ARM Cortex-M Instruction Set (introduction)

Makermax Education

11 Nov 201703:05

Summary

TLDRThe 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.

Outlines

00:00

💡 Evolution of ARM Instruction Sets

The paragraph discusses the evolution of ARM processors' instruction sets. Initially, ARM processors used a 32-bit instruction set known as the ARM instruction set, which was powerful but required more memory, leading to higher costs and power consumption. In 1995, ARM introduced the 16-bit Thumb instruction set to improve code density and reduce memory requirements. However, some functionalities still required the 32-bit ARM instruction set, necessitating support for both. To address this, ARM introduced the Thumb-2 instruction set in 2003, which included both 32-bit and 16-bit instructions, balancing code density with performance. Most Cortex-M processors, including the Cortex-M4, now only support Thumb-2 instructions. The paragraph also explains data size definitions in ARM processors: a byte is 8-bit, a half-word is 16-bit, a word is 32-bit, and a double word is 64-bit.

Mindmap

Keywords

💡Cortex-M

Cortex-M refers to a family of 32-bit RISC ARM processor cores licensed by ARM Holdings. These processors are designed for microcontroller applications and are optimized for low power consumption and cost-effectiveness. In the context of the video, Cortex-M is the subject of discussion as the video script explains the evolution of the instruction sets used in ARM processors, with a focus on how Cortex-M processors have adopted the Thumb-2 instruction set for better performance and code density.

💡Instruction Set

An instruction set is the part of the computer architecture and it is the set of all the commands (instructions) that a particular CPU can perform. In the video script, the evolution of ARM's instruction sets is discussed, starting from the 32-bit ARM instruction set to the 16-bit Thumb instruction set, and finally to the Thumb-2 instruction set which combines both 16-bit and 32-bit instructions for better performance and code density.

💡ARM Processors

ARM processors are a family of reduced instruction set computing (RISC) processors based on the ARM architecture, which is a 32-bit architecture. They are widely used in embedded systems and mobile devices. The video script discusses the ARM processors' transition from using a 32-bit instruction set to adopting the Thumb instruction sets for improved code density and reduced power consumption.

💡Thumb Instruction Set

The Thumb instruction set is a 16-bit instruction set developed by ARM Holdings for use in their processors. It was introduced to reduce the memory footprint of ARM code, which is particularly important in embedded systems where memory is a premium. The script explains how the Thumb instruction set was introduced in 1995 to address the issue of larger program memory required by the 32-bit ARM instruction set.

💡Code Density

Code density refers to the amount of functionality that can be packed into a given amount of memory. High code density means more instructions can be stored in less memory. The video script highlights the Thumb instruction set's ability to provide good code density, which is crucial for memory-constrained devices like microcontrollers found in Cortex-M processors.

💡Performance

In the context of processors, performance refers to how efficiently they can execute instructions. The video script discusses the trade-off between code density and performance. While the 16-bit Thumb instruction set improved code density, it initially had an impact on performance. The introduction of the Thumb-2 instruction set aimed to mitigate this by including 32-bit instructions to enhance performance.

💡Memory

Memory in computing refers to the hardware devices used to store information for immediate use by a computer's processor. The video script mentions memory as a critical factor in the evolution of ARM's instruction sets, as larger program memory was required by the initial 32-bit ARM instruction set, leading to the development of the more memory-efficient Thumb instruction sets.

💡Power Consumption

Power consumption is the amount of electrical energy used by a device over time. In the script, it is mentioned that the larger program memory of the 32-bit ARM instruction set also consumed more power, which is undesirable for battery-powered devices. The Thumb instruction sets were introduced to reduce power consumption by offering more efficient use of memory.

💡Data Size Definitions

Data size definitions in ARM processors refer to the standard sizes for data types: a byte is 8-bit, a half-word is 16-bit, a word is 32-bit, and a double word is 64-bit. The video script explains these definitions to provide a clear understanding of how data is processed and stored in ARM-based systems, which is essential for programmers working with ARM architecture.

💡Thumb-2 Instruction Set

The Thumb-2 instruction set is an extension of the Thumb instruction set that includes both 16-bit and 32-bit instructions. It was introduced in 2003 to combine the benefits of code density from the 16-bit Thumb instructions with the performance of the 32-bit ARM instructions. The video script emphasizes that most Cortex-M processors, including the Cortex-M4 discussed, only support the Thumb-2 instruction set.

Highlights

Before 1995, ARM processors used a 32-bit instruction set known as the ARM instruction set.

The ARM instruction set provided good performance but required larger program memory compared to 8-bit and 16-bit processors.

In 1995, ARM introduced the 16-bit Thumb instruction set to address the issue of memory size and power consumption.

The Thumb instruction set offered most of the functionality but not everything, necessitating some coding in the 32-bit ARM instruction set.

ARM processors started supporting both ARM and Thumb instruction sets to maintain functionality and performance.

The Thumb instruction set provided better code density compared to other 32-bit instruction sets.

The downside of the Thumb instruction set was its impact on performance due to handling two types of instructions.

In 2003, ARM introduced the Thumb-2 instruction set, which included both 32-bit and 16-bit Thumb instructions.

Thumb-2 maintained the code density of Thumb while improving performance to match the 32-bit instruction set.

Most Cortex-M processors, including the Cortex-M4, support only Thumb-2 instructions.

The Cortex-M3 and M7 processors also support only Thumb-2 instructions, not the ARM instruction set.

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

ARM processors define data sizes with a byte as 8-bit, a half-word as 16-bit, a word as 32-bit, and a double word as 64-bit.

Future lectures will delve into specific instructions within the ARM and Thumb-2 instruction sets.