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Microcode is a layer of hardware-level instructions that implement higher-level machine code instructions or internal state machine sequencing in many digital processing elements. Microcode is used in general central processing units, in more specialized processors such as microcontrollers, digital signal processors, channel controllers, disk controllers, network interface controllers, network processors, graphics processing units, and in other hardware. Microcode typically resides in special high-speed memory and translates machine instructions, state machine data or other input into sequences of detailed circuit-level operations. It separates the machine instructions from the underlying electronics so that instructions can be designed and altered more freely. It also facilitates the building of complex multi-step instructions, while reducing the complexity of computer circuits. Writing microcode is often called microprogramming and the microcode in a particular processor implementation is sometimes called a microprogram. More extensive microcoding allows small and simple microarchitectures to emulate more powerful architectures with wider word length, more execution units and so on, which is a relatively simple way to achieve software compatibility between different products in a processor family. Some hardware vendors, especially IBM, use the term "microcode" as a synonym for "firmware". In that way, all code within a device is termed "microcode" regardless of it being microcode or machine code; for example, hard disk drives are said to have their microcode updated, though they typically contain both microcode and firmware.〔(【引用サイトリンク】title=IBM pSeries Servers - Microcode Update for Ultrastar 73LZX (US73) 18/36 GB )〕 == Overview == When compared to normal application programs, the elements composing a microprogram exist on a lower conceptual level. To avoid confusion, each microprogram-related element is differentiated by the "micro" prefix: microinstruction, microassembler, microprogrammer, microarchitecture, etc. Engineers normally write the microcode during the design phase of a processor, storing it in a ROM (read-only memory) or PLA (programmable logic array) structure, or in a combination of both.〔Often denoted a ROM/PLA control store in the context of usage in a CPU; 〕 However, machines also exist that have some or all microcode stored in SRAM or flash memory. This is traditionally denoted as "writeable control store" in the context of computers, which can be either read-only or read-write memory. In the latter case, the CPU initialization process loads microcode into the control store from another storage medium, with the possibility of altering the microcode to correct bugs in the instruction set, or to implement new machine instructions. Complex digital processors may also employ more than one (possibly microcode-based) control unit in order to delegate sub-tasks that must be performed essentially asynchronously in parallel. A high-level programmer, or even an assembly programmer, does not normally see or change microcode. Unlike machine code, which often retains some compatibility among different processors in a family, microcode only runs on the exact electronic circuitry for which it is designed, as it constitutes an inherent part of the particular processor design itself. Microprograms consist of series of microinstructions, which control the CPU at a very fundamental level of hardware circuitry. For example, a single typical microinstruction might specify the following operations: * Connect Register 1 to the "A" side of the ALU * Connect Register 7 to the "B" side of the ALU * Set the ALU to perform two's-complement addition * Set the ALU's carry input to zero * Store the result value in Register 8 * Update the "condition codes" with the ALU status flags ("Negative", "Zero", "Overflow", and "Carry") * Microjump to MicroPC nnn for the next microinstruction To simultaneously control all processor's features in one cycle, the microinstruction is often wider than 50 bits, e.g., 128 bits on a 360/85 with an emulator feature. Microprograms are carefully designed and optimized for the fastest possible execution, as a slow microprogram would result in a slow machine instruction and degraded performance for related application programs that use such instructions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Microcode」の詳細全文を読む スポンサード リンク
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