User`s guide

The XMI

2.5.2 Read Transaction

Read (READ) transactions (see Figure 2–12) are used to transfer a

longword, quadword, octaword, or hexword of data from the responder

to the commander. The data is naturally aligned and delivered in

wraparound order. Wraparound reads are described in Section 2.1.5. A

Read transaction is initiated by a commander driving the XMI address and

function lines to represent a longword read, quadword read, octaword read,

or hexword read. The Read command cycle is decoded by all responder

nodes. The node that recognizes its own address latches that address and

command. This node is the responder.

Figure 2–12 Read Command

DON’T CARE ADDRESS<29:0>

LENGTH 00 = Hexword

01 = Longword

10 = Quadword

11 = Octaword

msb−p185−89

0001 MBZ

ADDRESS<39:30>

READ COMMAND

When the responder has the requested data, it initiates a return

data transfer. Multiple transfers may be necessary to transfer all the

quadwords in a given octaword or hexword transaction. The commander

monitors the bus trafﬁc waiting for its return data, and then latches the

information. The commander issues its own ID in the ID ﬁeld during the

command cycle. The responder returns this same ID with the return read

data so that the commander can recognize the return read data it had

requested.

Longword-length transactions can only be used in I/O space while

quadword-, octaword-, and hexword-length transactions can only be used

in memory space. The state of the memory block is transparent to the

commander. The memory controller treats Read commands differently

depending upon whether the memory block is free or owned. If free, the

address is accessed and data is returned with the appropriate GRD, CRD,

or RER response. If the block is owned, the memory controller stores the

command/address ﬁeld in a deferred queue and waits until the owner of

the blocks disown-writes the data back to memory. As soon as the memory

block becomes disowned, the memory controller executes the deferred Read

command and returns the data with the appropriate response.

2–34