For beginners new to hardware development, understanding the relationship between Cortex, ARM core, ARM architecture, ARM instruction set, and SoCs can be challenging. Neardi is here to clarify these ARM-related concepts for you.
ARM Overview: ARM can refer to the company, a type of microprocessor, or a technology.
ARM’s Role: ARM specializes in designing and developing chips based on RISC technology. As an intellectual property provider, ARM doesn’t manufacture chips directly but licenses its designs to other companies, who then produce chips with varying features.
ARM Processor Core: The ARM core is standardized and provided by ARM, while the on-chip components vary and are designed by different semiconductor companies. This allows for a wide range of embedded systems using a consistent core with diverse peripherals.
Let’s explore these concepts in detail.
ARM Company
ARM, short for Advanced RISC Machines, is primarily a company specializing in chip technology. Although it originally had a different name, the company is known for its significant role in designing and licensing chip technology rather than manufacturing or selling chips directly. For those interested, a deeper dive into ARM’s history is available, but we won’t cover it in detail here.
ARM Core and Architecture
Every ARM chip consists of two main components: the ARM core and peripherals.
**ARM Core**: This includes registers, an instruction set, buses, memory mapping, interrupt logic, and debugging components. The core is designed by ARM and licensed to chip manufacturers (ARM itself does not manufacture chips). For example, the Cortex-A8 and A9, designed for high performance, are built on the ARMv7a architecture. Cortex-M3 and M4 are based on ARMv7m. The former refers to the processor core, while the latter refers to the instruction set architecture (also known as the architecture).
**Peripherals**: These include timers, A/D converters, memory, I2C, UART, SPI, ROM, and more. These peripherals are designed by individual chip manufacturers and integrated with the ARM core. This diversity in peripherals across manufacturers has led to a large and varied ARM chip ecosystem.
### ARM Instruction Set Architecture (ISA)
The instruction set is the most critical part of a processor’s design, referred to as ISA (Instruction Set Architecture) in ARM terms. The instruction set is essentially the “soul” of CPU design, the key to unlocking the CPU’s functionality. To operate the CPU, we interact with it through these instructions.
For a 32-bit CPU, instructions are sequences of 32-bit binary values (0s and 1s), where different values represent different machine instructions. The CPU hardware can accurately interpret and execute these instructions for tasks like addressing, calculations, and exception handling.
When playing a game on your phone, every action you take is ultimately translated into a series of machine instructions.
Since the first ARMv1 architecture in 1985, ARM architecture has evolved, with the ARMv8 architecture introduced in 2011. ARMv8, used in Cortex-A32/35/53/57/72/73/77/78 processors, is ARM’s first architecture to support 64-bit instruction sets.
Processors and instruction set architectures before ARM11
Before the ARM11 chip, the architecture relationship of each chip was as follows:
| Architecture | Processor family |
| ARMv1 | ARM1 |
| ARMv2 | ARM2, ARM3 |
| ARMv3 | ARM6, ARM6OO, ARM610, ARM7, ARM700, ARM710 |
| ARMv4 | StrOngARM, ARM8, ARM810, ARM7-TDMI, ARM9-TDMI |
| ARMv5 | ARM7EJ, ARM9E, ARM10E, XScal |
| ARMv6 | ARM11 |
Processors and instruction set architectures after ARM11
After the ARM11 chip, that is, starting from the ARMv7 architecture, ARM’s naming method changed. The new processor family was renamed Cortex and divided into three series, namely Cortex-A, Cortex-R, and Cortex-M. Coincidentally, these three letters are A, R, and M.
Cortex-A Series (A: Application)
Designed for the growing consumer entertainment and wireless products, it is used in applications with high computing requirements, running rich operating systems and providing interactive media and graphics experience, such as smartphones, tablets, car entertainment systems, digital TVs, smart books, e-readers, home networks, home gateways and various other products.
Cortex-R Series (R: Real-time)
For system applications that need to run real-time operations, it is aimed at deep embedded real-time applications such as automotive braking systems, powertrain solutions, and large-capacity storage controllers.
Cortex-M Series (M: Microcontroller)
This series is aimed at the microcontroller field, mainly for cost- and power-sensitive applications, such as smart measurement, human-machine interface devices, automotive and industrial control systems, home appliances, consumer products and medical devices.
The ARM11 series includes the ARM11MPCore processor, ARM1176 processor, ARM1156 processor, and ARM1136 processor, which are based on the ARMv6 architecture.
ARM Cortex-A5 processor, Cortex-A7 processor, Cortex-A8 processor, Cortex-A9 processor, and Cortex-A15 processor belong to the Cortex-A series, based on the ARMv7-A architecture.
The Cortex-A53 and Cortex-A57 processors belong to the Cortex-A50 series, and for the first time use the 64-bit ARMv8 architecture.
In 2020, ARM recently released a new CPU architecture, the Cortex-A78, which is based on the ARMv8.2 instruction set.
What is SOC?
Let’s summarize these concepts below:
The name of ARM is ARM: Advanced RISC Machines;
The first microprocessor designed by Acorn, the predecessor of ARM, is called ARM: Acorn RISC Machine;
The name of ARM processor: formerly called ARM9 and ARM11, the new naming convention is changed to Cortex, namely Cortex-A, Cortex-R, Cortex-M; these three letters A, R, and M together are ARM.
The ARM instruction set is the ARM architecture, such as ARMv8. Each processor needs to rely on a certain ARM architecture to be designed;
SOC: major manufacturers buy ARM’s authorization, get the source code of the ARM processor, and then make some peripheral device IPs (either buy or design them themselves) to form a SOC.
Summary
The full name of SoC is: System-on-a-Chip, which means “the whole system is built on a chip” in Chinese.
SoC integrates many of the most critical components of mobile phones, such as CPU, GPU, memory, which means that although it exists as a chip on the motherboard, it is composed of many components packaged inside.
A classic ARM System-on-Chip or so-called Soc contains many components, only some of which are directly derived from ARM. First, the core itself is usually deeply embedded inside the device and is not usually directly visible from the device’s scope, while the debug port is usually the only exposed part connected to the core itself, with some glue logic such as clock and reset integrated circuits.
Since the ARM core only has two interrupt inputs, the most common peripheral is an interrupt controller of some kind. Inside the peripheral, the components are connected to each other through an on-chip interconnect bus architecture. For the vast majority of ARM-based devices, this is the standard AMBA interconnect.
AMBA specifies two buses, a high-performance system bus called AXI, and a low-power peripheral bus called APB. APB is usually used to connect all peripherals, while AXI is used for memory and other high-speed devices. Most devices have some amount of on-chip memory and interfaces to connect peripheral memory devices, but note that the external connection to the device is not an AMBA bus. This is only used internally in the device and is not exposed.