DIRECT (DCAM) - 2025.1 English - UG1308

Direct address match, unlike the other tables, does not involve a comparison but instead uses the key as the direct address to access the stored value at that location (the table is effectively a RAM).

• Key width: from 1 to 16 bits
• Response width: from 1 to 7k bits
• Table depth (max. number of entries): 2^KeyWidth
• Direct match: True/False
• Hit/miss flag: Yes

  Here is an example of how to instantiate a direct table in a P4 file:

      table forwardIPv4 {
          key             = { hdr.ipv4.tos : exact; }
          actions         = { forwardPacket; NoAction; }
          size            = 64;
          default_action  = NoAction;
          direct_match    = true;
      }

For more details on how the compiler selects a direct table, see Compiler Table Selection. Note that in this type of table the total number of entries specified (size field) is ignored and is instead calculated from 2^Keywidth.