property is 0.
Delay input data type is one of the following:
- Real scalar with a non-floating-point data type.
- Complex scalar with real and imaginary parts that are non-floating-point.
- Vector where each element is either a non-floating-point real scalar or complex scalar.
RAMSize is greater than or equal to the RAM Mapping Threshold value.
- RAMSize is the product Length * InputWordLength.
- InputWordLength is the number of bits that represent the input data type.

If any of the conditions are false, the dsp.Delay System object maps to registers in the HDL code.

off

A dsp.Delay System object maps to registers in the generated HDL code.