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Opcode
In computing, an opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine language instruction that specifies the operation to be performed. Beside the opcode itself, most instructions also specify the data they will process, in the form of operands. In addition to opcodes used in the instruction set architectures of various CPUs, which are hardware devices, they can also be used in abstract computing machines as part of their byte code specifications. Overview Specifications and format of the opcodes are laid out in the instruction set architecture (ISA) of the processor in question, which may be a general CPU or a more specialized processing unit. Opcodes for a given instruction set can be described through the use of an opcode table detailing all possible opcodes. Apart from the opcode itself, an instruction normally also has one or more specifiers fo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Illegal Opcode
An illegal opcode, also called an unimplemented operation, unintended opcode or undocumented instruction, is an instruction to a CPU that is not mentioned in any official documentation released by the CPU's designer or manufacturer, which nevertheless has an effect. Illegal opcodes were common on older CPUs designed during the 1970s, such as the MOS Technology 6502, Intel 8086, and the Zilog Z80. On these older processors, many exist as a side effect of the wiring of transistors in the CPU, and usually combine functions of the CPU that were not intended to be combined. On old and modern processors, there are also instructions intentionally included in the processor by the manufacturer, but that are not documented in any official specification. The effect of many illegal opcodes, on many processors, is just a trap to an error handler. However, some processors that trap for most illegal opcodes do not do so for some illegal opcodes, and some other processors do not check for ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Assembly Language
In computer programming, assembly language (or 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 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 Di ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Opcode Table
An opcode table (also called an opcode matrix) is a visual representation of all opcodes in an instruction set. It is arranged such that each axis of the table represents an upper or lower nibble In computing, a nibble (occasionally nybble, nyble, or nybl to match the spelling of byte) is a four- bit aggregation, or half an octet. It is also known as half-byte or tetrade. In a networking or telecommunication context, the nibble is oft ..., which combined form the full byte of the opcode. Additional opcode tables can exist for additional instructions created using an opcode prefix. Table values The structure and arrangement of an opcode table appears as follows: {, class="wikitable" style="text-align: center;" ! ! scope="column" , 0 , , 1 , , 2 , , 3 , , 4 , , 5 , , 6 , , 7 , , 8 , , 9 , , A , , B , , C , , D , , E , , F , - ! scope="row" , 0 , 00 , , 01 , , 02 , , 03 , , 04 , , 05 , , 06 , , 07 , , 08 , , 09 , , 0A , , 0B , , 0C , , 0D , , ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Opcode Prefix
In computing, an opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine language instruction that specifies the operation to be performed. Beside the opcode itself, most instructions also specify the data they will process, in the form of operands. In addition to opcodes used in the instruction set architectures of various CPUs, which are hardware devices, they can also be used in abstract computing machines as part of their byte code specifications. Overview Specifications and format of the opcodes are laid out in the instruction set architecture (ISA) of the processor in question, which may be a general CPU or a more specialized processing unit. Opcodes for a given instruction set can be described through the use of an opcode table detailing all possible opcodes. Apart from the opcode itself, an instruction normally also has one or more specifiers for ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Instruction (computer Science)
In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ''implementation''. In general, an ISA defines the supported instructions, data types, registers, the hardware support for managing main memory, fundamental features (such as the memory consistency, addressing modes, virtual memory), and the input/output model of a family of implementations of the ISA. 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 (among other things), but that are capable of running the same machine code, so th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Instruction Set Architecture
In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ''implementation''. In general, an ISA defines the supported instructions, data types, registers, the hardware support for managing main memory, fundamental features (such as the memory consistency, addressing modes, virtual memory), and the input/output model of a family of implementations of the ISA. 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 (among other things), but that are capable of running the same machine code, s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Machine Code
In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very specific task, such as a load, a store, a jump, or an arithmetic logic unit (ALU) operation on one or more units of data in the CPU's registers or memory. Early CPUs had specific machine code that might break backwards compatibility with each new CPU released. The notion of an instruction set architecture (ISA)Â defines and specifies the behavior and encoding in memory of the instruction set of the system, without specifying its exact implementation. This acts as an abstraction layer, enabling compatibility within the same family of CPUs, so that machine code written or generated according to the ISA for the family will run on all CPUs in the family, including future CPUs. In general, each architecture family (e.g. x86, ARM) has its ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Addressing Mode
Addressing modes are an aspect of the instruction set architecture in most central processing unit (CPU) designs. The various addressing modes that are defined in a given instruction set architecture 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. Caveats Note that there is no generally accepted way of naming the various addressing modes. In particular, different authors and computer manufacturers may give different names to the same addressing mode, or the sam ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gadget (machine Instruction Sequence)
Return-oriented programming (ROP) is a computer security exploit technique that allows an attacker to execute code in the presence of security defenses such as executable space protection and code signing. In this technique, an attacker gains control of the call stack to hijack program control flow and then executes carefully chosen machine instruction sequences that are already present in the machine's memory, called "gadgets". Each gadget typically ends in a return instruction and is located in a subroutine within the existing program and/or shared library code. Chained together, these gadgets allow an attacker to perform arbitrary operations on a machine employing defenses that thwart simpler attacks. Background Return-oriented programming is an advanced version of a stack smashing attack. Generally, these types of attacks arise when an adversary manipulates the call stack by taking advantage of a bug in the program, often a buffer overrun. In a buffer overrun, a functio ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Byte Code
Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (normally numeric addresses) that encode the result of compiler parsing and performing semantic analysis of things like type, scope, and nesting depths of program objects. The name bytecode stems from instruction sets that have one-byte opcodes followed by optional parameters. Intermediate representations such as bytecode may be output by programming language implementations to ease interpretation, or it may be used to reduce hardware and operating system dependence by allowing the same code to run cross-platform, on different devices. Bytecode may often be either directly executed on a virtual machine (a p-code machine, i.e., interpreter), or it may be further compiled into machine code for better performance. Since bytecode instructi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Central Processing Unit
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions in the program. This contrasts with external components such as main memory and I/O circuitry, and specialized processors such as graphics processing units (GPUs). The form, design, and implementation of CPUs have changed over time, but their fundamental operation remains almost unchanged. Principal components of a CPU include the arithmetic–logic unit (ALU) that performs arithmetic and logic operations, processor registers that supply operands to the ALU and store the results of ALU operations, and a control unit that orchestrates the fetching (from memory), decoding and execution (of instructions) by directing the coordinated operations of the ALU, registers and o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
CPUID
In the x86 architecture, the CPUID instruction (identified by a CPUID opcode) is a processor supplementary instruction (its name derived from CPU IDentification) allowing software to discover details of the processor. It was introduced by Intel in 1993 with the launch of the Pentium and SL-enhanced 486 processors. A program can use the CPUID to determine processor type and whether features such as MMX/ SSE are implemented. History Prior to the general availability of the CPUID instruction, programmers would write esoteric machine code which exploited minor differences in CPU behavior in order to determine the processor make and model. With the introduction of the 80386 processor, EDX on reset indicated the revision but this was only readable after reset and there was no standard way for applications to read the value. Outside the x86 family, developers are mostly still required to use esoteric processes (involving instruction timing or CPU fault triggers) to determine the va ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
