SC Item

The item interface provides communication between the system and the host. This module (named nuplus_item_interface) implements a control logic which interprets messages from the host and applies those requests on the nu+ core through a specific interface.

The supported messages are defined in the module:

typedef enum logic [HOST_COMMAND_WIDTH - 1 : 0] {
  HN_BOOT_COMMAND         = 0, 
  HN_BOOT_ACK             = 1, 
  HN_ENABLE_CORE_COMMAND  = 2, 
  HN_ENABLE_CORE_ACK      = 3, 
  HN_READ_STATUS_COMMAND  = 8, 
  HN_WRITE_STATUS_COMMAND = 9, 
  HN_LOG_REQUEST          = 10 
} hn_messages_t;

The command supported are:

• HN_BOOT_COMMAND: sets the PC of a given thread, the next item defines the ID of the thread, while the last item sets the PC value. The control logic gathers those information and sets the PC using a specific interface to the core:

output logic                                      hi_job_valid,
output address_t                                  hi_job_pc,
output thread_id_t                                hi_job_thread_id,

When all the required items are ready, the FSM sets the hi_job_valid bit for a clock cycle and forwards PC and thread ID respectively on hi_job_pc and hi_job_thread_id.

• HN_ENABLE_CORE_COMMAND: sets the active thread mask in the nu+ core. The value from the host is organized as a bitmask, if the i-th is high the i-th is active and it starts running. The bitmask is stored in a dedicated register:

always_ff @ ( posedge clk, posedge reset )
   if ( reset ) begin
      thread_en      <= {`THREAD_NUMB{1'b0}};
   end else begin
      if ( update_thread_en )
         thread_en      <= item_data_i[`THREAD_NUMB - 1 : 0];
end

and combinatorially propagated to the core:

output                 [`THREAD_NUMB - 1 : 0]     hi_thread_en,
assign hi_thread_en        = thread_en;

• HN_READ_STATUS_COMMAND: fetches a special purpose register from the core, such as performance counters and PCs. Some registers are shared others are private among threads, e.g. PCs. The control logic waits for two data from the host, which are both in the next item from the host. The low half of the data defines the register ID, while the remaining part stores the thread id:

• HN_WRITE_STATUS_COMMAND
• HN_LOG_REQUEST