這是一份對(duì)于ARM1176JZF-S芯片上MMU內(nèi)存地址轉(zhuǎn)換/映射過(guò)程的整理，內(nèi)容完全來(lái)自ARM官網(wǎng)的技術(shù)手冊(cè)。

ARM1176JZF-S屬于ARMv6產(chǎn)品系列，支持兩種頁(yè)表項(xiàng)的格式，一種是向ARMv4和ARMv5兼容的格式，另一新是ARMv6的新格式。v4/v5的格式在相應(yīng)的技術(shù)手冊(cè)里描述得非常清楚，但是對(duì)于v6格式的描述顯得有點(diǎn)散，所以本文只關(guān)注ARMv6格式。

內(nèi)容保持英文原文，沒(méi)有翻譯以免造成誤解弄巧成拙。

Level 1 Translation

When the CP15 Control Register c1 Bit 23 is set to 1 in the corresponding world, the subpage AP bits are disabled and the page tables have support for ARMv6 MMU features.

LEVEL 1 DESCRIPTOR FORMAT

As shown in Figure above, bits [1:0] of a level 1 page table entry determine the type of the entry.

Explanation of each bit:

Bits	Meaning
Bits [1:0] == b00	Translation fault.
Bits [1:0] == b01	The entry points to a second-level page table, called a Coarse page table. Figure 6.8 shows the formats of the possible entries in the Coarse page table.
Bits [1:0] == b10	The entry points to a either a 1MB Section of memory or a 16MB Supersection of memory. Bit [18] of the descriptor selects between a Section and a Supersection. For details of supersections see Supersections.
Bits [1:0] == b11	Reserved.
SBZ	SBZ = Should Be Zero, there bits should be set as 0 value.
NS	Defines the world for the processor:0 = Secure, reset value,1 = Non-secure.
Domain	A domain is a collection of memory regions. ARM1176JZF-S supports 16 Domains in the Secure world and 16 Domains in the Non-secure world. Domains provide support for multi-user operating systems. All regions of memory have an associated domain.
nG	The Not-Global (nG) bit, determines if the translation is marked as global (0), or process-specific (1) in the TLB. For process-specific translations the translation is inserted into the TLB using the current ASID, from the ContextID Register, CP15 c13.
S	The Shared (S) bit, determines if the translation is for Non-Shared (0), or Shared (1) memory. This only applies to Normal memory regions. Device memory can be Shared or Non-Shared as determined by the TEX bits and the C and B bits.
XN	The Execute-Never (XN) bit, determines if the region is Executable (0) or Not-executable (1).
APX	Three access permission bits.The access permissions extension (APX) bit, provides an extra access permission bit.See “Access Permissions”.
AP
P	ARM1176JZF-S processors do not support the P bit.
TEX	See “TEX, C and B bits”.
C
B

TEX, C AND B BITS

TEX, C AND B BITS PROVIDE OPTIONS FOR INNER AND OUTER CACHABILITY.

The ARMv6 MMU architecture originally defined five bits to describe all of the options for inner and outer cachability. These five bits, the Type Extension Field, TEX[2:0], Cacheable, C, and Bufferable, B bits, are set in the descriptors.

Few applications make use of all these options simultaneously. For this reason, a new configuration bit, TEX remap, bit [28] in the CP15 Control Register, permits the core to support a smaller number of options by using only the TEX[0], C and B bits.

The OS can configure this subset of options through a remap mechanism for these TEX[0], C, and B bits. The TEX[2:1] bits in the descriptor then become 2 OS managed page table bits.

See detail of ARM1176JZF-S Technical Reference Manual 6.6.1 for TexRemap=0 configuration/TexRemap=1 configuration.

ACCESS PERMISSIONS

The access permission bits control access to the corresponding memory region. If an access is made to an area of memory without the required permissions, then a permission fault is raised.

The access permissions are determined by a combination of the AP and APX bits in the page table, and the S and R bits in CP15 Control Register c1. For page tables not supporting the APX bit, the value 0 is used.

Access permission bit encoding:

VIRTUAL-PHISICAL ADDRESS TRANSLATION

Translation for a 1MB section, ARMv6 format

Level 2 Translation

LEVEL 2 DESCRIPTOR FORMAT

Explanation of each bit:

Bits	Meaning
Bits [1:0] == b00	Translation fault.
Bits [1:0] == b01	The entry points to a 64KB Large page in memory.
Bits [1:0] == b1x	The entry points to a 4KB Extended small page in memory.Bit [0] of the entry is the XN bit for the entry.

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VIRTUAL-PHISICAL ADDRESS TRANSLATION

Large page table walk, ARMv6 format:

4KB extended small page translations, ARMv6 format:

轉(zhuǎn)載于:https://blog.51cto.com/2ndmoon/1283723

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