Data Structures

To better organize the code, is useful to represent both the command and response via data structures, specifically C++ classes. The data structure for the command is in the file fifo_fun_vitis/src/cmd.h which contains the data structure:

class Cmd {
public:

    ap_int<16> trans_id; // Transaction ID
    ap_int<32> a; // Operand A
    ap_int<32> b; // Operand B
    ...
};

The structure has the two operands a and b along with a transaction ID trans_id.
Each job is given a transaction ID which is echoed back in the response. In this way, missing or out-of-order responses can be tracked.

Similarly, the response is given in the file fifo_fun_vitis/src/cmd.h which contains the data structure:

class Resp {
public:

    enum ErrCode : unsigned int {
        NO_ERR = 0,
        SYNC_ERR = 1
    };

    ap_int<16> trans_id; // Transaction ID
    ap_int<32> c; // Operand C
    ap_int<32> d; // Operand D
    ap_uint<8> err_code; // Error Code
    ...
};

Here, c and d are the outputs, trans_id is the echo of the ID from the command, and err_code is an error code that indicates if there was some problem processing the command.

In addition to the fields in the data structure, there are a large number of functions for processing the structure, particularly packing and unpacking the data from streams.

Code auto-generation

Transfer of data structures over streaming interfaces typically requires packing and unpacking of different fields in the structure into discrete words. The words many be 32 or 64 bits depending on the interface. Often designers hand write the packing and unpacking routines for the individual structures. This is too cumbersome. So, I have created a way that you can automatically generate the code.

In fifo_fun_vitis\python\datastructs.py, there is a Python description of the command data structure. For example, the command structure is described by a list of fields:

# Command structure
cmd_fields = [
    FieldInfo("trans_id", IntType(16), descr="Transaction ID"),
    FieldInfo("a", IntType(32), descr="Operand A"),
    FieldInfo("b", IntType(32), descr="Operand B")]

The response structure has a similar description.
We can then auto-generate the Vitis include files, cmd.h and result.h from these descriptions:

Activate the virtual environment with xilinxutils. See the instructions
Navigate to the directory with the command and response field descriptions:
```
  hwdesign/fifoif/fifo_fun_vitis/python
```
Run the command:
```
  python gen_vitis_code.py
```
This program will create cmd.h and result.h in the directory:
```
  wdesign/fifoif/fifo_fun_vitis/src
```

Go to AXI4-Stream interface.