Introduction
ARM and x86 are two of the most common CPU architectures used in devices today. Both have their own instruction sets and assembly languages that are fundamentally different. This article will provide an overview of the key differences between ARM and x86 assembly languages, their history, CPU architectures, and use cases.
History and Evolution
ARM originally stood for Acorn RISC Machine and was developed in the 1980s by Acorn Computers as a 32-bit RISC (Reduced Instruction Set Computer) processor for their personal computers. Since then, ARM has evolved into a widely licensed CPU architecture that powers billions of devices from smartphones to appliances.
x86 dates back to the 1970s and was originally created by Intel for their 8086 16-bit microprocessor. x86 evolved into a CISC (Complex Instruction Set Computer) architecture going up to 64-bit with the Pentium and Core series. AMD, VIA, and others have also produced x86-compatible CPUs.
So while both have a long history, ARM was designed from scratch as RISC while x86 evolved gradually from CISC. This leads to fundamental differences in their instruction sets.
Instruction Sets
The ARM instruction set is RISC-based, meaning the instructions are simpler and require fewer cycles to execute compared to CISC. ARM instructions are 32 bits or 64 bits in size.
The x86 CISC instruction set is larger and more complex. Instructions can vary from 8 bits to 64 bits in length. x86 supports variable length instructions while ARM only supports fixed length.
Some key differences in the instruction sets:
- ARM has 27 core registers while x86 has 8 general purpose registers. More registers means more data can be handled faster.
- ARM instructions operate on registers while x86 support memory-to-memory operations. ARM loads data from memory into registers before operating on it.
- ARM uses condition codes while x86 uses flags to handle program flow instructions like branches and loops.
- ARM has dedicated barrel shifter hardware for shift and rotate operations. These take 1 cycle compared to multiple cycles on x86.
- x86 has more complex instructions overall e.g. string copy, input/output, etc. ARM relies on simpler instructions in more steps.
So in summary, ARM instruction set is simpler, more regular while x86 is larger but more complex.
Architecture and Organization
The ARM architecture is based on RISC principles meaning it has fewer stages in the pipeline, simpler control logic, and no microcode. This increases efficiency and performance.
x86 has a longer pipeline, complex decoding and execution stages, and relies on microcode for some instructions. This can lower performance.
ARM also uses a load-store architecture where data is operated only in registers, not directly in memory. x86 allows direct memory access.
Both ARM and x86 use pipelining and superscalar execution for parallelism. But ARM pipelines tend to be shorter. x86 has longer pipelines but may stall more due to interdependencies.
ARM processors typically have fewer transistors, use less power, and generate less heat than x86 chips. This makes ARM popular in mobile and embedded use cases.
Assembly Languages
ARM and x86 naturally have different assembly languages reflecting the differences in their instruction sets.
Some key differences:
- ARM assembly code tends to be denser with fewer instructions needed to perform the same tasks.
- ARM syntax has a unified format with mnemonics like ADD, SUB, etc. x86 has instructions like ADD, ADDSUB, IMUL for different operations.
- ARM registers are referred to as R0-R12 while x86 uses names like EAX, EBX, etc.
- Addressing modes are also different. ARM uses register + offset while x86 has specialized addressing like based + scaled index.
- ARM branches use condition codes while x86 uses flags and jump instructions.
- Integer division is done in hardware on ARM but via software routines on x86.
So ARM assembly uses simpler, more uniform syntax reflecting its RISC origins while x86 assembly is more complex with many specialized instructions.
Use Cases
Due to its RISC design, smaller transistor count and lower power requirements, ARM is dominant in mobile devices from smartphones to tablets. ARM also powers many embedded systems like home appliances, toys and IoT devices.
x86 powers most laptops, desktops and workstations for its high performance. x86 servers and data centers benefit from fast single threaded performance. x86 also excels in applications like gaming.
ARM is making its way into PCs with Windows-on-ARM devices. Apple’s M1 ARM chips are now outperforming x86 chips in MacBooks. So the line between ARM and x86 is blurring with ARM gaining momentum with its power efficiency.
But x86 maintains an advantage in legacy software support. Most applications are still compiled for x86-64 so that inertia keeps x86 as the workhorse for PCs and servers.
Summary
In summary, key differences between ARM and x86 are:
- ARM uses 32/64 bit RISC while x86 has variable CISC instructions.
- ARM has fewer cores, simpler pipelines while x86 has longer pipelines and microcode.
- ARM uses condition codes, x86 uses flags for control flow.
- ARM has dedicated shifter unit, x86 does shifts/rotates in software.
- ARM assembly has simpler unified syntax vs. specialized x86 syntax.
ARM is dominant in mobile/embedded due to efficiency while x86 powers PCs and servers for performance. But ARM is gaining ground with impressive growth in performance per watt.
So while both ARM and x86 have carved out complementary roles, ARM’s RISC origins provide efficiency advantages that continue to push its adoption into new areas, even challenging x86 in its strongholds.
