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authorChris Johns <chrisj@rtems.org>2016-11-03 16:58:08 +1100
committerChris Johns <chrisj@rtems.org>2016-11-03 16:58:08 +1100
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+.. comment SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. COMMENT: COPYRIGHT (c) 1988-2009.
+.. COMMENT: On-Line Applications Research Corporation (OAR).
+.. COMMENT: All rights reserved.
+
+ARM Specific Information
+########################
+
+This chapter discusses the *ARM architecture*
+(http://en.wikipedia.org/wiki/ARM_architecture) dependencies in this port of
+RTEMS. The ARMv4T (and compatible), ARMv7-A, ARMv7-R and ARMv7-M architecture
+versions are supported by RTEMS. Processors with a MMU use a static
+configuration which is set up during system start. SMP is supported.
+
+**Architecture Documents**
+
+For information on the ARM architecture refer to the *ARM Infocenter*
+(http://infocenter.arm.com/).
+
+CPU Model Dependent Features
+============================
+
+This section presents the set of features which vary across ARM implementations
+and are of importance to RTEMS. The set of CPU model feature macros are
+defined in the file :file:`cpukit/score/cpu/arm/rtems/score/arm.h` based upon
+the particular CPU model flags specified on the compilation command line.
+
+CPU Model Name
+--------------
+
+The macro ``CPU_MODEL_NAME`` is a string which designates the architectural
+level of this CPU model. See in :file:`cpukit/score/cpu/arm/rtems/score/arm.h`
+for the values.
+
+Count Leading Zeroes Instruction
+--------------------------------
+
+The ARMv5 and later has the count leading zeroes ``clz`` instruction which
+could be used to speed up the find first bit operation. The use of this
+instruction should significantly speed up the scheduling associated with a
+thread blocking. This is currently not used.
+
+Floating Point Unit
+-------------------
+
+The following floating point units are supported.
+
+- VFPv3-D32/NEON (for example available on Cortex-A processors)
+
+- VFPv3-D16 (for example available on Cortex-R processors)
+
+- FPv4-SP-D16 (for example available on Cortex-M processors)
+
+Multilibs
+=========
+
+The following multilibs are available:
+
+#. ``.``: ARMv4T, ARM instruction set
+
+#. ``thumb``: ARMv4T, Thumb-1 instruction set
+
+#. ``thumb/armv6-m``: ARMv6M, subset of Thumb-2 instruction set
+
+#. ``thumb/armv7-a``: ARMv7-A, Thumb-2 instruction set
+
+#. ``thumb/armv7-a/neon/hard``: ARMv7-A, Thumb-2 instruction set with
+ hard-float ABI Neon and VFP-D32 support
+
+#. ``thumb/armv7-r``: ARMv7-R, Thumb-2 instruction set
+
+#. ``thumb/armv7-r/vfpv3-d16/hard``: ARMv7-R, Thumb-2 instruction set with
+ hard-float ABI VFP-D16 support
+
+#. ``thumb/armv7-m``: ARMv7-M, Thumb-2 instruction set with hardware
+ integer division (SDIV/UDIV)
+
+#. ``thumb/armv7-m/fpv4-sp-d16``: ARMv7-M, Thumb-2 instruction set with
+ hardware integer division (SDIV/UDIV) and hard-float ABI FPv4-SP support
+
+#. ``eb/thumb/armv7-r``: ARMv7-R, Big-endian Thumb-2 instruction set
+
+#. ``eb/thumb/armv7-r/vfpv3-d16/hard``: ARMv7-R, Big-endian Thumb-2 instruction
+ set with hard-float ABI VFP-D16 support
+
+Multilib 1. and 2. support the standard ARM7TDMI and ARM926EJ-S targets.
+
+Multilib 3. supports the Cortex-M0 and Cortex-M1 cores.
+
+Multilib 8. supports the Cortex-M3 and Cortex-M4 cores, which have a special
+hardware integer division instruction (this is not present in the A and R
+profiles).
+
+Multilib 9. supports the Cortex-M4 cores with a floating point unit.
+
+Multilib 4. and 5. support the Cortex-A processors.
+
+Multilib 6., 7., 10. and 11. support the Cortex-R processors. Here also
+big-endian variants are available.
+
+Use for example the following GCC options:
+
+.. code-block:: shell
+
+ -mthumb -march=armv7-a -mfpu=neon -mfloat-abi=hard -mtune=cortex-a9
+
+to build an application or BSP for the ARMv7-A architecture and tune the code
+for a Cortex-A9 processor. It is important to select the options used for the
+multilibs. For example:
+
+.. code-block:: shell
+
+ -mthumb -mcpu=cortex-a9
+
+alone will not select the ARMv7-A multilib.
+
+Calling Conventions
+===================
+
+Please refer to the *Procedure Call Standard for the ARM Architecture*
+(http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042c/IHI0042C_aapcs.pdf).
+
+Memory Model
+============
+
+A flat 32-bit memory model is supported. The board support package must take
+care about the MMU if necessary.
+
+Interrupt Processing
+====================
+
+The ARMv4T (and compatible) architecture has seven exception types:
+
+- Reset
+
+- Undefined
+
+- Software Interrupt (SWI)
+
+- Prefetch Abort
+
+- Data Abort
+
+- Interrupt (IRQ)
+
+- Fast Interrupt (FIQ)
+
+Of these types only the IRQ has explicit operating system support. It is
+intentional that the FIQ is not supported by the operating system. Without
+operating system support for the FIQ it is not necessary to disable them during
+critical sections of the system.
+
+The ARMv7-M architecture has a completely different exception model. Here
+interrupts are disabled with a write of 0x80 to the ``basepri_max`` register.
+This means that all exceptions and interrupts with a priority value of greater
+than or equal to 0x80 are disabled. Thus exceptions and interrupts with a
+priority value of less than 0x80 are non-maskable with respect to the operating
+system and therefore must not use operating system services. Several support
+libraries of chip vendors implicitly shift the priority value somehow before
+the value is written to the NVIC IPR register. This can easily lead to
+confusion.
+
+Interrupt Levels
+----------------
+
+There are exactly two interrupt levels on ARM with respect to RTEMS. Level
+zero corresponds to interrupts enabled. Level one corresponds to interrupts
+disabled.
+
+Interrupt Stack
+---------------
+
+The board support package must initialize the interrupt stack. The memory for
+the stacks is usually reserved in the linker script.
+
+Default Fatal Error Processing
+==============================
+
+The default fatal error handler for this architecture performs the following
+actions:
+
+- disables operating system supported interrupts (IRQ),
+
+- places the error code in ``r0``, and
+
+- executes an infinite loop to simulate a halt processor instruction.
+
+Symmetric Multiprocessing
+=========================
+
+SMP is supported on ARMv7-A. Available platforms are the Altera Cyclone V and
+the Xilinx Zynq.
+
+Thread-Local Storage
+====================
+
+Thread-local storage is supported.