Instruction set architecture in the context of Lower level language

The IBM System/360 architecture is the model independent architecture for the entire S/360 line of mainframe computers, including but not limited to the instruction set architecture. The elements of the architecture are documented in the IBM System/360 Principles of Operation and the IBM System/360 I/O Interface Channel to Control Unit Original Equipment Manufacturers' Information manuals.

In computer science and computer engineering, a computer architecture is the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, the description may include the instruction set architecture design, microarchitecture design, logic design, and implementation.

macOS (previously OS X and originally Mac OS X) is a proprietary Unix-based operating system, derived from OPENSTEP for Mach and FreeBSD, which has been marketed and developed by Apple since 2001. It is the current operating system for Apple's Mac computers. Within the market of desktop and laptop computers, it is the second most widely used desktop OS, after Microsoft Windows and ahead of all Linux distributions, including ChromeOS and SteamOS. As of 2025, the most recent release of macOS is macOS 26 Tahoe, the 22nd major version of macOS.

Mac OS X succeeded the classic Mac OS, the primary Macintosh operating system from 1984 to 2001. Its underlying architecture came from NeXT's NeXTSTEP, as a result of Apple's acquisition of NeXT, which also brought Steve Jobs back to Apple. The first desktop version, Mac OS X 10.0, was released on March 24, 2001. Mac OS X Leopard and all later versions of macOS, other than OS X Lion, are UNIX 03 certified. Each of Apple's other contemporary operating systems, including iOS, iPadOS, watchOS, tvOS, audioOS and visionOS, are derivatives of macOS. Throughout its history, macOS has supported three major processor architectures: the initial version supported PowerPC-based Macs only, with support for Intel-based Macs beginning with OS X Tiger 10.4.4 and support for ARM-based Apple silicon Macs beginning with macOS Big Sur. Support for PowerPC-based Macs was dropped with OS X Snow Leopard, and it was announced at the 2025 Worldwide Developers Conference that macOS Tahoe will be the last to support Intel-based Macs.

C is a general-purpose programming language. It was created in the 1970s by Dennis Ritchie and remains widely used and influential. By design, C gives the programmer relatively direct access to the features of the typical CPU architecture, customized for the target instruction set. It has been and continues to be used to implement operating systems (especially kernels), device drivers, and protocol stacks, but its use in application software has been decreasing. C is used on computers that range from the largest supercomputers to the smallest microcontrollers and embedded systems.

A successor to the programming language B, C was originally developed at Bell Labs by Ritchie between 1972 and 1973 to construct utilities running on Unix. It was applied to re-implementing the kernel of the Unix operating system. During the 1980s, C gradually gained popularity. It has become one of the most widely used programming languages, with C compilers available for practically all modern computer architectures and operating systems. The book The C Programming Language, co-authored by the original language designer, served for many years as the de facto standard for the language. C has been standardized since 1989 by the American National Standards Institute (ANSI) and, subsequently, jointly by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC).

In electronics, computer science and computer engineering, microarchitecture, also called computer organization and sometimes abbreviated as μarch or uarch, is the way a given instruction set architecture (ISA) is implemented in a particular processor. A given ISA may be implemented with different microarchitectures; implementations may vary due to different goals of a given design or due to shifts in technology.

Computer architecture is the combination of microarchitecture and instruction set architecture.

In computing, assembly language (alternatively assembler language or symbolic machine code), often referred to simply as assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence between the instructions in the language and the architecture's machine code instructions. Assembly language usually has one statement per machine code instruction (1:1), but constants, comments, assembler directives, symbolic labels of, e.g., memory locations, registers, and macros are generally also supported.

The first assembly code in which a language is used to represent machine code instructions is found in Kathleen and Andrew Donald Booth's 1947 work, Coding for A.R.C.. Assembly code is converted into executable machine code by a utility program referred to as an assembler. The term "assembler" is generally attributed to Wilkes, Wheeler and Gill in their 1951 book The Preparation of Programs for an Electronic Digital Computer, who, however, used the term to mean "a program that assembles another program consisting of several sections into a single program". The conversion process is referred to as assembly, as in assembling the source code. The computational step when an assembler is processing a program is called assembly time.

A low-level programming language is a programming language that provides little or no abstraction from a computer's instruction set architecture, memory or underlying physical hardware; commands or functions in the language are structurally similar to a processor's instructions. These languages provide the programmer with full control over program memory and the underlying machine code instructions. Because of the low level of abstraction (hence the term "low-level") between the language and machine language, low-level languages are sometimes described as being "close to the hardware".

In processor design, microcode serves as an intermediary layer situated between the central processing unit (CPU) hardware and the programmer-visible instruction set architecture of a computer. It consists of a set of hardware-level instructions that implement the higher-level machine code instructions or control internal finite-state machine sequencing in many digital processing components. While microcode is utilized in Intel and AMD general-purpose CPUs in contemporary desktops and laptops, it functions only as a fallback path for scenarios that the faster hardwired control unit is unable to manage.

Housed in special high-speed memory, microcode translates machine instructions, state machine data, or other input into sequences of detailed circuit-level operations. It separates the machine instructions from the underlying electronics, thereby enabling greater flexibility in designing and altering instructions. Moreover, it facilitates the construction of complex multi-step instructions, while simultaneously reducing the complexity of computer circuits. The act of writing microcode is often referred to as microprogramming, and the microcode in a specific processor implementation is sometimes termed a microprogram.

Addressing modes are an aspect of the instruction set architecture in most central processing unit (CPU) designs. Addressing modes define how the machine language instructions in that architecture identify the operand(s) of each instruction. An addressing mode specifies how to calculate the effective memory address of an operand by using information held in registers and/or constants contained within a machine instruction or elsewhere.

In computer programming, addressing modes are primarily of interest to those who write in assembly languages and to compiler writers. For a related concept see orthogonal instruction set which deals with the ability of any instruction to use any addressing mode.

PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple–IBM–Motorola alliance, known as AIM. PowerPC, as an evolving instruction set, has been named Power ISA since 2006, while the old name lives on as a trademark for some implementations of Power Architecture–based processors.

Originally intended for personal computers, the architecture is well known for being used by Apple's desktop and laptop lines from 1994 until 2006, and in several videogame consoles including Microsoft's Xbox 360, Sony's PlayStation 3, and Nintendo's GameCube, Wii, and Wii U. PowerPC was also used for the Curiosity and Perseverance rovers on Mars and a variety of satellites. It has since become a niche architecture for personal computers, particularly with AmigaOS 4 implementations, but remains popular for embedded systems.

LLVM is a set of compiler and toolchain technologies that can be used to develop a frontend for any programming language and a backend for any instruction set architecture. LLVM is designed around a language-independent intermediate representation (IR) that serves as a portable, high-level assembly language that can be optimized with a variety of transformations over multiple passes. The name LLVM originally stood for Low Level Virtual Machine. However, the project has since expanded, and the name is no longer an acronym but an orphan initialism.

LLVM is written in C++ and is designed for compile-time, link-time, and runtime optimization. Originally implemented for C and C++, the language-agnostic design of LLVM has since spawned a wide variety of frontends: languages with compilers that use LLVM (or which do not directly use LLVM but can generate compiled programs as LLVM IR) include ActionScript, Ada, C# for .NET, Common Lisp, PicoLisp, Crystal, CUDA, D, Delphi, Dylan, Forth, Fortran, FreeBASIC, Free Pascal, Halide, Haskell, Idris, Jai (only for optimized release builds), Java bytecode, Julia, Kotlin, LabVIEW's G language, Objective-C, OpenCL, PostgreSQL's SQL and PL/pgSQL, Ruby, Rust, Scala, Standard ML, Swift, Xojo, and Zig.

The Mac transition to Intel processors was the process of switching the central processing units (CPUs) of Apple's line of Mac and Xserve computers from PowerPC processors over to Intel's x86-64 processors. The change was announced at the 2005 Worldwide Developers Conference (WWDC) by then-Apple CEO Steve Jobs, who said Apple would gradually stop using PowerPC microprocessors supplied by Freescale (formerly Motorola) and IBM.

The transition was the second time Apple had switched the processor instruction set architecture of its personal computers, after the Mac transition to PowerPC processors in 1994 from the original Motorola 68000 series architecture.

x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel, based on the 8086 microprocessor and its 8-bit-external-bus variant, the 8088. The 8086 was introduced in 1978 as a fully 16-bit extension of Intel's 8-bit 8080 microprocessor, with memory segmentation as a solution for addressing more memory than can be covered by a plain 16-bit address. The term "x86" came into being because the names of several successors to Intel's 8086 processor end in "86", including the 80186, 80286, 80386 and 80486. Colloquially, their names were "186", "286", "386" and "486".

The term is not synonymous with IBM PC compatibility, as this implies a multitude of other computer hardware. Embedded systems and general-purpose computers used x86 chips before the PC-compatible market started, some of them before the IBM PC (1981) debut.

SuperH (or SH) is a 32-bit reduced instruction set computing (RISC) instruction set architecture (ISA) developed by Hitachi and currently produced by Renesas. It is implemented by microcontrollers and microprocessors for embedded systems.

At the time of introduction, SuperH was notable for having fixed-length 16-bit instructions in spite of its 32-bit architecture. Using smaller instructions had consequences: the register file was smaller and instructions were generally two-operand format. However, for the market the SuperH was aimed at, this was a small price to pay for the improved memory and processor cache efficiency.

MIPS (Microprocessor without Interlocked Pipelined Stages) is a family of reduced instruction set computer (RISC) instruction set architectures (ISA) developed by MIPS Computer Systems, now MIPS Technologies, based in the United States.

There are multiple versions of MIPS, including MIPS I, II, III, IV, and V, as well as five releases of MIPS32/64 (for 32- and 64-bit implementations, respectively). The early MIPS architectures were 32-bit; 64-bit versions were developed later. As of April 2017, the current version of MIPS is MIPS32/64 Release 6. MIPS32/64 primarily differs from MIPS I–V by defining the privileged kernel mode System Control Coprocessor in addition to the user mode architecture.

ARM (stylised in lowercase as arm) is a family of RISC instruction set architectures for computer processors. Arm Holdings develops the instruction set architecture and licenses them to other companies, who build the physical devices that use the instruction set. It also designs and licenses cores that implement these instruction set architectures.

Due to their low costs, low power consumption, and low heat generation, ARM processors are useful for light, portable, battery-powered devices, including smartphones, laptops, and tablet computers, as well as embedded systems. However, ARM processors are also used for desktops and servers, including Fugaku, the world's fastest supercomputer from 2020 to 2022. With over 230 billion ARM chips produced, since at least 2003, and with its dominance increasing every year, ARM is the most widely used family of instruction set architectures.