In PCI-X, before the Address Phase, there’s an extra time slot called the Attribute Phase.

Here, the requester provides:

• Byte Count (how much data will be transferred).
• Requester ID (Bus:Device:Function number → who’s making the request).
• Extra control bits, like:
  • No Snoop (NS)
  • Relaxed Ordering (RO)

These attributes let the system make smarter decisions about how to handle the transaction.

No Snoop (NS) Bit

Normally, when memory is accessed:

1. The CPU must check ("snoop") its internal caches to see if that memory address is already cached.
2. If cached:
   • Cache may need to write back (if dirty).
   • Or invalidate the cached copy (to avoid stale data).
3. This snooping adds latency.

👉 But if the requester knows the target memory region is uncacheable (like DMA buffers for I/O devices), snooping is a waste.

• Setting NS = 1 tells the system:

  “Skip cache snooping — this memory access won’t hit CPU caches anyway.”

✅ Benefit: Lower latency, faster DMA.

Relaxed Ordering (RO) Bit

By default, PCI/PCI-X follows Strong Ordering:

• Transactions must complete in the same order they were issued.
• This prevents hazards like:
  • Write → Read to the same address (must keep order).

But not all transactions depend on each other. For example:

• Write packet to NIC buffer A.
• Write packet to NIC buffer B.
  These two are independent.

👉 Setting RO = 1 tells bridges/buffers:

“This transaction doesn’t depend on the earlier ones — it can be reordered.”

✅ Benefit: Improves throughput, especially under heavy I/O loads, because the bus/bridges can reorder independent transactions for efficiency.