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In computer science, a tagged pointer is a pointer (concretely a memory address) with additional data associated with it, such as an indirection bit or reference count. This additional data is often "folded" into the pointer, meaning stored inline in the data representing the address, taking advantage of certain properties of memory addressing. The name comes from "tagged union", and the additional data is called a "tag" or "tags", though strictly speaking "tag" refers to data specifying a ''type,'' not other data; however, the usage "tagged pointer" is standard. ==Folding tags into the pointer== There are various techniques for folding tags into a pointer.〔(Friday Q&A 2012-07-27: Let's Build Tagged Pointers ), by Mike Ash〕 Most architectures are byte-addressable (memory addresses are in bytes), but certain types of data will often be ''aligned'' to the size of the data, often a word or multiple thereof. This discrepancy leaves a few of the least significant bits of the pointer unused, which can be used for tags – most often as a bit field (each bit a separate tag) – as long as code that uses the pointer masks out these bits before accessing memory. E.g., on a 32-bit architecture (for both addresses and word size), a word is 32 bits = 4 bytes, so word-aligned addresses are always a multiple of 4, hence end in 00, leaving the last 2 bits available; while on a 64-bit architecture, a word is 64 bits word = 8 bytes, so word-aligned addresses end in 000, leaving the last 3 bits available. In cases where data is aligned at a multiple of word size, further bits are available. In case of word-addressable architectures, word-aligned data does not leave any bits available, as there is no discrepancy between alignment and addressing, but data aligned at a multiple of word size does. Conversely, in some operating systems, virtual addresses are narrower than the overall architecture width, which leaves the most significant bits available for tags; this can be combined with the previous technique in case of aligned addresses. This is particularly the case on 64-bit architectures, as 64 bits of address space are far above the data requirements of all but the largest applications, and thus many practical 64-bit processors have narrower addresses. Note that the virtual address width may be narrower than the physical address width, which in turn may be narrower than the architecture width; for tagging of pointers in user space, the virtual address space provided by the operating system (in turn provided by the memory management unit) is the relevant width. In fact, some processors specifically forbid use of such tagged pointers at the processor level, notably x86-64, which requires the use of canonical form addresses by the operating system, with most significant bits all 0s or all 1s. Lastly, the virtual memory system in most modern operating systems reserves a block of logical memory around address 0 as unusable. This means that, for example, a pointer to 0 is never a valid pointer and can be used as a special null pointer value. Unlike the previously mentioned techniques, this only allows a single special pointer value, not extra data for pointers generally. 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「tagged pointer」の詳細全文を読む スポンサード リンク
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