doom_riscv/cores/usb/doc/microcode.md

USB Transaction microcode

The upper 4 bits of the word determine the operation type.

Data / Ops (0xxx )

0x0: NOP - No OPeration

0x1: LD - LoaD

    [2:0] - Source
            000 - evt          - Pending Events
                                 bit 0 = RX OK
                                 bit 1 = RX Error
                                 bit 2 = TX Done
                                 bit 3 = Timeout

            010 - pkt_pid      - Packet PID
            011 - pkt_pid_chk  - Packet PID (DATA0/DATA1 check)
            100 - ep_type      - End Point type
            110 - bd_state     - State of Buffer Descriptor

0x2: EP - End Point operation

      [8] - Set Control Endpoint Lockout bit
      [7] - Issue Write Back
    [5:3] - New Buffer Descriptor State value
      [2] - Set Buffer Descriptor State
      [1] - Flip Buffer index bit (active only if EP is dual buffered)
      [0] - Flip Data Toggle bit

0x3: ZL - Zero Length

0x4: TX - Transmit packet

      [4] - Auto-Set DataToggle (DATA0/DATA1)
    [3:0] - Packet PID

0x5: NOTIFY - Notify Host

    [3:0] - Notify code

0x6: EVT_CLR - EVenT CLeaR

    [3:0] - Bit Mask of events to clear

0x7: EVT_RTO - EVenT Receive Time Out

    [7:0] - Timeout value

Control flow ( 1xxx )

JMP / JEQ / JNE

      [15] - Set to 1 to denote control flow operation
      [14] - Invert the condition
    [13:8] - Target address (divided by 4)
     [7:0] - Condition Mask
     [3:0] - Condition Value

This performs conditional jumps to any address where the two LSBs are clear (i.e. aligned to 4).