In the PCIe Data Link Layer, formatting packets and storing them in the Replay Buffer is only half the battle. The transmitter also needs a reliable way to monitor which of those packets have been successfully received and acknowledged by the neighboring device. This vital tracking task is handled by a specialized hardware component known as the ACKD_SEQ register.
Here is a closer look at how this register tracks sequence numbers, verifies forward progress, and prevents the system from overflowing with unacknowledged data.
What is the ACKD_SEQ Register?
The ACKD_SEQ (Acknowledged Sequence) register is a 12-bit register located within the transmitter. Its sole purpose is to store the sequence number of the most recently received Ack or Nak Data Link Layer Packet (DLLP).
When the system undergoes a reset, or whenever the Data Link Layer becomes inactive, this register is initialized to all 1s. During normal operation, every time a new Ack or Nak successfully arrives, the register is updated using the 12-bit AckNak_Seq_Num field found inside that incoming DLLP.
Evaluating Forward Progress
The primary function of the ACKD_SEQ register is to help the transmitter determine if the Link is making “forward progress”.
Whenever an Ack or Nak is received, the transmitter compares the sequence number stored inside the DLLP against the value currently held in the ACKD_SEQ register. If the sequence number in the newly arrived DLLP is “later” than the number stored in the register, the transmitter officially knows that forward progress has occurred.
Because PCIe sequence numbers are generated by a 12-bit counter that maxes out at 4095 and rolls over to zero, the system’s logic is designed to understand that a “later” sequence number is not always a numerically higher value. It properly accounts for this wrap-around effect when performing the comparison.
Triggering Buffer Updates
Confirming forward progress via the ACKD_SEQ register triggers immediate action within the transmitter:
- Purging Packets: It purges all Transaction Layer Packets (TLPs) from the Replay Buffer that possess sequence numbers equal to or older than the value in the received DLLP.
- Resetting Timers: It resets the internal
REPLAY_TIMERand theREPLAY_NUMcounters, as successful communication has been verified.
Conversely, if the comparison shows no forward progress has been made, no TLPs can be safely purged from the buffer. In that scenario, the transmitter simply checks to see if the incoming packet is a Nak that necessitates a replay of the buffer’s contents.
The Ultimate Safeguard: The 2048 Rule
The ACKD_SEQ register also plays a starring role in preventing catastrophic sequence number overlap. Theoretically, a transmitter could send out thousands of TLPs faster than the receiver can acknowledge them, causing the 12-bit sequence counter to lap itself and create massive confusion.
To prevent this, the PCIe specification dictates that the number of unacknowledged packets in flight can never exceed half the maximum count value, which is 2048. The hardware enforces this by constantly monitoring the distance between two components:
- The
NEXT_TRANSMIT_SEQcounter (tracking the next packet to be sent). - The
ACKD_SEQregister (tracking the last packet acknowledged).
If the numerical distance between what has been sent and what has been acknowledged ever reaches 2048, the transmitter automatically blocks the acceptance of any new TLPs from the Transaction Layer. It will stubbornly hold this block until the receiver catches up and returns more Acks, keeping the pipeline perfectly stable.
Summary The ACKD_SEQ register serves as the transmitter’s reliable memory for successful data deliveries. By constantly updating with the latest acknowledged sequence numbers and verifying forward progress, it safely manages the Replay Buffer and ensures the transmission pipeline never becomes dangerously overloaded.