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Descriptors are an architectural feature of Burroughs large systems, including the current (as of 2006) Unisys Clearpath/MCP systems. Apart from being stack- and tag-based, a notable architectural feature of these systems is that it is descriptor-based. Descriptors are the means of having data that does not reside on the stack as for arrays and objects. Descriptors are also used for string data as in compilers and commercial applications. ==Details== Descriptors describe data blocks. Each descriptor contains a 20-bit address field referencing the data block. Each block has a length which is stored in the descriptor, also 20 bits. The size of the data is also given, being 4-, 6-, 8- or 48-bit data in a three bit field. The first computer with this architecture was the B5000. in that implementation, the meaning of the various status bits was: * Bit 47 — The presence bit (P-Bit) * Bit 46 — The copy bit * Bit 45 — The indexed bit * Bit 44 — The paged bit * Bit 43 — The read only bit In later implementations these status bits evolved to keep up with growing memory sizes and gained insights. Bit 47 is probably the most interesting bit in the system – it is the way the architecture implements virtual memory. Virtual memory was originally developed for the Atlas project at the University of Manchester in the late 1950s. Keen to see this used in commercial applications, they invited engineers from several computer companies to a seminar, including those from Burroughs and IBM. The Burroughs engineers saw the significance of virtual memory and put it into the B5000. The IBM engineers weren't interested and IBM did not "invent" virtual memory for another ten years. When a descriptor is referenced, the hardware checks bit 47. If it is 1, the data is present in memory at the location indicated in the address field. If bit 47 is 0, the data block is not present and an interrupt (p-bit interrupt) is raised and MCP code entered to make the block present. In this case, if the address field is 0, the data block has not been allocated (init p-bit) and the MCP searches for a free block the size of which is given in the length field. The last p-bit scenario is when bit 47 is 0, indicating that the data is not in memory, but the address is non-zero, indicating that the data has been allocated and in this case the address represents a disk address in the virtual memory area on disk. In this case a p-bit interrupt is raised and it is noted as an 'other' p-bit. 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「Burroughs large systems descriptors」の詳細全文を読む スポンサード リンク
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