## Assembly language (asm)

An assembly (or assembler) language, often abbreviated asm, is a low-level programming language for a computer, or other programmable device, in which there is a very strong (generally one-to-one) correspondence between the language and the architecture's machine code instructions. Each assembly language is specific to a particular computer architecture, in contrast to most high-level programming languages, which are generally portable across multiple architectures, but require interpreting or compiling.

## Application Binary Interface (ABI)

An application binary interface (ABI) is the interface between two program modules, one of which is often a library or operating system, at the level of machine code. An ABI determines such details as how functions are called and in which binary format information should be passed from one program component to the next, or to the operating system in the case of a system call.

## Array

An array is an contiguous sequence of memory divided into equal sized slots. Given an index number can compute which slot you're in and get the value back.

## Associative Array

An associative array, map, symbol table, dictionary (Swift), associative containers (C++) object literal (JavaScript) or hash (Ruby) is an abstract data type composed of a collection of (key, value) pairs, such that each possible key appears just once in the collection.

## Bitmap or Bit array

A bitmap is a mapping from some domain (for example, a range of integers) to bits, that is, values which are zero or one. It is also called a bit array or bitmap index.

In computer graphics, when the domain is a rectangle (indexed by two coordinates) a bitmap gives a way to store a binary image, that is, an image in which each pixel is either black or white (or any two colors).

A bit array is an array data structure that compactly stores bits. It can be used to implement a simple set data structure. A bit array is effective at exploiting bit-level parallelism in hardware to perform operations quickly.

## Bitwise operation

A bitwise operation operates on one or more bit patterns or binary numerals at the level of their individual bits. It is a fast, simple action directly supported by the processor, and is used to manipulate values for comparisons and calculations.

## Bitwise operators

### Bitwise NOT (`~`

)

The bitwise NOT, or complement, is a unary operation that performs logical negation on each bit, forming the ones' complement of the given binary value.

### Bitwise AND (`&`

)

The bitwise AND takes two equal-length binary representations and performs the logical AND operation on each pair of the corresponding bits, by multiplying them.

The operation may be used to determine whether a particular bit is set (1) or clear (0). For example, given a bit pattern 0011 (decimal 3), to determine whether the second bit is set we use a bitwise AND with a bit pattern containing 1 only in the second bit. Because the result 0010 is non-zero, we know the second bit in the original pattern was set. This is often called bit masking. (By analogy, the use of masking tape covers, or masks, portions that should not be altered or portions that are not of interest. In this case, the 0 values mask the bits that are not of interest.)

The bitwise AND may be used to clear selected bits (or flags) of a register in which each bit represents an individual Boolean state. For example, 0110 (decimal 6) can be considered a set of four flags, where the first and fourth flags are clear (0), and the second and third flags are set (1). The second bit may be cleared by using a bitwise AND with the pattern that has a zero only in the second bit.

It becomes easy to check the parity of a binary number by checking the value of the lowest valued bit. Because 0110 (6) AND 0001 (1) is 0000 (zero), 6 is divisible by two and therefore even.

### Bitwise OR (`|`

)

A bitwise OR takes two bit patterns of equal length and performs the logical inclusive OR operation on each pair of corresponding bits. The result in each position is 0 if both bits are 0, otherwise the result is 1.

The bitwise OR may be used to set to 1 the selected bits of the register that bitwise AND clears. For example, the fourth bit of 0010 (decimal 2) may be set by performing a bitwise OR with the pattern with only the fourth bit set.

### Bitwise XOR (`^`

)

A bitwise XOR takes two bit patterns of equal length and performs the logical exclusive OR operation on each pair of corresponding bits. The result in each position is 1 if only the first bit is 1 or only the second bit is 1, but will be 0 if both are 0 or both are 1.

In this we perform the comparison of two bits, being 1 if the two bits are different, and 0 if they are the same. The bitwise XOR may be used to invert selected bits in a register (also called toggle or flip). Any bit may be toggled by XORing it with 1. For example, given the bit pattern 0010 the second and fourth bits may be toggled by a bitwise XOR with a bit pattern containing 1 in the second and fourth positions.

Assembly language programmers sometimes use XOR as a short-cut to setting the value of a register to zero. Performing XOR on a value against itself always yields zero, and on many architectures this operation requires fewer clock cycles and memory than loading a zero value and saving it to the register.

`x ^ y = (x & ~y) | (~x & y)`

## Communicating Sequential Processes

Communicating sequential processes (CSP) is a formal language for describing patterns of interaction in concurrent systems.

CSP was first described in a 1977 paper by Tony Hoare "Communicating Sequential Processes".

## Composition

### Object Composition

Object composition is a way to combine simple objects or data types into more complex ones. Compositions are a critical building block of many basic data structures, including the tagged union, the linked list, and the binary tree, as well as the object used in object-oriented programming.

### Function Composition

Function composition is an act or mechanism to combine simple functions to build more complicated ones. The result of each function is passed as the argument of the next, and the result of the last one is the result of the whole.

## Control Flow

Control flow (or alternatively, flow of control) refers to the specification of the order in which the individual statements, instructions or function calls of an imperative program are executed or evaluated.

## Cooperative Multitasking

Cooperative multitasking, also known as non-preemptive multitasking, is a style of computer multitasking in which the operating system never initiates a context switch from a running process to another process. Instead, processes voluntarily yield control periodically or when idle in order to enable multiple applications to be run simultaneously. This type of multitasking is called "cooperative" because all programs must cooperate for the entire scheduling scheme to work. In this scheme, the process scheduler of an operating system is known as a cooperative scheduler, having its role reduced down to starting the processes and letting them return control back to it voluntarily.

## Coroutines

Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing multiple entry points for suspending and resuming execution at certain locations.

## Cryptographic hash function

A cryptographic hash function is a special class of hash function that has certain properties which make it suitable for use in cryptography. It is a mathematical algorithm that maps data of arbitrary size to a bit string of a fixed size (a hash function) which is designed to also be a one-way function, that is, a function which is infeasible to invert.

The only way to recreate the input data from an ideal cryptographic hash function's output is to attempt a brute-force search of possible inputs to see if they produce a match, or use a rainbow table of matched hashes. The input data is often called the message, and the output (the hash value or hash) is often called the message digest or simply the digest.

## Declarative Programming

Declarative programming is a programming paradigm that expresses the logic of a computation without describing its control flow.

Declarative programming is when you say what you want.

## Daemon

In multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user. Traditionally daemon names end with the letter d. For example, syslogd is the daemon that implements the system logging facility and sshd is a daemon that services incoming SSH connections.

## Data Buffer

In computer science, a data buffer (or just buffer) is a region of a physical memory storage used to temporarily store data while it is being moved from one place to another.

Typically, the data is stored in a buffer as it is retrieved from an input device (such as a microphone) or just before it is sent to an output device (such as speakers). However, a buffer may be used when moving data between processes within a computer. This is comparable to buffers in telecommunication. Buffers can be implemented in a fixed memory location in hardware—or by using a virtual data buffer in software, pointing at a location in the physical memory. In all cases, the data stored in a data buffer are stored on a physical storage medium. A majority of buffers are implemented in software, which typically use the faster RAM to store temporary data, due to the much faster access time compared with hard disk drives. Buffers are typically used when there is a difference between the rate at which data is received and the rate at which it can be processed, or in the case that these rates are variable, for example in a printer spooler or in online video streaming.

## Digital signatures

An analogy for digital signatures is the sealing of an envelope with a personal wax seal. The message can be opened by anyone, but the presence of the unique seal authenticates the sender.

In a public key signature system, a person can combine a message with a private key to create a short digital signature on the message. Anyone with the corresponding public key can combine a message, a putative digital signature on it, and a known public key to verify whether the signature was valid—made by the owner of the corresponding private key.

A digital signature scheme typically consists of 3 algorithms:

- A key generation algorithm that selects a private key uniformly at random from a set of possible private keys. The algorithm outputs the private key and a corresponding public key.

- A signing algorithm that, given a message and a private key, produces a signature.

- A signature verifying algorithm that, given the message, public key and signature, either accepts or rejects the message's claim to authenticity.

Two main properties are required. First, the authenticity of a signature generated from a fixed message and fixed private key can be verified by using the corresponding public key. Secondly, it should be computationally infeasible to generate a valid signature for a party without knowing that party's private key.

## Encoding - bits and bytes

Bit is a portmanteau of *binary digit*. A bit can have only one of two values, most commonly represented as either a 0 or 1.

Byte is a unit of digital information in computing, most commonly consisting of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit of memory in many computer architectures.

For example, see *ASCII* (American Standard Code for Information Interchange) which encodes 128 (English) characters and Unicode which is the standard for encoding. UTF-8 (U from Universal Character Set + Transformation Format—8-bit) is a character encoding capable of encoding all possible characters (called code points) in Unicode. The encoding is variable-length and uses 8-bit code units.

Generally, bytes are used for storage and bits for speed.

## Functional Programming

Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data.

## Fold (higher-order functions)

In functional programming, fold (also termed reduce, accumulate, aggregate, compress, or inject) refers to a family of higher-order functions that analyze a recursive data structure and through use of a given combining operation, recombine the results of recursively processing its constituent parts, building up a return value. Typically, a fold is presented with a combining function, a top node of a data structure, and possibly some default values to be used under certain conditions. The fold then proceeds to combine elements of the data structure's hierarchy, using the function in a systematic way.

## Glob

Glob patterns specify sets of filenames with wildcard characters.

`*`

is a wildcard standing for "any string of characters" and `*.txt`

is a glob pattern. `?`

is a wildcard standing for one character.

## Hash function

A hash function is any function that can be used to map digital data of arbitrary size to digital data of fixed size. The values returned by a hash function are called hash values, hash codes, hash sums, or simply hashes.

## Hash table

A hash table (hash map) is a data structure which implements an associative array abstract data type, a structure that can map keys to values. A hash table uses a hash function to compute an index into an array of buckets or slots, from which the desired value can be found.

## Hexadecimal

Hexadecimal (also base 16, or hex) is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F (or alternatively a, b, c, d, e, f) to represent values ten to fifteen. Hexadecimal numerals are widely used by computer systems designers and programmers.

## Higher-order Functions

A higher-order function (also functional, functional form or functor) is a function that does at least one of the following:

- takes one or more functions as arguments (i.e., procedural parameters);
- returns a function as its result.

All other functions are first-order functions.

## Imperative Programming

Imperative programming is a programming paradigm that defines computation as statements that change a program state.

Imperative language is when you say how to get what you want.

## Instruction Set Architecture

*x86* is a family of backward compatible instruction set architectures (ISA) based on the Intel 8086 CPU and its Intel 8088 variant. x86 includes 16-bit, 32-bit and 64-bit (x86-64) versions.

*A64* and *A32 (ARM)* are ISAs developed by ARM and used in smartphones and tablets such as "System in Package" (SiP) designs found in Apple Watch.

Instruction set architecture (ISA), is the part of the computer architecture related to *programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O*.

ISA refers to the set of predefined opcodes that are valid for the given CPU architecture.

x86 ISA: https://en.wikipedia.org/wiki/X86_instruction_listings

ARM ISA: https://www.arm.com/products/processors/instruction-set-architectures/index.php

ARM ISA includes ARMv8-A architecture for "applications" (high performance markets such as mobile and enterprise), ARMv8-R for "real-time" (embedded applications in automotive and industrial control) and ARMv8-M architecture for "microcontrollers" (embedded and IoT applications).

A system on a chip or system on chip (SoC or SOC) is an integrated circuit (also known as an "IC" or "chip") that integrates all components of a computer or other electronic systems.

## Machine code

Machine code or machine language is a set of instructions executed directly by a computer's central processing unit (CPU). Each instruction performs a very specific task, such as a load, a jump, or an Arithmetic/Logic Unit (ALU) operation on a unit of data in a CPU register or memory. Every program directly executed by a CPU is made up of a series of such instructions.

Numerical machine code (i.e., not assembly code) may be regarded as the lowest-level representation of a compiled or assembled computer program or as a primitive and hardware-dependent programming language.

Machine code: IA32 is the machine code that has become the de facto standard for a wide range of systems. x86-64 is an extension of IA32 to enable programs to operate on larger data and to reference a wider range of memory addresses. Since x86-64 systems are able to run IA32 code, both of these forms of machine code will see widespread use for the foreseeable future.

Machine code instructions generally map one-to-one to assembly instructions. Assembly code is the easy-to-read (for humans) interpretaion of machine code.

## Microarchitecture

ISA is distinguished from the microarchitecture, which is the set of processor design techniques used to implement the instruction set. Computers 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 have radically different internal designs.

Arm architectures

List of ARM microarchitectures.

Comparison of ARMv8-A cores.

## Octal

The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7. Octal numerals can be made from binary numerals by grouping consecutive binary digits into groups of three (starting from the right). For example, the binary representation for decimal 74 is 1001010, which can be grouped into (00)1 001 010 – so the octal representation is 112.

## Ordinal numbers

Ordinal numbers tell the order of things in a set—first, second, third, etc. Ordinal numbers do not show quantity. They only show rank or position.

## Operand

In mathematics, an operand is the object of a mathematical operation, a quantity on which an operation is performed.

## Public key encryption and digital signatures

An analogy to public key encryption is that of a locked mail box with a mail slot. The mail slot is exposed and accessible to the public – its location (the street address) is, in essence, the public key. Anyone knowing the street address can go to the door and drop a written message through the slot. However, only the person who possesses the key can open the mailbox and read the message.

In a public key encryption system, any person can encrypt a message using the public key of the receiver, but such a message can be decrypted only with the receiver's private key. For this to work it must be computationally easy for a user to generate a public and private key-pair to be used for encryption and decryption. The strength of a public key cryptography system relies on the degree of difficulty (computational impracticality) for a properly generated private key to be determined from its corresponding public key. Security then depends only on keeping the private key private, and the public key may be published without compromising security.

## Recursive data type

A recursive data type (also known as a recursively-defined, inductively-defined or inductive data type) is a data type for values that may contain other values of the same type. Data of recursive types are usually viewed as directed graphs.

## Reference

A reference is a value that enables a program to indirectly access a particular datum, such as a variable or a record, in the computer's memory or in some other storage device. The reference is said to refer to the datum, and accessing the datum is called dereferencing the reference.

A reference is distinct from the data itself. Typically, for references to data stored in memory on a given system, a reference is implemented as the physical address of where the data is stored in memory or in the storage device. For this reason, a reference is often erroneously confused with a pointer or address, and is said to "point to" the data. However a reference may also be implemented in other ways, such as the offset (difference) between the datum's address and some fixed "base" address, as an index into an array, or more abstractly as a handle.

The concept of reference must not be confused with other values (keys or identifiers) that uniquely identify the data item, but give access to it only through a non-trivial lookup operation in some table data structure.

Pointers are the most primitive. Due to their intimate relationship with the underlying hardware, they are one of the most powerful and efficient types of references.

## Secure Hash Algorithm (SHA)

SHA-1 is a cryptographic hash function designed by the United States National Security Agency and is a U.S. Federal Information Processing Standard published by the United States NIST.

SHA-1 produces a 160-bit (20-byte) hash value known as a message digest. A SHA-1 hash value is typically rendered as a hexadecimal number, 40 digits long. SHA stands for "secure hash algorithm". The four SHA algorithms are structured differently and are named SHA-0, SHA-1, SHA-2, and SHA-3.

SHA-1 is to be sunset by Microsoft, Google and Mozilla in browsers in 2017. Revision control systems such as Git and Mercurial use SHA-1 not for security but for ensuring that the data has not changed due to accidental corruption.

SHA-2 includes significant changes from its predecessor, SHA-1. The SHA-2 family consists of six hash functions with digests (hash values) that are 224, 256, 384 or 512 bits: SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, SHA-512/256.

SHA-256 and SHA-512 are novel hash functions computed with 32-bit and 64-bit words, respectively. They use different shift amounts and additive constants, but their structures are otherwise virtually identical, differing only in the number of rounds. SHA-224 and SHA-384 are simply truncated versions of the first two, computed with different initial values.

The SHA-2 hash function is implemented in some widely used security applications and protocols, including TLS and SSL, PGP, SSH, S/MIME, and IPsec.

## Unary operation

A unary operation is an operation with only one operand, i.e. a single input.

In the C family of languages, the following operators are unary:

Increment: `++x`

, `x++`

Decrement: `−−x`

, `x−−`

Address: `&x`

Indirection: `*x`

Positive: `+x`

Negative: `−x`

Ones' complement: `~x`

Logical negation: `!x`

Sizeof: `sizeof x`

, `sizeof(type-name)`

Cast: `(type-name) cast-expression`