The debug engine initiates all transactions with the kernel debugger, normally by sending a command while the kernel debugger is waiting to receive one. In this case, the kernel debugger expects to receive a CVK_HDR_DATA packet and will discard any CVK_HDR_ACK or CVK_HDR_NACK packets it receives. (The kernel debugger also discards a packet if its header cannot be unbitstuffed or has a bad checksum. The kernel issues a CVK_HDR_NACK packet containing the sequence number and index number from the original CVK_HDR_DATA packet if the packet's body cannot be unbitstuffed or has a bad checksum.)

Once the kernel receives a valid CVK_HDR_DATA packet, it extracts the sequence number and index number and uses them to construct a CVK_HDR_ACK packet, which it returns to the debug engine. (The flast flag in the CVK_HDR_DATA packet is ignored.) The kernel debugger then performs the action requested by the command and returns the result.

In general, the result is returned in a single CVK_HDR_DATA packet whose sequence number matches the sequence number contained in the debug engine's original command packet, whose index number is zero, and whose flast flag is FALSE. After the result is transmitted, the kernel debugger waits for a response from the debug engine. The kernel is expecting either a CVK_HDR_ACK packet whose sequence number and index number match those sent in the result _or_ a CVK_HDR_DATA packet (containing the next command). If the kernel debugger receives any other response, it resends the CVK_HDR_DATA packet containing the result.

If the debug engine sends a KDP break character while the victim machine is running, either to initiate a transaction or to regain control after the victim machine has resumed execution, the kernel debugger responds with a CVK_HDR_DATA packet whose sequence number matches the sequence number from the CVK_HDR_DATA packet that caused the system to resume. There is no such packet when the kernel debugger responds to the first KDP break sent by the debug engine. The sequence number in that case contains garbage.

The kernel debugger does not generate replies for some commands, such as reboot, and the replies to commands that cause the victim machine to resume execution, such as resume or step, are not sent until an event such as a breakpoint, module load, or break signal from the debug engine, has caused the victim machine to quiesce.

The kernel debugger responds somewhat differently to the CVK_CMD_RAW command, which is used to issue arbitrary kernel debugger commands while in packet mode. Each line in the response is returned in a separate CVK_HDR_DATA packet whose sequence number matches the sequence number in the CVK_CMD_RAW command's header. The index number in the first reply packet is 0; the index number increases by 1 in each successive reply packet (and wraps from 63 to 0). The debug engine should return a CVK_HDR_ACK packet with the appropriate sqeuence number and index number after each reply packet is received.

The kernel debugger does not manipulate or increment sequence numbers and uses them only to generate ACKs and NACKs and to match ACKs with replies. A debug engine could use the same sequence number for every request, but this is not recommended.