TMPR4956C datasheet(82/286 Pages) TOSHIBA

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Chapter 3 TX49/H4 Core’s Registers

3-22

3.2.17

XContext Register (Reg

#20)

The XContext register is a read/write register, and 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. When

there is a TLB miss, the operating system software loads the TLB with the missing translation from the

PTE array. However the contents of this register duplicates some information of the BadVAddr register, it

is arranged in a form that is more useful for TLB exception handler by a software. This register is for use

with the XTLB refill handler, which loads TLB entries for references to a 64-bit address space, and is

included solely for operating system use. The operating system sets the PTE base field in the register, as

needed. Normally, the operating system uses this register to address the current page map which resides in

the Kernel mapped segment, kseg3.

The BadVPN2 field of 27 bits has bit 39-13 of the virtual address that caused the TLB miss; bit 12 is

excluded because a single TLB entry maps to an even-odd page pair. For a 4 KByte page size, this format

may be used directly to access the pair-table of 8 Byte PTEs. For other page sizes and PTE sizes, shifting

and masking this value produces the appropriate address.

Figure 3.2.17 shows the format of the XContext register and Table 3.2.18 describes the XContext

33 32 31 30

4 3

PTEBase

BadVPN2

Figure 3.2.17 XContext Register Format

Table 3.2.18 XContext Register Field Description

Bit(s)

Field Name

Description

Cold Reset

Read/Write

63:33

PTEBase

Page table entry base pointer

This field is normally written with a value that allows the

operation system to use the Context register as a pointer into

the current PTE array in memory.

Undefined

Read/Write

32:31

The Region field contains bits 63-62 of the virtual address.

00: user

01: supervisor

11: kernel

Undefined

Read/Write

30:4

BadVPN2

Bad virtual page number divided by two.

This field is written by hardware on a miss. It contains the

VPN of the most recent invalidly translated virtual address.

Undefined

Read

3:0

Reserved

0x0

Read

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