In Sv39, Sv48, and Sv57, bits 62-61 of a leaf page table entry indicate the use of page-based memory types that override the PMA(s) for the associated memory pages. The encoding for the PBMT bits is captured in Encodings for PBMT field in Sv39, Sv48, and Sv57 PTEs..

The Svpbmt extension depends on Sv39.

Implementations may override additional PMAs not explicitly listed in Encodings for PBMT field in Sv39, Sv48, and Sv57 PTEs.. For example, to be consistent with the characteristics of a typical I/O region, a misaligned memory access to a page with PBMT=IO might raise an exception, even if the underlying region were main memory and the same access would have succeeded for PBMT=PMA.

For non-leaf PTEs, bits 62-61 are reserved for future standard use. Until their use is defined by a standard extension, they must be cleared by software for forward compatibility, or else a page-fault exception is raised.

For leaf PTEs, setting bits 62-61 to the value 3 is reserved for future standard use. Until this value is defined by a standard extension, using this reserved value in a leaf PTE raises a page-fault exception.

If the underlying physical memory attribute for a page is vacant, the PBMT settings do not override that.

When PBMT settings override a main memory page into I/O or vice versa, memory accesses to such pages obey the memory ordering rules of the final effective attribute, as follows.

If the underlying physical memory attribute for a page is I/O, and the page has PBMT=NC, then accesses to that page obey RVWMO. However, accesses to such pages are considered to be both I/O and main memory accesses for the purposes of FENCE, .aq, and .rl.

If the underlying physical memory attribute for a page is main memory, and the page has PBMT=IO, then accesses to that page obey strong channel 0 I/O ordering rules. However, accesses to such pages are considered to be both I/O and main memory accesses for the purposes of FENCE, .aq, and .rl.

When Svpbmt is used with non-zero PBMT encodings, it is possible for multiple virtual aliases of the same physical page to exist simultaneously with different memory attributes. It is also possible for a U-mode or S-mode mapping through a PTE with Svpbmt enabled to observe different memory attributes for a given region of physical memory than a concurrent access to the same page performed by M-mode or when MODE=Bare. In such cases, the behaviors dictated by the attributes (including coherence, which is otherwise unaffected) may be violated.

Accessing the same location using different attributes that are both non-cacheable (e.g., NC and IO) does not cause loss of coherence, but might result in weaker memory ordering than the stricter attribute ordinarily guarantees. Executing a fence iorw, iorw instruction between such accesses suffices to prevent loss of memory ordering.

Accessing the same location using different cacheability attributes may cause loss of coherence. Executing the following sequence between such accesses prevents both loss of coherence and loss of memory ordering: fence iorw, iorw, followed by cbo.flush to an address of that location, followed by a fence iorw, iorw.

When two-stage address translation is enabled within the H extension, the page-based memory types are also applied in two stages. First, if hgatp.MODE is not equal to zero, non-zero G-stage PTE PBMT bits override the attributes in the PMA to produce an intermediate set of attributes. Otherwise, the PMAs serve as the intermediate attributes. Second, if vsatp.MODE is not equal to zero, non-zero VS-stage PTE PBMT bits override the intermediate attributes to produce the final set of attributes used by accesses to the page in question. Otherwise, the intermediate attributes are used as the final set of attributes.