opcode

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      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.


      CPUs


      Opcodes are found in the machine language instructions of CPUs as well as in some abstract computing machines. In CPUs, an opcode may be referred to as an instruction machine code, instruction code, instruction syllable, instruction parcel, or opstring. For any particular processor (which may be a general CPU or a more specialized processing unit), the opcodes are defined by the processor's instruction set architecture (ISA). They can be described using an opcode table. The types of operations may include arithmetic, data copying, logical operations, program control, and special instructions (e.g., CPUID).
      In addition to the opcode, many instructions specify the data (known as operands) the operation will act upon, although some instructions may have implicit operands or none. Some instruction sets have nearly uniform fields for opcode and operand specifiers, whereas others (e.g., x86 architecture) have a less uniform, variable-length structure. Instruction sets can be extended through opcode prefixes, which add a subset of new instructions made up of existing opcodes following reserved byte sequences.


      Software instruction sets


      Opcodes can be found in so-called byte codes and other representations intended for a software interpreter rather than a hardware device. These software-based instruction sets often employ slightly higher-level data types and operations than most hardware counterparts but are nevertheless constructed along similar lines. Examples include the byte code found in Java class files, which are then interpreted by the Java Virtual Machine (JVM), the byte code used in GNU Emacs for compiled Lisp code, NET Common Intermediate Language (CIL), and many others.


      See also



      Gadget (machine instruction sequence)
      Illegal opcode
      Syllable (computing)
      Fused operation


      References

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    Difference between: Opcode, byte code, mnemonics, machine …

    Jul 14, 2013 · Opcode: Opcode is short for operation code. As its name suggests, the opcode is a type of code that tells the machine what to do, i.e. what operation to perform. Opcode is a type of machine language instruction. Bytecode: Bytecode is similar to opcode in nature, as it also tells the machine what to do.

    How to read the Intel Opcode notation - Stack Overflow

    Feb 22, 2013 · The opcode determines if the operand is a signed value. All words, doublewords and quadwords are given with the low-order byte first. • +rb, +rw, +rd, +ro — Indicates the lower 3 bits of the opcode byte is used to encode the register operand without a modR/M byte. The instruction lists the corresponding hexadecimal value of the opcode byte ...

    assembly - Intel x86 Opcode Reference? - Stack Overflow

    Jun 19, 2011 · While Intel Software Developer's Manual itself is definitely not very convenient to search through, the opcode tables in this manual could help. Take a look at the Appendix A "Opcode Map" in the volume 2A, 2B, 2C, and 2D of the manual, it might be useful:

    How does an instruction decoder tell the difference between a …

    Aug 23, 2021 · The decoder has an undocumented (but easy to reverse) algorithm when picking up these prefixes. This shows that some opcode listed in the manual is actually a prefix+opcode. N.B. Is syntactically valid to add an unneeded prefix to an opcode, but it's a reserved use. –

    How to interpret the opcode manually? - Stack Overflow

    May 16, 2011 · look for mov dword ptr [ebp-4],eax you have 8 bits of code.you can get it easily here is the procedure first six bits are given or should be memorized for mov command and then add on LSB the destination bit(D) where d=1 when there is a register in the destination or d=0 when the register is in source.Here the register eax is in source side so should add 0 and then …

    What is the difference between machine code and opcode?

    The question is mostly related to PHP because IMHO opcode is mostly mentioned in PHP context. In fact Java is more popular for its byte-/opcode. 1) Is opcode just a portion of machine code, does machine code consist of opcodes? Yes, but this opcodes are machine-dependent.

    Why call instruction opcode is represented as FF15?

    Apr 24, 2015 · Have a look at this question: what does opcode FF350E204000 do? It explains that an entire group of instructions starts with FF: INC , DEC , CALLN , CALLF , JMPN , JMPF , PUSH . The instruction is determined by looking at bits 5 through 3 of the ModR/M byte (see e.g. here if you want to avoid the official intel manual), that is in your case ...

    optimization - What opcode dispatch strategies are used in …

    Nov 11, 2010 · opcode_targets maps the instruction in the language's bytecode to the location in memory of the opcode implementation. This is faster because the processor's branch predictor can make a different prediction for each bytecode, in contrast to a switch statement that has only one branch instruction.

    What does 'opcode -1' mean in Chrome? - Stack Overflow

    Oct 21, 2018 · Chrome failing to connect to websocket server (Opcode -1) "handshake was canceled" 15 What does the 'X-DevTools-Emulate-Network-Conditions-Client-Id' header key represent?

    how to fetch the MIPS-opcode from the hex - Stack Overflow

    Nov 24, 2013 · Anyway if the opcode is in a fixed position (26-31) the opcode can be simply obtained through a mask and shift right as follow: opcode=(reg>>26)&0x3F An algorithm to print a binary number can be like this: