Why Can't ARM CPUs Natively Run x86 Applications?

ARM CPUs are widely recognized for their efficiency, low power consumption, and extensive use in mobile devices, servers, and embedded systems. Despite their popularity, one common question often arises: Why can't ARM CPUs run x86 applications natively? This article explores the reasons behind this limitation and discusses the solutions that make cross-platform execution possible.

Key Reasons ARM CPUs Can't Run x86 Applications Natively

The inability of ARM CPUs to execute x86 applications natively is primarily due to fundamental differences in their architecture and instruction sets. Let's delve into the specific reasons:

Different Instruction Sets

ARM and x86 architectures have fundamentally different instruction sets. The x86 architecture, a Complex Instruction Set Computing (CISC) design, supports a wide variety of complex instructions. On the other hand, the ARM architecture is a Reduced Instruction Set Computing (RISC) design, focusing on simpler and more efficient instructions. This architectural difference means that x86 binary code, compiled for the x86 instruction set, cannot be directly executed by an ARM processor. The instruction sets are not compatible, making the direct execution impossible.

Binary Compatibility

Applications compiled for x86 are in a format specific to the x86 architecture. Since ARM processors lack the necessary hardware to understand or execute this binary format, some form of translation is required. This translation layer ensures that the binary code is converted to a format that can be executed by ARM processors. Without this translation, ARM would be unable to run x86 applications directly.

System Calls and ABI Differences

Beyond the instruction set, the Application Binary Interface (ABI) and system calls also differ between x86 and ARM architectures. These differences mean that even if the application can execute its instructions, it wouldn't interact correctly with the operating system and hardware on an ARM system. System calls are specific to the architecture, and the ABI defines the calling conventions and data types, all of which are architecture-specific. Therefore, running an x86 application on ARM directly would result in incorrect system call handling and data format issues.

Emulation and Translation Layers

To overcome these limitations, techniques such as emulation and translation layers have been developed. Emulators like QEMU and translation layers like Rosetta 2 for Apple Silicon can translate x86 instructions into ARM instructions on the fly, allowing x86 applications to run on ARM CPUs. However, this process introduces overhead, leading to potential performance loss compared to running native ARM applications.

For example, when you install Android developer tools and start an on-screen ARM phone, it is indeed running ARM code on an x86 processor. This is made possible through emulation. Similarly, x86 processors can also run ARM emulators, effectively translating and running ARM applications on their own x86 architecture.

In summary, the fundamental architectural differences and incompatibilities between ARM and x86 architectures prevent ARM CPUs from executing x86 applications natively. While software solutions like emulation and translation layers enable cross-platform execution, they introduce performance overhead and may not offer the same efficiency as native execution.