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Procedural programming could be considered a step toward declarative programming. A programmer can often tell, simply by looking at the names, arguments, and return types of procedures , what a particular procedure is supposed to do, without necessarily looking at the details of how it achieves its result. At the same time, a complete program is still imperative since it fixes the statements to be executed and their order of execution to a large extent.
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The programming paradigm used to build programs for almost all computers typically follows an imperative model. Digital computer hardware is designed to execute machine code, which is native to the computer and is usually written in the imperative style, although low-level compilers and interpreters using other paradigms exist for some architectures such as lisp machines.
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From this low-level perspective, the program state is defined by the contents of memory, and the statements are instructions in the native machine language of the computer. Higher-level imperative languages use variables and more complex statements, but still follow the same paradigm. Recipes and process checklists, while not computer programs, are also familiar concepts that are similar in style to imperative programming; each step is an instruction, and the physical world holds the state. Since the basic ideas of imperative programming are both conceptually familiar and directly embodied in the hardware, most computer languages are in the imperative style.
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Assignment statements, in imperative paradigm, perform an operation on information located in memory and store the results in memory for later use. High-level imperative languages, in addition, permit the evaluation of complex expressions, which may consist of a combination of arithmetic operations and function evaluations, and the assignment of the resulting value to memory. Looping statements allow a sequence of statements to be executed multiple times. Loops can either execute the statements they contain a predefined number of times, or they can execute them repeatedly until some condition is met. Conditional branching statements allow a sequence of statements to be executed only if some condition is met. Otherwise, the statements are skipped and the execution sequence continues from the statement following them. Unconditional branching statements allow an execution sequence to be transferred to another part of a program. These include the jump , switch, and the subprogram, subroutine, or procedure call .
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Early in the development of high-level programming languages, the introduction of the block enabled the construction of programs in which a group of statements and declarations could be treated as if they were one statement. This, alongside the introduction of subroutines, enabled complex structures to be expressed by hierarchical decomposition into simpler procedural structures.
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Many imperative programming languages are abstractions of assembly language.
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The earliest imperative languages were the machine languages of the original computers. In these languages, instructions were very simple, which made hardware implementation easier but hindered the creation of complex programs. FORTRAN, developed by John Backus at International Business Machines starting in 1954, was the first major programming language to remove the obstacles presented by machine code in the creation of complex programs. FORTRAN was a compiled language that allowed named variables, complex expressions, subprograms, and many other features now common in imperative languages. The next two decades saw the development of many other major high-level imperative programming languages. In the late 1950s and 1960s, ALGOL was developed in order to allow mathematical algorithms to be more easily expressed and even served as the operating system's target language for some computers. MUMPS carried the imperative paradigm to a logical extreme, by not having any statements at all, relying purely on commands, even to the extent of making the IF and ELSE commands independent of each other, connected only by an intrinsic variable named $TEST. COBOL and BASIC were both attempts to make programming syntax look more like English. In the 1970s, Pascal was developed by Niklaus Wirth, and C was created by Dennis Ritchie while he was working at Bell Laboratories. Wirth went on to design Modula-2 and Oberon. For the needs of the United States Department of Defense, Jean Ichbiah and a team at Honeywell began designing Ada in 1978, after a 4-year project to define the requirements for the language. The specification was first published in 1983, with revisions in 1995, 2005, and 2012.
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The 1980s saw a rapid growth in interest in object-oriented programming. These languages were imperative in style, but added features to support objects. The last two decades of the 20th century saw the development of many such languages. Smalltalk-80, originally conceived by Alan Kay in 1969, was released in 1980, by the Xerox Palo Alto Research Center . Drawing from concepts in another object-oriented language—Simula —Bjarne Stroustrup designed C++, an object-oriented language based on C. Design of C++ began in 1979 and the first implementation was completed in 1983. In the late 1980s and 1990s, the notable imperative languages drawing on object-oriented concepts were Perl, released by Larry Wall in 1987; Python, released by Guido van Rossum in 1990; Visual Basic and Visual C++ 2.0), released by Microsoft in 1991 and 1993 respectively; PHP, released by Rasmus Lerdorf in 1994; Java, by James Gosling in 1995, JavaScript, by Brendan Eich , and Ruby, by Yukihiro "Matz" Matsumoto, both released in 1995. Microsoft's .NET Framework is imperative at its core, as are its main target languages, VB.NET and C# that run on it; however Microsoft's F#, a functional language, also runs on it.
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FORTRAN was unveiled as "The IBM Mathematical FORmula TRANslating system." It was designed for scientific calculations, without string handling facilities. Along with declarations, expressions, and statements, it supported:
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It succeeded because:
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However, non IBM vendors also wrote Fortran compilers, but with a syntax that would likely fail IBM's compiler. The American National Standards Institute developed the first Fortran standard in 1966. In 1978, Fortran 77 became the standard until 1991. Fortran 90 supports:
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COBOL stands for "COmmon Business Oriented Language." Fortran manipulated symbols. It was soon realized that symbols did not need to be numbers, so strings were introduced. The US Department of Defense influenced COBOL's development, with Grace Hopper being a major contributor. The statements were English-like and verbose. The goal was to design a language so managers could read the programs. However, the lack of structured statements hindered this goal.
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COBOL's development was tightly controlled, so dialects did not emerge to require ANSI standards. As a consequence, it was not changed for 15 years until 1974. The 1990s version did make consequential changes, like object-oriented programming.
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ALGOL stands for "ALGOrithmic Language." It had a profound influence on programming language design. Emerging from a committee of European and American programming language experts, it used standard mathematical notation and had a readable structured design. Algol was first to define its syntax using the Backus–Naur form. This led to syntax-directed compilers. It added features like:
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Algol's direct descendants include Pascal, Modula-2, Ada, Delphi and Oberon on one branch. On another branch there's C, C++ and Java.
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BASIC stands for "Beginner's All Purpose Symbolic Instruction Code." It was developed at Dartmouth College for all of their students to learn. If a student did not go on to a more powerful language, the student would still remember Basic. A Basic interpreter was installed in the microcomputers manufactured in the late 1970s. As the microcomputer industry grew, so did the language.
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Basic pioneered the interactive session. It offered operating system commands within its environment:
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However, the Basic syntax was too simple for large programs. Recent dialects added structure and object-oriented extensions. Microsoft's Visual Basic is still widely used and produces a graphical user interface.
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C programming language got its name because the language BCPL was replaced with B, and AT&T Bell Labs called the next version "C." Its purpose was to write the UNIX operating system. C is a relatively small language -- making it easy to write compilers. Its growth mirrored the hardware growth in the 1980s. Its growth also was because it has the facilities of assembly language, but uses a high-level syntax. It added advanced features like:
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C allows the programmer to control in which region of memory data is to be stored. Global variables and static variables require the fewest clock cycles to store. The stack is automatically used for the standard variable declarations. Heap memory is returned to a pointer variable from the malloc function.
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In the 1970s, software engineers needed language support to break large projects down into modules. One obvious feature was to decompose large projects physically into separate files. A less obvious feature was to decompose large projects logically into abstract datatypes. At the time, languages supported concrete datatypes like integer numbers, floating-point numbers, and strings of characters. Concrete datatypes have their representation as part of their name. Abstract datatypes are structures of concrete datatypes — with a new name assigned. For example, a list of integers could be called integer_list.
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In object-oriented jargon, abstract datatypes are called classes. However, a class is only a definition; no memory is allocated. When memory is allocated to a class, it's called an object.
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Object-oriented imperative languages developed by combining the need for classes and the need for safe functional programming. A function, in an object-oriented language, is assigned to a class. An assigned function is then referred to as a method, member function, or operation. Object-oriented programming is executing operations on objects.
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Object-oriented languages support a syntax to model subset/superset relationships. In set theory, an element of a subset inherits all the attributes contained in the superset. For example, a student is a person. Therefore, the set of students is a subset of the set of persons. As a result, students inherit all the attributes common to all persons. Additionally, students have unique attributes that other persons don't have. Object-oriented languages model subset/superset relationships using inheritance. Object-oriented programming became the dominant language paradigm by the late 1990s.
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C++ was originally called "C with Classes." It was designed to expand C's capabilities by adding the object-oriented facilities of the language Simula.
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An object-oriented module is composed of two files. The definitions file is called the header file. Here is a C++ header file for the GRADE class in a simple school application:
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A constructor operation is a function with the same name as the class name. It is executed when the calling operation executes the new statement.
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A module's other file is the source file. Here is a C++ source file for the GRADE class in a simple school application:
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Here is a C++ header file for the PERSON class in a simple school application:
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Here is a C++ source file for the PERSON class in a simple school application:
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Here is a C++ header file for the STUDENT class in a simple school application:
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Here is a C++ source file for the STUDENT class in a simple school application:
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Here is a driver program for demonstration:
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Here is a makefile to compile everything:
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In general, an ISA defines the supported instructions, data types, registers, the hardware support for managing main memory, fundamental features , and the input/output model of a family of implementations of the ISA.
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An ISA specifies the behavior of machine code running on implementations of that ISA in a fashion that does not depend on the characteristics of that implementation, providing binary compatibility between implementations. This enables multiple implementations of an ISA that differ in characteristics such as performance, physical size, and monetary cost , but that are capable of running the same machine code, so that a lower-performance, lower-cost machine can be replaced with a higher-cost, higher-performance machine without having to replace software. It also enables the evolution of the microarchitectures of the implementations of that ISA, so that a newer, higher-performance implementation of an ISA can run software that runs on previous generations of implementations.
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If an operating system maintains a standard and compatible application binary interface for a particular ISA, machine code will run on future implementations of that ISA and operating system. However, if an ISA supports running multiple operating systems, it does not guarantee that machine code for one operating system will run on another operating system, unless the first operating system supports running machine code built for the other operating system.
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An ISA can be extended by adding instructions or other capabilities, or adding support for larger addresses and data values; an implementation of the extended ISA will still be able to execute machine code for versions of the ISA without those extensions. Machine code using those extensions will only run on implementations that support those extensions.
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The binary compatibility that they provide makes ISAs one of the most fundamental abstractions in computing.
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An instruction set architecture is distinguished from a microarchitecture, which is the set of processor design techniques used, in a particular processor, to implement the instruction set. Processors with different microarchitectures can share a common instruction set. For example, the Intel Pentium and the AMD Athlon implement nearly identical versions of the x86 instruction set, but they have radically different internal designs.
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The concept of an architecture, distinct from the design of a specific machine, was developed by Fred Brooks at IBM during the design phase of System/360.
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Some virtual machines that support bytecode as their ISA such as Smalltalk, the Java virtual machine, and Microsoft's Common Language Runtime, implement this by translating the bytecode for commonly used code paths into native machine code. In addition, these virtual machines execute less frequently used code paths by interpretation . Transmeta implemented the x86 instruction set atop VLIW processors in this fashion.
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An ISA may be classified in a number of different ways. A common classification is by architectural complexity. A complex instruction set computer has many specialized instructions, some of which may only be rarely used in practical programs. A reduced instruction set computer simplifies the processor by efficiently implementing only the instructions that are frequently used in programs, while the less common operations are implemented as subroutines, having their resulting additional processor execution time offset by infrequent use.
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Other types include very long instruction word architectures, and the closely related long instruction word and explicitly parallel instruction computing architectures. These architectures seek to exploit instruction-level parallelism with less hardware than RISC and CISC by making the compiler responsible for instruction issue and scheduling.
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Architectures with even less complexity have been studied, such as the minimal instruction set computer and one-instruction set computer . These are theoretically important types, but have not been commercialized.
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Machine language is built up from discrete statements or instructions. On the processing architecture, a given instruction may specify:
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More complex operations are built up by combining these simple instructions, which are executed sequentially, or as otherwise directed by control flow instructions.
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Examples of operations common to many instruction sets include:
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Processors may include "complex" instructions in their instruction set. A single "complex" instruction does something that may take many instructions on other computers. Such instructions are typified by instructions that take multiple steps, control multiple functional units, or otherwise appear on a larger scale than the bulk of simple instructions implemented by the given processor. Some examples of "complex" instructions include:
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Complex instructions are more common in CISC instruction sets than in RISC instruction sets, but RISC instruction sets may include them as well. RISC instruction sets generally do not include ALU operations with memory operands, or instructions to move large blocks of memory, but most RISC instruction sets include SIMD or vector instructions that perform the same arithmetic operation on multiple pieces of data at the same time. SIMD instructions have the ability of manipulating large vectors and matrices in minimal time. SIMD instructions allow easy parallelization of algorithms commonly involved in sound, image, and video processing. Various SIMD implementations have been brought to market under trade names such as MMX, 3DNow!, and AltiVec.
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On traditional architectures, an instruction includes an opcode that specifies the operation to perform, such as add contents of memory to register—and zero or more operand specifiers, which may specify registers, memory locations, or literal data. The operand specifiers may have addressing modes determining their meaning or may be in fixed fields. In very long instruction word architectures, which include many microcode architectures, multiple simultaneous opcodes and operands are specified in a single instruction.
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Some exotic instruction sets do not have an opcode field, such as transport triggered architectures , only operand.
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Most stack machines have "0-operand" instruction sets in which arithmetic and logical operations lack any operand specifier fields; only instructions that push operands onto the evaluation stack or that pop operands from the stack into variables have operand specifiers. The instruction set carries out most ALU actions with postfix operations that work only on the expression stack, not on data registers or arbitrary main memory cells. This can be very convenient for compiling high-level languages, because most arithmetic expressions can be easily translated into postfix notation.
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Conditional instructions often have a predicate field—a few bits that encode the specific condition to cause an operation to be performed rather than not performed. For example, a conditional branch instruction will transfer control if the condition is true, so that execution proceeds to a different part of the program, and not transfer control if the condition is false, so that execution continues sequentially. Some instruction sets also have conditional moves, so that the move will be executed, and the data stored in the target location, if the condition is true, and not executed, and the target location not modified, if the condition is false. Similarly, IBM z/Architecture has a conditional store instruction. A few instruction sets include a predicate field in every instruction; this is called branch predication.
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Instruction sets may be categorized by the maximum number of operands explicitly specified in instructions.
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addresses referring to memory cells, while reg1 and so on refer to machine registers.)
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Due to the large number of bits needed to encode the three registers of a 3-operand instruction, RISC architectures that have 16-bit instructions are invariably 2-operand designs, such as the Atmel AVR, TI MSP430, and some versions of ARM Thumb. RISC architectures that have 32-bit instructions are usually 3-operand designs, such as the ARM, AVR32, MIPS, Power ISA, and SPARC architectures.
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Each instruction specifies some number of operands explicitly. Some instructions give one or both operands implicitly, such as by being stored on top of the stack or in an implicit register. If some of the operands are given implicitly, fewer operands need be specified in the instruction. When a "destination operand" explicitly specifies the destination, an additional operand must be supplied. Consequently, the number of operands encoded in an instruction may differ from the mathematically necessary number of arguments for a logical or arithmetic operation . Operands are either encoded in the "opcode" representation of the instruction, or else are given as values or addresses following the opcode.
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Register pressure measures the availability of free registers at any point in time during the program execution. Register pressure is high when a large number of the available registers are in use; thus, the higher the register pressure, the more often the register contents must be spilled into memory. Increasing the number of registers in an architecture decreases register pressure but increases the cost.
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While embedded instruction sets such as Thumb suffer from extremely high register pressure because they have small register sets, general-purpose RISC ISAs like MIPS and Alpha enjoy low register pressure. CISC ISAs like x86-64 offer low register pressure despite having smaller register sets. This is due to the many addressing modes and optimizations that CISC ISAs offer.
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The size or length of an instruction varies widely, from as little as four bits in some microcontrollers to many hundreds of bits in some VLIW systems. Processors used in personal computers, mainframes, and supercomputers have minimum instruction sizes between 8 and 64 bits. The longest possible instruction on x86 is 15 bytes . Within an instruction set, different instructions may have different lengths. In some architectures, notably most reduced instruction set computers , instructions are a fixed length, typically corresponding with that architecture's word size. In other architectures, instructions have variable length, typically integral multiples of a byte or a halfword. Some, such as the ARM with Thumb-extension have mixed variable encoding, that is two fixed, usually 32-bit and 16-bit encodings, where instructions cannot be mixed freely but must be switched between on a branch .
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Fixed-length instructions are less complicated to handle than variable-length instructions for several reasons , and are therefore somewhat easier to optimize for speed.
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In early 1960s computers, main memory was expensive and very limited, even on mainframes. Minimizing the size of a program to make sure it would fit in the limited memory was often central. Thus the size of the instructions needed to perform a particular task, the code density, was an important characteristic of any instruction set. It remained important on the initially-tiny memories of minicomputers and then microprocessors. Density remains important today, for smartphone applications, applications downloaded into browsers over slow Internet connections, and in ROMs for embedded applications. A more general advantage of increased density is improved effectiveness of caches and instruction prefetch.
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Computers with high code density often have complex instructions for procedure entry, parameterized returns, loops, etc. . However, more typical, or frequent, "CISC" instructions merely combine a basic ALU operation, such as "add", with the access of one or more operands in memory . Certain architectures may allow two or three operands directly in memory or may be able to perform functions such as automatic pointer increment, etc. Software-implemented instruction sets may have even more complex and powerful instructions.
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Reduced instruction-set computers, RISC, were first widely implemented during a period of rapidly growing memory subsystems. They sacrifice code density to simplify implementation circuitry, and try to increase performance via higher clock frequencies and more registers. A single RISC instruction typically performs only a single operation, such as an "add" of registers or a "load" from a memory location into a register. A RISC instruction set normally has a fixed instruction length, whereas a typical CISC instruction set has instructions of widely varying length. However, as RISC computers normally require more and often longer instructions to implement a given task, they inherently make less optimal use of bus bandwidth and cache memories.
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Certain embedded RISC ISAs like Thumb and AVR32 typically exhibit very high density owing to a technique called code compression. This technique packs two 16-bit instructions into one 32-bit word, which is then unpacked at the decode stage and executed as two instructions.
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Minimal instruction set computers are commonly a form of stack machine, where there are few separate instructions , so that multiple instructions can be fit into a single machine word. These types of cores often take little silicon to implement, so they can be easily realized in an FPGA or in a multi-core form. The code density of MISC is similar to the code density of RISC; the increased instruction density is offset by requiring more of the primitive instructions to do a task.
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There has been research into executable compression as a mechanism for improving code density. The mathematics of Kolmogorov complexity describes the challenges and limits of this.
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In practice, code density is also dependent on the compiler. Most optimizing compilers have options that control whether to optimize code generation for execution speed or for code density. For instance GCC has the option -Os to optimize for small machine code size, and -O3 to optimize for execution speed at the cost of larger machine code.
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The instructions constituting a program are rarely specified using their internal, numeric form ; they may be specified by programmers using an assembly language or, more commonly, may be generated from high-level programming languages by compilers.
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The design of instruction sets is a complex issue. There were two stages in history for the microprocessor. The first was the CISC , which had many different instructions. In the 1970s, however, places like IBM did research and found that many instructions in the set could be eliminated. The result was the RISC , an architecture that uses a smaller set of instructions. A simpler instruction set may offer the potential for higher speeds, reduced processor size, and reduced power consumption. However, a more complex set may optimize common operations, improve memory and cache efficiency, or simplify programming.
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Some instruction set designers reserve one or more opcodes for some kind of system call or software interrupt. For example, MOS Technology 6502 uses 00H, Zilog Z80 uses the eight codes C7,CF,D7,DF,E7,EF,F7,FFH while Motorola 68000 use codes in the range A000..AFFFH.
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Fast virtual machines are much easier to implement if an instruction set meets the Popek and Goldberg virtualization requirements.
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The NOP slide used in immunity-aware programming is much easier to implement if the "unprogrammed" state of the memory is interpreted as a NOP.
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On systems with multiple processors, non-blocking synchronization algorithms are much easier to implement if the instruction set includes support for something such as "fetch-and-add", "load-link/store-conditional" , or "atomic compare-and-swap".
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A given instruction set can be implemented in a variety of ways. All ways of implementing a particular instruction set provide the same programming model, and all implementations of that instruction set are able to run the same executables. The various ways of implementing an instruction set give different tradeoffs between cost, performance, power consumption, size, etc.
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When designing the microarchitecture of a processor, engineers use blocks of "hard-wired" electronic circuitry such as adders, multiplexers, counters, registers, ALUs, etc. Some kind of register transfer language is then often used to describe the decoding and sequencing of each instruction of an ISA using this physical microarchitecture.
There are two basic ways to build a control unit to implement this description :
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Some computer designs "hardwire" the complete instruction set decoding and sequencing .
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Other designs employ microcode routines or tables to do this, using ROMs or writable RAMs , PLAs, or both.
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Some microcoded CPU designs with a writable control store use it to allow the instruction set to be changed .
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CPUs designed for reconfigurable computing may use field-programmable gate arrays .
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An ISA can also be emulated in software by an interpreter. Naturally, due to the interpretation overhead, this is slower than directly running programs on the emulated hardware, unless the hardware running the emulator is an order of magnitude faster. Today, it is common practice for vendors of new ISAs or microarchitectures to make software emulators available to software developers before the hardware implementation is ready.
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Often the details of the implementation have a strong influence on the particular instructions selected for the instruction set. For example, many implementations of the instruction pipeline only allow a single memory load or memory store per instruction, leading to a load–store architecture . For another example, some early ways of implementing the instruction pipeline led to a delay slot.
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The demands of high-speed digital signal processing have pushed in the opposite direction—forcing instructions to be implemented in a particular way. For example, to perform digital filters fast enough, the MAC instruction in a typical digital signal processor must use a kind of Harvard architecture that can fetch an instruction and two data words simultaneously, and it requires a single-cycle multiply–accumulate multiplier.
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IBM PC compatible computers are similar to the original IBM PC, XT, and AT, all from computer giant IBM, that are able to use the same software and expansion cards. Such computers were referred to as PC clones, IBM clones or IBM PC clones. The term "IBM PC compatible" is now a historical description only, since IBM no longer sells personal computers after it sold its personal computer division in 2005 to Chinese technology company Lenovo. The designation "PC", as used in much of personal computer history, has not meant "personal computer" generally, but rather an x86 computer capable of running the same software that a contemporary IBM PC could. The term was initially in contrast to the variety of home computer systems available in the early 1980s, such as the Apple II, TRS-80, and Commodore 64. Later, the term was primarily used in contrast to Apple's Macintosh computers.
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These "clones" duplicated almost all the significant features of the original IBM PC architectures. This was facilitated by IBM's choice of commodity hardware components, which were cheap, and by various manufacturers' ability to reverse-engineer the BIOS firmware using a "clean room design" technique. Columbia Data Products built the first clone of the IBM personal computer, the MPC 1600 by a clean-room reverse-engineered implementation of its BIOS. Other rival companies, Corona Data Systems, Eagle Computer, and the Handwell Corporation were threatened with legal action by IBM, who settled with them. Soon after in 1982, Compaq released the very successful Compaq Portable, also with a clean-room reverse-engineered BIOS, and also not challenged legally by IBM.
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Early IBM PC compatibles used the same computer buses as their IBM counterparts, switching from the 8-bit IBM PC and XT bus to the 16-bit IBM AT bus with the release of the AT. IBM's introduction of the proprietary Micro Channel architecture in its PS/2 series resulted in the establishment of the Extended Industry Standard Architecture bus open standard by a consortium of IBM PC compatible vendors, redefining the 16-bit IBM AT bus as the Industry Standard Architecture bus. Additional bus standards were subsequently adopted to improve compatibility between IBM PC compatibles, including the VESA Local Bus , Peripheral Component Interconnect , and the Accelerated Graphics Port .
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Descendants of the x86 IBM PC compatibles, namely 64-bit computers based on "x86-64/AMD64" chips comprise the majority of desktop computers on the market as of 2021, with the dominant operating system being Microsoft Windows. Interoperability with the bus structure and peripherals of the original PC architecture may be limited or non-existent. Many modern computers are unable to use old software or hardware that depends on portions of the IBM PC compatible architecture which are missing or do not have equivalents in modern computers. For example, computers which boot using Unified Extensible Firmware Interface-based firmware that lack a Compatibility Support Module, or CSM, required to emulate the old BIOS-based firmware interface, or have their CSMs disabled, cannot natively run MS-DOS since MS-DOS depends on a BIOS interface to boot.
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Only the Macintosh had kept significant market share without having compatibility with the IBM PC, although that changed during the Intel Macs era running Mac OS X, often dual-booting Windows with Boot Camp.
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IBM decided in 1980 to market a low-cost single-user computer as quickly as possible. On August 12, 1981, the first IBM PC went on sale. There were three operating systems available for it. The least expensive and most popular was PC DOS made by Microsoft. In a crucial concession, IBM's agreement allowed Microsoft to sell its own version, MS-DOS, for non-IBM computers. The only component of the original PC architecture exclusive to IBM was the BIOS .
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IBM at first asked developers to avoid writing software that addressed the computer's hardware directly and to instead make standard calls to BIOS functions that carried out hardware-dependent operations. This software would run on any machine using MS-DOS or PC DOS. Software that directly addressed the hardware instead of making standard calls was faster, however; this was particularly relevant to games. Software addressing IBM PC hardware in this way would not run on MS-DOS machines with different hardware . The IBM PC was sold in high enough volumes to justify writing software specifically for it, and this encouraged other manufacturers to produce machines that could use the same programs, expansion cards, and peripherals as the PC. The 808x computer marketplace rapidly excluded all machines which were not hardware- and software-compatible with the PC. The 640 KB barrier on "conventional" system memory available to MS-DOS is a legacy of that period; other non-clone machines, while subject to a limit, could exceed 640 KB.
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Rumors of "lookalike," compatible computers, created without IBM's approval, began almost immediately after the IBM PC's release. InfoWorld wrote on the first anniversary of the IBM PC that
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By June 1983 PC Magazine defined "PC 'clone'" as "a computer accommodate the user who takes a disk home from an IBM PC, walks across the room, and plugs it into the 'foreign' machine". Because of a shortage of IBM PCs that year, many customers purchased clones instead. Columbia Data Products produced the first computer more or less compatible with the IBM PC standard during June 1982, soon followed by Eagle Computer. Compaq announced its first product, an IBM PC compatible in November 1982, the Compaq Portable. The Compaq was the first sewing machine-sized portable computer that was essentially 100% PC-compatible. The court decision in Apple v. Franklin, was that BIOS code was protected by copyright law, but it could reverse-engineer the IBM BIOS and then write its own BIOS using clean room design. Note this was over a year after Compaq released the Portable. The money and research put into reverse-engineering the BIOS was a calculated risk.
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2,694
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At the same time, many manufacturers such as Tandy/RadioShack, Xerox, Hewlett-Packard, Digital Equipment Corporation, Sanyo, Texas Instruments, Tulip, Wang and Olivetti introduced personal computers that supported MS-DOS, but were not completely software- or hardware-compatible with the IBM PC.
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2,695
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Tandy described the Tandy 2000, for example, as having a "'next generation' true 16-bit CPU", and with "More speed. More disk storage. More expansion" than the IBM PC or "other MS-DOS computers". While admitting in 1984 that many PC DOS programs did not work on the computer, the company stated that "the most popular, sophisticated software on the market" was available, either immediately or "over the next six months".
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2,696
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Like IBM, Microsoft's apparent intention was that application writers would write to the application programming interfaces in MS-DOS or the firmware BIOS, and that this would form what would now be termed a hardware abstraction layer. Each computer would have its own Original Equipment Manufacturer version of MS-DOS, customized to its hardware. Any software written for MS-DOS would operate on any MS-DOS computer, despite variations in hardware design.
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2,697
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This expectation seemed reasonable in the computer marketplace of the time. Until then Microsoft's business was based primarily on computer languages such as BASIC. The established small system operating software was CP/M from Digital Research which was in use both at the hobbyist level and by the more professional of those using microcomputers. To achieve such widespread use, and thus make the product viable economically, the OS had to operate across a range of machines from different vendors that had widely varying hardware. Those customers who needed other applications than the starter programs could reasonably expect publishers to offer their products for a variety of computers, on suitable media for each.
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2,698
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Microsoft's competing OS was intended initially to operate on a similar varied spectrum of hardware, although all based on the 8086 processor. Thus, MS-DOS was for several years sold only as an OEM product. There was no Microsoft-branded MS-DOS: MS-DOS could not be purchased directly from Microsoft, and each OEM release was packaged with the trade dress of the given PC vendor. Malfunctions were to be reported to the OEM, not to Microsoft. However, as machines that were compatible with IBM hardware—thus supporting direct calls to the hardware—became widespread, it soon became clear that the OEM versions of MS-DOS were virtually identical, except perhaps for the provision of a few utility programs.
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2,699
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MS-DOS provided adequate functionality for character-oriented applications such as those that could have been implemented on a text-only terminal. Had the bulk of commercially important software been of this nature, low-level hardware compatibility might not have mattered. However, in order to provide maximum performance and leverage hardware features , PC applications quickly developed beyond the simple terminal applications that MS-DOS supported directly. Spreadsheets, WYSIWYG word processors, presentation software and remote communication software established new markets that exploited the PC's strengths, but required capabilities beyond what MS-DOS provided. Thus, from very early in the development of the MS-DOS software environment, many significant commercial software products were written directly to the hardware, for a variety of reasons:
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2,700
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In May 1983, Future Computing defined four levels of compatibility:
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