Dma Read Data Return Packet Description - DEC AlphaServer 8200 Technical Manual

The DMA Read Data Return packet is supported by the Mailbox Only, I/O
Window, Full, and Memory Channel variants of the hose protocol.
Table 6-16 gives the description of the DMA Read Data Return packet.
Table 6-16 DMA Read Data Return Packet Description
Field Description
Tag <7:0> associates the DMA read data return with the corresponding
Clock 1, <31:24>
DMA Read packet on the Up Hose. The tag is generated by the I/O bus
adapter and sent to the I/O port as part of a DMA Read packet.
Clock 1, <23> The Error bit. Set if an error has been detected on this packet.
These bits are always zero.
Clock 1, <22:14>
Clock 1, <13:12> DND<13:12> of the first cycle of a DMA Read Data Return packet is driven
with a 01 by the I/O port to indicate the packet type to the I/O adapter.
Is always zero.
Clock 1, <11>
The length field indicates the length of this packet. A DMA Read Data Re-
Clock 1, <10:8>
turn packet has three packet lengths: octaword, hexword, and double
hexword. However, the length code for the octaword packet may indicate
that only a longword or quadword of data is needed. This length code is
looped back from the initiating DMA read packet and allows the I/O bus
adapter to use the length code to extract the correct amount of information
from the octaword DMA Read Data Return packet.
Length
Code
001
010
011
000
100
1 Even though the length code in an octaword packet may be for a longword or quadword,
the parity across the hose must be good for all cycles of the DMA Read Data Return packet.
Are always zero.
Clock 1, <7:0>
Return data longwords 0 through 15 (1 to 16), respectively.
Clock 2 through
17, <31:0>
DMA Read Data Return Packet with Error
If the I/O port detects an error while trying to process a DMA read packet,
it logs the error and generates an error interrupt to the CPU(s), if inter-
rupts are enabled. A DMA Read Data Return packet with the error bit set
is sent across the Down Hose if not prohibited by the error condition. The
packet is one hose cycle long. It is shown in Figure 6-16. Any Up Hose or
Turbo Vortex errors detected by the I/O port prevent the I/O port from re-
turning a DMA Read Data Return packet. If an error occurs after the
DMA Read packet has successfully made it to the IDR and ICR chips, a
Data Valid in
Packet Data Length Packet
1
Octaword Longword
1
Octaword Quadword
Octaword Octaword
Hexword Hexword
Double hexword Double hexword
Number of Down
Hose Cycles
Required
5
5
5
9
17
I/O Port 6-43