# K1431–K1435 PCI capability chain contract

The caller first checks the PCI status capability-list bit and supplies the
header-specific first pointer. A zero pointer means an empty list. Every
nonzero pointer must be DWORD aligned and within the conventional PCI
configuration header area `0x40..=0xfc`. A capability DWORD contains the ID
in bits 0–7 and the next pointer in bits 8–15.

Traversal keeps a visited bitmap and a fixed maximum number of entries. It
rejects a repeated pointer, including a self-loop, an unaligned or out-of-
range pointer, a read error, and capacity exhaustion. A valid chain ending in
zero is published only after every node has been read and checked. A failure
does not publish a partial chain or change an existing discovery inventory.

The positive local fixture has two linked capabilities ending in zero. The
negative fixture changes the first node's next pointer to point to itself;
discovery returns `Cycle` and preserves the prior inventory. A guest probe
must read the physical PCI capability chain from a DTB-bounded ECAM mapping,
show a bounded ret path for the injected cycle, and keep all resource claims
balanced before development guest acceptance is recorded.