This presentation contains an overview of novelties in ARMv8-A and details on application binary interface (ABI), memory management unit (MMU), caches and interrupts

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Embedded TechTalk #2:
ARM Architecture for Kernel Development
Andrew Lukin
2017-08-10

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In general

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RISC -- Reduced Instructions Set Computer
● Small set of simple and general instructions
● Fixed length instructions
● Simpler processor’s core logic
● Harvard architecture -- architecture with physically separate storage and
signal pathways for instructions and data
● Load/Store architecture -- separate instructions for memory access
● A lot of general purpose registers or even register files

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Evolution of the ARM architecture

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Registers ARMv7

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Registers AArch64 (ARMv8)

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PSTATE at AArch32

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OS-specific

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ARMv7 exceptions

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AArch64 exceptions model

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Exceptions table

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Exceptions
● A synchronous exception if it is generated as a result of execution or attempted
execution of the instruction stream, and where the return address provides
details of the instruction that caused it.
● An asynchronous exception is not generated by executing instructions, while the
return address might not always provide details of what caused the exception.
● In the ARMv7-A architecture, the prefetch abort, Data Abort and undef
exceptions are separate items.
● In AArch64, all of these events generate a Synchronous abort. The exception
handler may then read the syndrome and FAR registers to obtain the necessary
information to distinguish between them.

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Interrupts

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Execution states

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Execution states - Registers mapping

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MMU

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MMU ARMv7

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AArch64 MMU Support

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MMU - Caches
● Point of Coherency (PoC) -- is the point at which all observers, for example,
cores, DSPs, or DMA engines, that can access memory, are guaranteed to
see the same copy of a memory location. Typically, this is the main external
system memory.
● Point of Unification (PoU) -- is the point at which the instruction and data
caches and translation table walks of the core are guaranteed to see the
same copy of a memory location

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MMU + ASID

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MMU - Normal memory
● Normal memory -- The processor can re-order, repeat, and merge accesses
to it.
Furthermore, address locations that are marked as Normal can be accessed
speculatively by the processor, so that data or instructions can be read from
memory without being explicitly referenced in the program, or in advance of
the actual execution of an explicit reference. Such speculative accesses can
occur as a result of branch prediction, speculative cache linefills, out-of-order
data loads, or other hardware optimizations.

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MMU - Device memory
● Device memory --
○ Device-nGnRnE most restrictive (equivalent to Strongly Ordered
memory in the ARMv7 architecture).
○ Device-nGnRE
○ Device-nGRE
○ Device-GRE least restrictive
● Gathering of non Gathering (G or nG) -- whether multiple accesses can be
merged into a single bus transaction for this memory region.
● Re-ordering (R or nR) -- whether accesses to the same device can be re-
ordered with respect to each other.
● Early Write Acknowledgement (E or nE) -- whether an intermediate write
buffer between the processor and the slave device being accessed is allowed
to send an acknowledgement of a write completion

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Instructions

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Instruction changes

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Instruction changes

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Instruction changes

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Multiplication instructions in assembly language

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ABI

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ABI for AArch64

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PCS

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Links
1. Programmer’s Guide for ARMv8-A (DEN0024A)
2. ARM® Architecture Reference Manual
ARMv8, for ARMv8-A architecture profile
DDI0487B_a_armv8.pdf
3. ARM® Architecture Reference Manual
ARMv7-A and ARMv7-R edition
DDI0406C_C_arm_architecture_reference_manual.pdf

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Questions?

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Thank you
Andrew Lukin
Sr Embedded Developer
andrii.lukin@globallogic.com
+380-95-303-43-76