Pass by Value

• ap_none: No I/O protocol. This is the default for inputs.
• ap_stable: No I/O protocol.
• ap_ack: Implemented with an associated output acknowledge port.
• ap_vld: Implemented with an associated input valid port.
• ap_hs: Implemented with both input valid and output acknowledge ports.

Pass by Reference

• ap_none: No I/O protocol. This is the default for inputs.
• ap_stable: No I/O protocol.
• ap_ack: Implemented with an associated input acknowledge port.
• ap_vld: Implemented with an associated output valid port. This is the default for outputs.
• ap_ovld: Implemented with an associated output valid port (no valid port
  for the input part of any inout ports).
• ap_hs: Implemented with both input valid port and output acknowledge
  ports
• ap_fifo: A FIFO interface with associated output write and input FIFO full ports.
• ap_bus: A Vivado HLS bus interface protocol.