VR4102 datasheet(169/701 Pages) NEC

169 / 701 page

CHAPTER 6 EXCEPTION PROCESSING

169

6.3.9 XContext Register (20)

The read/write XContext register contains a pointer to an entry in the page table entry (PTE) array, an operating

system data structure that stores virtual-to-physical address translations. If a TLB miss occurs, the operating system

loads the untranslated data from the PTE into the TLB to handle the software error.

The XContext register is used by the XTLB Refill exception handler to load TLB entries in 64-bit addressing

mode.

The XContext register duplicates some of the information provided in the BadVAddr register, and puts it in a form

useful for the XTLB exception handler.

This register is included solely for operating system use. The operating system sets the PTEBase field in the

Figure 6-10. XContext Register Format

35 34 33

PTEBase

BadVPN2

PTEBase : The PTEBase field is a read/write field, and is used by software as the pointer to the base

address of the PTE table in the current user address space.

BadVPN2 : The BadVPN2 field is written by hardware if a TLB miss occurs. This field holds the value (VPN2)

obtained by halving the virtual page number of the most recent virtual address for which

translation failed.

: Space type (00

o User, 01o Supervisor, 11 o Kernel). The setting of this field matches virtual

address bits 63 and 62.

: Reserved for future use. Write 0 in a write operation. When this field is read, 0 is read.

The 29-bit BadVPN2 field has bits 39 to 11 of the virtual address that caused the TLB miss; bit 10 is excluded

because a single TLB entry maps to an even-odd page pair. For a 1-Kbyte page size, this format may be used

directly to address the pair-table of 8-byte PTEs. For 4-Kbyte-or-more page and PTE sizes, shifting or masking this

value produces the appropriate address.

VR4102 Datasheet(PDF) 169 Page - NEC