User`s guide

DC 900-1516D 83

Chapter

ICP Packet Formats

This chapter describes the packet formats used by Protogate protocols. The packet

formats that are written to the ICP2432 are the same whether the ICP is attached to a

Freeway server or a PCI bus in your VMS system. Because most Protogate protocol pro-

grammer’s guides mention commands and responses as they apply to the Freeway

server, this chapter covers both Freeway and device driver use of packets.

5.1 DLI Packet Format

The OpenVMS ICP driver QIO interface provides a block-transfer interface between a

client application and the protocol software resident on the embedded ICP. From the

application’s perspective, these packets consist of message blocks composed of an ICP

header structure followed by a protocol header structure followed by an optional data

array. Figure 5–1 shows the “C” definition of this ICP packet structure.

When preparing a packet to write to the ICP, the application must initialize the

usICPCount field with the size in bytes of the PROT_HDR structure (16) plus the size of

the data array that follows it. After reading a packet from the driver, the application may

compute the size of the data array that follows the

PROT_HDR structure by subtracting

16 from the value of the usICPCount field in the

ICP_HDR structure.

Note that the

ICP_HDR structure must be in network byte-order (Big Endian). This

means that the VMS program must swap bytes in the ICP header before writing packets

to the ICP. The VMS program must also swap bytes in the ICP header after reading each

packet from the ICP. The

PROT_HDR structure remains in the client computer’s natural

byte order, which is Little Endian for AXP systems.

Roger — Bill

Reid’s review

comment

was “What is

this and why

is it?” Maybe

it’s

redundant

now that we

are putting

all the packet

info in the

protocol

documents

...Leslie