ARM Cortex-M Instruction Set (introduction)

Summary

The video script discusses the evolution of ARM processor instruction sets. Initially, ARM used a 32-bit instruction set, which was powerful but required large, expensive program memory. In 1995, ARM introduced the 16-bit Thumb instruction set for better code density and cost-efficiency. However, some functionalities still required the 32-bit ARM instruction set. To address this, ARM launched the Thumb-2 instruction set in 2003, which included both 16-bit and 32-bit instructions, balancing code density and performance. Most Cortex-M processors, including the M4, now only support Thumb-2 instructions. The script also explains data size definitions in ARM processors, such as bytes, half-words, words, and double words.

Takeaways

• 📚 Before 1995, ARM processors used a 32-bit instruction set known as the ARM instruction set.
• 💡 The ARM instruction set was powerful and provided good performance but required larger program memory, leading to higher costs and power consumption.
• 🔑 In 1995, ARM introduced the 16-bit Thumb instruction set to address the high memory requirements and power consumption.
• 🌟 The Thumb instruction set offered better code density compared to 32-bit instruction sets but had a performance trade-off.
• 🚀 In 2003, ARM introduced Thumb-2, which included both 32-bit and 16-bit instructions, combining the benefits of code density and performance.
• 💻 Most modern Cortex-M processors, including the Cortex-M4, support only Thumb-2 instructions and not the original ARM instruction set.
• 🔄 The Cortex-M3 and M7 processors also support Thumb-2 instructions exclusively, while the M0 and M0+ processors partially support 32-bit Thumb instructions but fully support 16-bit Thumb instructions.
• 📏 ARM processors define data sizes as follows: a byte is 8-bits, a half-word is 16-bits, a word is 32-bits, and a double word is 64-bits.
• 📈 The script suggests that future lectures will delve into specific instructions within the ARM and Thumb-2 instruction sets.

Q & A

What was the primary issue with the original 32-bit ARM instruction set before 1995?

The original 32-bit ARM instruction set, while powerful and providing good performance, required larger program memory compared to 8-bit and 16-bit processors. This was problematic due to the high cost and power consumption associated with larger memory sizes.

Why did ARM introduce the 16-bit Thumb instruction set in 1995?

ARM introduced the 16-bit Thumb instruction set in 1995 to address the issue of high memory consumption and cost associated with the 32-bit instruction set. The Thumb instruction set provided better code density and reduced memory requirements.

What was the main limitation of the 16-bit Thumb instruction set introduced in 1995?

The 16-bit Thumb instruction set could not perform all the functionalities that the 32-bit ARM instruction set could. There were certain tasks that still required the use of the 32-bit ARM instruction set, which is why ARM processors had to support both instruction sets.

How did the introduction of the Thumb-2 instruction set in 2003 improve upon the Thumb instruction set?

The Thumb-2 instruction set introduced in 2003 included both 32-bit and 16-bit Thumb instructions, allowing ARM to maintain the high code density of the Thumb instruction set while also achieving the performance benefits of the 32-bit instruction set.

Which ARM processors only support Thumb-2 instructions?

Most Cortex-M processors, including the Cortex-M4, Cortex-M3, and Cortex-M7, only support Thumb-2 instructions and do not support the original ARM instruction set.

What is the difference in instruction set support between the Cortex-M0 and Cortex-M0+ processors?

The Cortex-M0 and Cortex-M0+ processors partially support the 32-bit Thumb instruction set but fully support the 16-bit Thumb instruction set.

What are the data size definitions in ARM processors?

In ARM processors, a byte is defined as 8-bits, a half-word as 16-bits, a word as 32-bits, and a double word as 64-bits.

Why is code density important in embedded systems like those using Cortex-M processors?

Code density is important in embedded systems because it directly affects the amount of memory required to store the program. Higher code density allows for more efficient use of memory, which is often a limited and expensive resource in embedded systems.

How does the support of both 16-bit and 32-bit Thumb instructions in Thumb-2 affect performance?

Supporting both 16-bit and 32-bit Thumb instructions in Thumb-2 allows for a balance between code density and performance. The 16-bit instructions save space, while the 32-bit instructions can provide the necessary performance for more complex tasks.

What are the implications of a processor only supporting Thumb-2 instructions for software development?

For software development, a processor that only supports Thumb-2 instructions means that developers must use this instruction set for all their code. This can simplify development by reducing the need to switch between different instruction sets but may also impose certain limitations or require specific optimization strategies.