Programming in Assembler for Mobiles and ARM

Let's take a look at the mobile devices market. Mobile phones, tablets, and other devices are built on ARM processor architecture. For example, take Snapdragon SoC (System-on-Chip) designed by Qualcomm – this chip integrates a CPU based on the ARM architecture. Of course, that chip may have an additional graphics processing unit(GPU) and even a digital signal processor(DSP) for faster signal processing. Similarly, Apple A18 processors are based on ARM architecture. The only difference between all these mobile devices is the ARM version on which the processor is designed.

ARM stands for Advanced RISC Machine.

ARM has multiple processor architecture series: Cortex-M, Cortex-R, Cortex-A, and Cortex-X, as well as other series, not only generalised Central Processing Units - CPU. Cortex-X series processors are made for performance on smartphones and laptops. It supports up to 14 cores, but that number may change over time. Similarly, the Cortex-A series is designed for devices that execute complex computational tasks. These processors are designed to deliver power-efficient performance, increasing battery life. Like the Cortex-X series, these processors also may have up to 14 cores. The cortex-R series is made for real-time operations, where reaction to events is crucial. These are designed specifically for use in time-sensitive and safety-critical environments. The architecture is very similar to the Cortex-A processors, with some exceptions that will not be discussed here. The Cortex-M series of microcontrollers requires low power consumption and high computational power. Many wearable devices, IoT devices, and embedded devices use ARM processors. The ARM processor “world” is huge – only through individual investigation can it be explored and its potential realised. In this book, we will present key ideas for what needs to be investigated in more depth to unlock the exact processor's full potential and features. The primary resource for this book is the ARM official homepage at https://developer.arm.com/documentation, which includes all its resources and documentation.

Mobile devices tend to use Cortex-A series processors. As the instruction set is very similar to that of the Cortex series (of course, with minor differences), we will use a Raspberry Pi 5 as a mobile device to learn its architecture. The A64 instruction set will be used in assembly language. The Raspberry Pi 5 uses BCM2712, an ARM Cortex-A76 processor based on the ARMv8.2 architecture. The processor architecture includes many useful features to speed up applications. Currently, the Raspberry Pi 5 has NEON with an integrated floating-point unit, but only half-precision. The architecture has an optional L3 cache, and the BCM2712 on the Raspberry Pi 5 has 2MB of it. All specifics are available in the BCM2712 datasheet. Note that ARM typically uses registers to manipulate data. There are no operations that directly manipulate data in memory. All the data must be loaded into the processor's general-purpose registers. All manipulations are performed directly on those registers containing data, and the data can finally be stored back in memory.