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1.1. About the MMU-500

The MMU-500 is a system-level Memory Management Unit (MMU) that translates an input address to an output address, based on address mapping and memory attribute information available in the MMU-500 internal registers and translation tables.

An address translation from an input address to an output address is described as a stage of address translation.

The MMU-500 supports the translation table formats defined by the ARM architecture, ARMv7 and ARMv8, and can perform:

  • Stage 1 translations that translate an input virtual address (VA) to an output physical address (PA) or intermediate physical address (IPA).

  • Stage 2 translations that translate an input IPA to an output PA.

  • Combined stage 1 and stage 2 translations that translate an input VA to an output IPA, and then translate that IPA to a PA. The MMU-500 performs translation table walks for each stage of the translation.

Address translation can span over two stages, namely stage 1 and stage 2. Address translation can require multiple translation table lookups. Each translation table lookup is described as a level of address lookup. Each level of stage 1 translation might require additional stage 2 translation.

In addition to translating an input address to an output address, a stage of address translation also defines the memory attributes of the output address. With a two-stage translation, the stage 2 translation can modify the attributes defined by the stage 1 translation.

A stage of address translation can be disabled or bypassed, and the MMU-500 can define memory attributes for disabled and bypassed stages of translation.

The MMU-500 uses inputs from the requesting master to identify a context. This context tells the MMU-500 what resources to use for the translation including which translation tables to use.

For the stage 1 translations that are typically associated with application and OS-level operation, the VA range can be split into two subranges, translated by Translation Table Base registers, TTBR0 and TTBR1, each with associated translation tables and control registers.

These features mean the MMU-500 can perform address translations with the following page size limitations, for memory accesses from either AArch32 state or from AArch64 state:

ARMv7 architecture

The MMU-500 supports all page sizes.

ARMv8 architecture

Apart from the 16KB page granule, the MMU-500 supports all page sizes.

Stage 1 translations are supported for both Secure and Non-secure translation contexts. Usually, the appropriate OS:

  • Defines the translation tables, in memory, for the stage 1 translations for its security state.

  • Programs the MMU-500 to configure stage 1 translations, and then enables the translations.

Stage 2 translations are supported only for Non-secure translation contexts. The typical usage model for two stages of address translation is as follows:

  • The Non-secure operating system defines the stage 1 address translations for application-level and OS-level operation. It does this assuming it is mapping from the VAs used by the processors to PAs in the physical memory system. However, it actually maps VAs to IPAs.

    Note

    This means that all the addresses the OS uses in the translation tables that it defines are in the IPA address space, and require a stage 2 translation to map them to the PA address space.

  • The hypervisor defines the stage 2 address translations that map the IPAs to PAs. It does this as part of its virtualization of one or more Non-secure guest operating systems.

The MMU-500 can cache the result of a translation table lookup in a translation lookaside buffer (TLB) that means the MMU-500 also supports TLB maintenance operations.

For more information about:

  • The supported architectural features of the MMU-500, see the ARM® System Memory Management Architecture Specification.

  • Address translation, including the translation table formats and TLB maintenance operations, see:

    • The ARM® Architecture Reference Manual, ARMv7-A and ARMv-7 R edition.

    • The ARM® Architecture Reference Manual, ARMv8, for ARMv8-A architecture profile.

The MMU-500 has the following key components:

Translation buffer unit (TBU)

The TBU contains a TLB that caches page tables. The MMU-500 implements a TBU for each connected master, and the TBU is designed, so that it is local to the master.

Translation control unit (TCU)

Controls and manages the address translations. The MMU-500 implements a single TCU.

Interconnect

Connects multiple TBUs to the TCU.

Figure 1.1 shows the MMU-500 block diagram.

Figure 1.1. MMU-500 block diagram

Figure 1.1. MMU-500 block diagram

For more information about logical processing steps, interfaces, and operational features, see Chapter 2 Functional Description.

The following are example masters for the MMU-500:

  • Graphics processor unit (GPU).

  • Video engines.

  • Direct memory access (DMA) controllers.

  • Color LCD (CLCD) controllers.

  • Network controllers.

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