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A status register, flag register, or condition code register is a collection of status flag bits for a processor. An example is the FLAGS register of the x86 architecture. The flags might be part of a larger register, such as a program status word (PSW) register. The status register is a hardware register that contains information about the state of the processor. Individual bits are implicitly or explicitly read and/or written by the machine code instructions executing on the processor. The status register lets an instruction take action contingent on the outcome of a previous instruction. For example, an instruction combines or compares operands and a subsequent instruction changes the execution sequence based on the outcome of the earlier instruction summarized in the status register. The status register in a traditional processor design includes at least three central flags: Zero, Carry, and Overflow, which are set or cleared automatically as effects of arithmetic and bit manipulation operations. One or more of the flags may then be read by a subsequent conditional jump instruction (including conditional calls, returns, etc. in some machines) or by some arithmetic, shift/rotate or bitwise operation, typically using the carry flag as input in addition to any explicitly given operands. There are also processors where other classes of instructions may read or write the fundamental zero, carry or overflow flags, such as block-, string- or dedicated input/output instructions, for instance. Some CPU architectures, such as the MIPS and Alpha, do not use a dedicated flag register. Others do not implicitly set and/or read flags. Such machines either do not pass ''implicit'' status information between instructions at all, or they pass it in an explicitly selected general purpose register. A status register may often have other fields as well, such as more specialized flags, interrupt enable bits, and similar types of information. During an interrupt, the status of the thread currently executing can be preserved (and later recalled) by storing the current value of the status register along with the program counter and other active registers into the machine stack or some other reserved area of memory. ==Common flags== This is a list of the most common CPU status register flags, implemented in almost all modern processors. 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「status register」の詳細全文を読む スポンサード リンク
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