Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification
Rev. 1.0
102
4.1.6 Isochronous Request/Status Encoding
The AGP3.0 specification includes four new isochronous transaction codes in addition to those required
by the AGP Interface Specification V2.0. The AGP3.0 Master device must place one of these four new
codes on the “CCCC” wires to initiate an isochronous transaction. The new isochronous transaction
codes are shown in Table 50. Highlighting implies a change from AGP.
Table 50: AGP3.0 Interconnect Requests
CCCC AGP3.0 Operation Description
0000 Read Asynchronous Read (formerly Low Priority Read)
0001 Reserved AGP high priority read is unused in AGP3.0
0010 Reserved
0011 ISOCH Read Isochronous Read command – uses the Length Field LLL in Type
1 command to specify transaction size as follows:
L-L-L Size
0-0-0 32Bytes
0-0-1 64Bytes
0-1-0 128Bytes
0-1-1 256Bytes
0100 Write Asynchronous Write (formerly Low Priority Write)
0101 Reserved AGP3.0 does not use AGP High Priority Write
0110 ISOCH Write/Unfenced Isochronous Write with out-of-order completion. Uses Length Field
LLL in Type 1 request to specify transaction size as follows:
L-L-L Size
0-0-0 32Bytes
0-0-1 64Bytes
0-1-0 Reserved
0-1-1 Reserved
No byte masking allowed so Isoch Master must assert C#/BE[3:0]
lines high during data transfer to indicate that all byte lanes are
active.
0111 ISOCH Write/Fenced Isochronous Write with ordered completion. Implicit Fence forces
all preceding Isoch writes (fenced and unfenced) to complete first.
Isoch Writes following it cannot bypass. Size is determined the
same as in Isoch Write/unfenced.
1000 Reserved AGP3.0 does not implement Long Read
1001 Reserved AGP3.0 does not implement Long High Priority Read