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.
Read more about this topic: Burroughs Large Systems Descriptors
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