ARM Processor Architecture

ARM, generically known as A32, is a fixed-length (32-bit) instruction set. It is the base 32-bit ISA used in the ARMv4T, ARMv5TEJ and ARMv6 architectures.  In these architectures it is used in applications requiring high performance, or for handling hardware exceptions such as interrupts and processor start-up.

The ARM ISA is also supported in the Cortex™-A and Cortex-R profiles of the Cortex architecture for performance critical applications, and for legacy code.  Most of its functionality is subsumed into the Thumb instruction set with the introduction of Thumb-2 technology. Thumb (T32) benefits from improved code density.

ARM instructions are 32-bits wide, and are aligned on 4-byte boundaries.

Most ARM instructions can be "conditionalised" to only execute when previous instructions have set a particular condition code. This means that instructions only have their normal effect on the programmers’ model operation, memory and coprocessors if the N, Z, C and V flags in the Application Program Status Register satisfy a condition specified in the instruction. If the flags do not satisfy this condition, the instruction acts as a NOP, that is, execution advances to the next instruction as normal, including any relevant checks for exceptions being taken, but has no other effect.  This conditionalisation of instructions allows small sections of if- and while-statements to be encoded without the use of branch instructions.  

The condition codes are:

Condition Code	Meaning
N	Negative condition code, set to 1 if  result is negative
Z	Zero condition code, set to 1 if the result of the instruction is 0
C	Carry condition code, set to 1 if the instruction results in a carry condition
V	Overflow condition code, set to 1 if the instruction results in an overflow condition.

 

 

Cost-sensitive embedded control applications such as cell phones, disk drives, modems and pagers are always looking for ways to achieve 32-bit performance and address space at minimal cost with respect to memory footprint.    

The Thumb (T32) instruction set provides a subset of the most commonly used 32-bit ARM instructions which have been compressed into 16-bit wide opcodes. On execution, these 16-bit instructions are decompressed transparently to full 32-bit ARM instructions in real time without performance loss.

Thumb offers the designer:

• Excellent code-density for minimal system memory size and cost
  • 32-bit performance from 8- or16-bit memory on an 8- or 16-bit bus for low system cost.
• Plus the established ARM features
• Industry-leading MIPS/Watt for maximum battery life and RISC performance
• Small die size for integration and minimum chip cost
• Global multi-partner sourcing for secure supply.

 Designers can use both 16-bit Thumb and 32-bit ARM instructions sets and therefore have the flexibility to emphasize performance or code size on a sub-routine level as their applications require.

The Thumb ISA is widely supported by the ARM ecosystem, including a complete Windows software development environment as well as development and evaluation cards.

Improved Code Density with Performance and Power Efficiency

Thumb-2 technology made Thumb a mixed (32- and 16-bit) length instruction set, and is the instruction set common to all ARMv7 compliant ARM Cortex implementations. Thumb-2 provides enhanced levels of performance, energy efficiency, and code density for a wide range of embedded applications.

The technology is backwards compatible with existing ARM and Thumb solutions, while significantly extending the features available in the Thumb instructions set, allowing more of the application to benefit from the best in class code density of Thumb. For performance optimised code Thumb-2 technology uses 31 percent less memory to reduce system cost, while providing up to 38 percent higher performance than existing high density code, which can be used to prolong battery-life or to enrich the product feature set.