To:  The SCSI Committee                       X3T9.2/88-064 Rev 0

From: Jeff Stai, Western Digital Corp.

Subject: Processor Device Model

     Well, here goes... Add the following as section 11.1:

11.1. Processor Device Model

     The SCSI processor device is a simple target that is 
designed to receive and send data packets with SCSI initiators. 
The contents of these data packets are not defined by this 
standard. A processor device may also use the COPY command to 
move data between itself and another SCSI device, or it may move 
data between two other SCSI devices (this function is not unique 
to processor devices).

     A processor device typically is implemented for one or more 
of the purposes described below.


11.1.1. Host to Host Communication

     A "Host" is defined here as a "primary computing device", 
such as a personal computer or minicomputer. In this application 
of a processor device, two or more host systems use the SCSI bus 
to pass packets amongst themselves as in a Very Local Area 
Network (VLAN). The data packets will typically carry system 
requests, system status, and "user" data.

     As an example of system behavior, a host system (Host A) 
would take the initiator role, select another host (Host B) as a 
target, and issue a packet (via the SEND command) which contains 
an operating system call that requests data from Host B's local 
storage. Host B would process the request, and prepare a data 
packet with the requested data and also the completion status of 
the system call. Host B would then take the initiator role, 
select Host A as a target, and transfer the packet (again, via 
the SEND command) to Host A.

     If an error occurred for the operating system call sent by 
Host A, the error information would be reported via the 
completion status information in the data packet. In this model, 
the REQUEST SENSE command is used to report standardized 
conditions (such as NOT READY) and failures related to the SCSI 
protocol (such as path failures or ILLEGAL REQUEST).

     Note that in the above example, only the SEND command is 
used. This illustrates why the RECEIVE command is optional: the 
device with a packet to send takes the initiator role and SENDs 
it to the processor target. An initiator asking a target for data 
in this scenario implies that the initiator somehow knows the 
target has data to send.

     This VLAN application of processor devices differs from the 
communication device in that no other foreign protocol is 
implied. The host-to-host path is the network. In a communication 
device, the packet is taken from the initiator and translated to 
another communcation path. [this is real shaky; why do we have a 
separate comm device?]


11.1.2. "Exotic" Devices

     Historically, in SCSI, the processor device was intended for 
use by "exotic" devices which do not fit into any of the other 
device types. The implementor is urged to consider the other 
device types very carefully before deciding to use the processor 
device; often a standard device can be used with some vendor 
unique functions. Device which require several unique commands, 
and would not find using data packets for control and status 
convenient, should consider using a vendor unique device type. 
An implementor who feels that the new device is of general 
interest is encouraged to have that device standardized by the 
SCSI committee.

     One "non-host" device which can use the processor device 
command set is a graphics display terminal. The initiator sends 
data packets which contain the data defining the image to be 
displayed. Data packets which control attributes of the display 
may also be sent. In this model, the REQUEST SENSE command would 
also indicate error in the device (such as HARDWARE ERROR).

     Another example would be a data gathering device, such as a 
data acquisition peripheral. In this example, the data may be 
transfered to the initiator via the RECEIVE command, since the 
initiator knows that data is accumulated over a time period. 
Control information packets are transfered via the SEND command.