Low-level programming language - AbsoluteAstronomy.com

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

, a low-level programming language is a programming language

Programming language

A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

that provides little or no abstraction

Abstraction (computer science)

In computer science, abstraction is the process by which data and programs are defined with a representation similar to its pictorial meaning as rooted in the more complex realm of human life and language with their higher need of summarization and categorization , while hiding away the...

from a computer's instruction set architecture. Generally this refers to either machine code

Machine code

Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...

or assembly language

Assembly language

An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...

. The word "low" refers to the small or nonexistent amount of abstraction

Abstraction (computer science)

between the language and machine language; because of this, low-level languages are sometimes described as being "close to the hardware."

Low-level languages can be converted to machine code without using a compiler or interpreter, and the resulting code runs directly on the processor. A program written in a low-level language can be made to run very fast, and with a very small memory footprint; an equivalent program in a high-level language will be more heavyweight. Low-level languages are simple, but are considered difficult to use, due to the numerous technical details which must be remembered.

By comparison, a high-level programming language

High-level programming language

A high-level programming language is a programming language with strong abstraction from the details of the computer. In comparison to low-level programming languages, it may use natural language elements, be easier to use, or be from the specification of the program, making the process of...

isolates the execution semantics of a computer architecture from the specification of the program, which simplifies development.

Low-level programming languages are sometimes divided into two categories: first generation, and second generation.

Machine code

is the only language a microprocessor can process directly without a previous transformation. Currently, programmers almost never write programs directly in machine code, because it requires attention to numerous details which a high-level language would handle automatically, and also requires memorizing or looking up numerical codes for every instruction that is used. For this reason, second generation programming languages provide one abstraction level on top of the machine code.

Example: A function in 32-bit x86 machine code to calculate the nth Fibonacci number

Fibonacci number

In mathematics, the Fibonacci numbers are the numbers in the following integer sequence:0,\;1,\;1,\;2,\;3,\;5,\;8,\;13,\;21,\;34,\;55,\;89,\;144,\; \ldots\; ....



8B542408 83FA0077 06B80000 0000C383

FA027706 B8010000 00C353BB 01000000

B9010000 008D0419 83FA0376 078BD98B

C84AEBF1 5BC3

Assembly

Assembly language

is considered a low-level language because while it is not a microprocessor's native language, an assembly language programmer must still understand the microprocessor's unique architecture (such as its registers and instructions). These simple instructions are then assembled directly into machine code. The assembly code can also be abstracted to another layer in a similar manner as machine code is abstracted into assembly code.

Example: The same Fibonacci number

Fibonacci number

In mathematics, the Fibonacci numbers are the numbers in the following integer sequence:0,\;1,\;1,\;2,\;3,\;5,\;8,\;13,\;21,\;34,\;55,\;89,\;144,\; \ldots\; ....

calculator as above, but in x86 assembly language using MASM syntax:



fib:

    mov edx, [esp+8]

    cmp edx, 0

    ja @f

    mov eax, 0

    ret

 

    @@:

    cmp edx, 2

    ja @f

    mov eax, 1

    ret

 

    @@:

    push ebx

    mov ebx, 1

    mov ecx, 1

 

    @@:

        lea eax, [ebx+ecx]

        cmp edx, 3

        jbe @f

        mov ebx, ecx

        mov ecx, eax

        dec edx

    jmp @b

 

    @@:

    pop ebx

    ret

Low-level programming in high-level languages

Experiments with hardware support in high-level languages in the late 1960s led to such languages as PL/S, BLISS

BLISS

BLISS is a system programming language developed at Carnegie Mellon University by W. A. Wulf, D. B. Russell, and A. N. Habermann around 1970. It was perhaps the best known systems programming language right up until C made its debut a few years later. Since then, C took off and BLISS faded into...

, BCPL

BCPL

BCPL is a procedural, imperative, and structured computer programming language designed by Martin Richards of the University of Cambridge in 1966.- Design :...

, and extended ALGOL

ALGOL

ALGOL is a family of imperative computer programming languages originally developed in the mid 1950s which greatly influenced many other languages and became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years...

for Burroughs large systems being used for low-level programming. Forth also has applications as a systems language. However, the language that became pre-eminent in systems programming was C

C (programming language)

C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

.

C is considered a third generation

Third-generation programming language

A third-generation programming language is a refinement of a second-generation programming language. The second generation of programming languages brought logical structure to software. The third generation brought refinements to make the languages more programmer-friendly...

programming language, since it is structured

Structured programming

Structured programming is a programming paradigm aimed on improving the clarity, quality, and development time of a computer program by making extensive use of subroutines, block structures and for and while loops - in contrast to using simple tests and jumps such as the goto statement which could...

and abstracts from machine code (historically, no second generation programming language emerged that was particularly suitable for low-level programming). However, many programmers today might refer to C as low-level, as it lacks a large runtime-system (no garbage collection etc.), basically supports only scalar operations, and provides direct memory addressing. It therefore readily blends with assembly language and the machine level of CPUs and microcontroller

Microcontroller

A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...

s. C's ability to abstract from the machine level means that the same code can be compiled for different hardware platforms; however, fine-grained control at the systems level is still possible providing that certain the target platform has certain broadly-defined features in place, such as a flat memory model

Flat memory model

Flat memory model or linear memory model refers to a memory addressing paradigm in low-level software design such that the CPU can directly address all of the available memory locations without having to resort to any sort of memory segmentation or paging schemes.Memory management and...

, and memory that is divided into byte

Byte

The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...

s. C programmes may require a certain amount of 'tweaking', often implemented by conditional compilation

Conditional compilation

In computer programming, conditional compilation is compilation implementing methods which allow the compiler to produce differences in the executable produced controlled by parameters that are provided during compilation...

, for different target platforms. The process of adapting a systems programme for a different platform is known as porting

Porting

In computer science, porting is the process of adapting software so that an executable program can be created for a computing environment that is different from the one for which it was originally designed...

.

Example - a function that calculates the nth Fibonacci number

Fibonacci number

In mathematics, the Fibonacci numbers are the numbers in the following integer sequence:0,\;1,\;1,\;2,\;3,\;5,\;8,\;13,\;21,\;34,\;55,\;89,\;144,\; \ldots\; ....

in C



unsigned int fib(unsigned int n)

{

    if (n <= 0)

        return 0;

    else if (n <= 2)

        return 1;

    else {

        int a,b,c;

        a = 1;

        b = 1;

        while (1) {

            c = a + b;

            if (n <= 3) return c;

            a = b;

            b = c;

            n--;

        }

    }

}

Relative meaning

The terms high-level and low-level are inherently relative. Some decades ago, the C language

C (programming language)

C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

, and similar languages, were most often considered "high-level" Many programmers today might refer to C as low-level.

Assembly language may itself be regarded as a higher level (but often still one-to-one if used without macros) representation of machine code

Machine code

, as it supports concepts such as constants and (limited) expressions, sometimes even variables, procedures, and data structure

Data structure

In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...

s. Machine code

Machine code

, in its turn, is inherently at a slightly higher level than the microcode

Microcode

Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...

or micro-operation

Micro-operation

In computer central processing units, micro-operations are detailed low-level instructions used in some designs to implement complex machine instructions .Various forms of μops have long been the basis for traditional microcode routines used to simplify the implementation of a...

s used internally in many processors.

Machine code

Assembly

Low-level programming in high-level languages

Relative meaning

See also