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
Famous quotes containing the word details:
“There was a time when the average reader read a novel simply for the moral he could get out of it, and however naïve that may have been, it was a good deal less naïve than some of the limited objectives he has now. Today novels are considered to be entirely concerned with the social or economic or psychological forces that they will by necessity exhibit, or with those details of daily life that are for the good novelist only means to some deeper end.”
—Flannery O’Connor (1925–1964)
“Anyone can see that to write Uncle Tom’s Cabin on the knee in the kitchen, with constant calls to cooking and other details of housework to punctuate the paragraphs, was a more difficult achievement than to write it at leisure in a quiet room.”
—Anna Garlin Spencer (1851–1931)