Specifications

6-24 I/O Port
Table 6-7 Transaction Types Supported by the I/O Port
The I/O port performs three primary functions on the TLSB:
• DMA transactions
• Interrupt transactions
• CSR transactions
The I/O port uses the mailbox structure to access the XMI I/O bus and the
Futurebus+. It uses the I/O window space to access the PCI bus.
The I/O port can pipeline up to two transactions at a time. Transactions
are serviced on a first in, first out basis regardless of their source or desti-
nation. This applies to both CPU-initiated transactions and I/O DMA or
interrupt traffic.
6.5.1.1 DMA Transactions
DMA transactions transfer data between memory and an I/O device. They
are of the following types:
• Read transactions
• Interlocked Read/Unlock Write transactions
• Unmasked Write transactions
• Masked Write transactions
DMA Read Transactions
The I/O port supports octaword, hexword, and double hexword reads from
memory. However, reads of all lengths look like double hexword reads on
the TLSB.
Wrapped reads on hexword boundaries are permitted on the TLSB. The
I/O port uses wrapped reads on the TLSB when doing so will decrease the
latency perceived by the I/O device that is requesting the data. Whenever
an I/O bus adapter requests an octaword or hexword of data from an ad-
dress with bit <5> asserted, the I/O port issues a wrapped read on the
TLSB. Table 6-8 shows the hose transaction lengths and memory ad-
dresses for which the I/O port uses a wrapped read.
TLSB_CMD<2:0> Initiates Responds to Command
0
1
2
3
4
5
6
7
Yes
No
Yes
Yes
Yes
Yes
No
1
Yes
2
No
No
No
No
No
No
Yes
Yes
No-op
Victim
Read
Write
Read Bank Lock
Write Bank Unlock
CSR Read
CSR Write
1
If the I/O port is in debug mode, it can initiate CSR reads and writes.
2
The I/O port initiates Write Broadcast CSR transactions only (unless in debug mode).