NonStop S-Series Server Description Manual (G06.24+)

Principles of System Operation

HP NonStop S-Series Server Description Manual—520331-003

2-14

Sequence for Incoming Requests

Figure 2-7 illustrates the sequence of operations for an incoming request, one that did

not originate in this ServerNet device. In this case, the remote access logic is used

instead of the transfer initiation logic (shaded out) that was used in Figure 2-5 and

Figure 2-6.

1. Prior to being able to handle incoming requests, an access validation and

translation table must have been set up, as well as buffer space for each

ServerNet device that can possibly make requests to this (illustrated) device. The

example shown arbitrarily assumes that “device 3” is sending the request. Thus

ServerNet services will have set up the access validation and translation table

(AVTT) for that device. Entries in the AVTT contain information for virtual-to-

physical address translation.

2. The incoming request packet is received by the processor ServerNet interface.

Upon examining the header of the packet, the processor ServerNet interface

determines that this is a request packet. It is unexpected in the sense that this

device (specifically the BTE logic) did not originate this transaction. The processor

ServerNet interface therefore passes control to the access validation and

translation (AVT) hardware.

3. The AVT hardware accesses the AVTT to determine whether the address supplied

in the header of the packet is valid—that is, that it actually addresses some

location in the buffer space allocated for “device 3.” Virtual addresses are being

compared at this time.

4. If the access is valid, the AVT hardware reads the table entry information and

translates the address supplied by the packet into a physical address. The AVT

hardware at this point also returns a response packet (see next topic). If the

access is not valid, the response packet includes a negative acknowledge in the

header.

5. If the incoming valid request is a write request, the AVT hardware writes the packet

data into the addressed locations within whatever buffer space allocated for

“device 3.” The addressed locations may be an in-request buffer, as shown, or in

some cases can be the client’s buffer.

6. If a dedicated in-buffer is used, the client process periodically must copy the data

to its own (or some other) buffer.

7. If the incoming request address is valid but addresses a particular location that has

been agreed upon as an interrupt location (the AVTT entry at that location has its

interrupt bit set on), the packet is placed in an interrupt queue. A typical reason for

doing this might be to signify that all packets for the current request have been

exchanged.

8. In queue order, the packet is examined by exception-handler millicode, and the

appropriate interrupt is passed up to a higher-level interrupt handler.