3. ARM Specific Information — RTEMS CPU Architecture Supplement 5.4-rc2 (21st August 2026) documentation

# 3. 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/).

## 3.1. 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 `cpukit/score/cpu/arm/rtems/score/arm.h` based upon the particular CPU model flags specified on the compilation command line.

### 3.1.1. CPU Model Name

The macro `CPU_MODEL_NAME` is a string which designates the architectural level of this CPU model. See in `cpukit/score/cpu/arm/rtems/score/arm.h` for the values.

### 3.1.2. 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.

### 3.1.3. Floating Point Unit

The following floating point units are supported:

- VFPv2 (for example available on ARM926EJ-S processors)

- 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)

- FPv5-D16 (for example available on Cortex-M7 processors)

## 3.2. Multilibs

The following multilibs are available:

1. `.`: ARMv4T, ARM instruction set

2. `vfp/hard`: ARMv4T, ARM instruction set with hard-float ABI and VFPv2 support

3. `thumb`: ARMv4T, Thumb-1 instruction set

4. `thumb/armv6-m`: ARMv6M, subset of Thumb-2 instruction set

5. `thumb/armv7-a`: ARMv7-A, Thumb-2 instruction set

6. `thumb/armv7-a/neon/hard`: ARMv7-A, Thumb-2 instruction set with hard-float ABI Neon and VFP-D32 support

7. `thumb/armv7-r`: ARMv7-R, Thumb-2 instruction set

8. `thumb/armv7-r/vfpv3-d16/hard`: ARMv7-R, Thumb-2 instruction set with hard-float ABI VFP-D16 support

9. `thumb/cortex-m3`: Cortex-M3, Thumb-2 instruction set with hardware integer division (SDIV/UDIV) and a fix for Cortex-M3 Errata 602117.

10. `thumb/cortex-m4`: Cortex-M4, Thumb-2 instruction set with hardware integer division (SDIV/UDIV) and DSP instructions

11. `thumb/cortex-m4/fpv4-sp-d16`: Cortex-M4, Thumb-2 instruction set with hardware integer division (SDIV/UDIV), DSP instructions and hard-float ABI FPv4-SP support

12. `thumb/cortex-m7/fpv5-d16`: Cortex-M7, Thumb-2 instruction set with hard-float ABI and FPv5-D16 support

13. `eb/thumb/armv7-r`: ARMv7-R, Big-endian Thumb-2 instruction set

14. `eb/thumb/armv7-r/vfpv3-d16/hard`: ARMv7-R, Big-endian Thumb-2 instruction set with hard-float ABI VFP-D16 support

Multilib 1., 2. and 3. support the legacy ARM7TDMI and ARM926EJ-S processors.

Multilib 4. supports the Cortex-M0 and Cortex-M1 cores.

Multilib 5. and 6. support the Cortex-A processors.

Multilib 7., 8., 13. and 14. support the Cortex-R processors. Here also big-endian variants are available.

Use for example the following GCC options:

    -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:

    -mthumb -mcpu=cortex-a9

alone will not select the ARMv7-A multilib.

## 3.3. 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).

## 3.4. Memory Model

A flat 32-bit memory model is supported. The board support package must take care about the MMU if necessary.

## 3.5. 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.

### 3.5.1. 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.

### 3.5.2. Interrupt Stack

The board support package must initialize the interrupt stack. The memory for the stacks is usually reserved in the linker script.

## 3.6. 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.

## 3.7. Symmetric Multiprocessing

SMP is supported on ARMv7-A. Available platforms are

- Altera Cyclone V,

- NXP i.MX 7, and

- Xilinx Zynq.

## 3.8. Thread-Local Storage

Thread-local storage is supported.