Instruction (computer science) in the context of Call stack

In computing, an executable is a resource that a computer can use to control its behavior. As with all information in computing, it is data, but distinct from data that does not imply a flow of control. Terms such as executable code, executable file, executable program, and executable image describe forms in which the information is represented and stored. A native executable is machine code and is directly executable at the instruction level of a CPU. A script is also executable although indirectly via an interpreter. Intermediate executable code (such as bytecode) may be interpreted or converted to native code at runtime via just-in-time compilation.

In computing, an opcode (abbreviated from operation code) is an enumerated value that specifies the operation to be performed. Opcodes are employed in hardware devices such as arithmetic logic units (ALUs), central processing units (CPUs), and software instruction sets. In ALUs, the opcode is directly applied to circuitry via an input signal bus. In contrast, in CPUs, the opcode is the portion of a machine language instruction that specifies the operation to be performed.

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.

CPU time (or process time) is the amount of time that a central processing unit (CPU) was used for processing a specific set of instructions of a computer program or operating system, as opposed to total time from start to finish, where the CPU might have spent time on other processes. See the figure. CPU time is measured in clock ticks or seconds. Sometimes it is useful to convert CPU time into a percentage of the CPU capacity, giving the CPU usage.

Measuring CPU time for two functionally identical programs that process identical inputs can indicate which program is faster, but it is a common misunderstanding that CPU time can be used to compare algorithms. Comparing programs by their CPU time compares specific implementations of algorithms. (It is possible to have both efficient and inefficient implementations of the same algorithm.) Algorithms are more commonly compared using measures of time complexity and space complexity.

In computer programming, a branch table or jump table is a method of transferring program control (branching) to another part of a program (or a different program that may have been dynamically loaded) using a table of branch or jump instructions. It is a form of multiway branch. The branch table construction is commonly used when programming in assembly language but may also be generated by compilers, especially when implementing optimized switch statements whose values are densely packed together.

Access time is the time delay or latency between a request to an electronic system, and the access being initiated or the requested data returned.

In computer and software systems, it is the time interval between the point where an instruction control unit initiates a call to retrieve data or a request to store data, and the point at which delivery of the data is completed or the storage is started. Note that in distributed software systems or other systems with stochastic processes, access time or latency should be measured at the 99th percentile.

A program counter (PC) is a register that stores where a computer program is being executed by a processor. It is also commonly called the instruction pointer (IP) in Intel x86 and Itanium microprocessors, and sometimes called the instruction address register (IAR), the instruction counter, or just part of the instruction sequencer.

Usually, a PC stores the memory address of an instruction. Further, it usually is incremented after fetching an instruction, and therefore points to the next instruction to be executed. For a processor that increments before fetch, the PC points to the instruction being executed. In some processors, the PC points some distance beyond the current instruction. For instance, in the ARM7, the value of PC visible to the programmer reflects instruction prefetching and reads as the address of the current instruction plus 8 in ARM State, or plus 4 in Thumb State. For modern processors, the location of execution in the program is complicated by instruction-level parallelism and out-of-order execution.