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 storage areas, I/O requests and I/O results. They contain fields indicating, e.g. the type of descriptor, the address, the length, whether the data are present in storage. The details differ depending on the product line and the type of descriptor. The following text numbers the leftmost (most significant) bit as 0, in accordance with the Burroughs documentation.
Program and data descriptors have a bit called the "presence bit" or "p-bit"; it indicates whether the object referred to by the descriptor is currently in memory or in secondary storage. This bit is used to 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 B5000, released in 1961, was the first commercial computer to provide virtual memory. In the mid-1960s, other vendors, including IBM, RCA, and General Electric developed machines that supported virtual memory, with operating systems oriented towards time-sharing; IBM didn't support virtual memory in machines intended for general data processing use until 1972.
When a descriptor is referenced, the hardware checks the p-bit. If it is 1, the data is present in memory at the location indicated in the address field. If the p-bit 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 the p-bit 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.
B5000, B5500 and B5700
The B5000 was the first descriptor-based computer. Every descriptor has a flag (bit 0) of 1. Data descriptors contain a 15-bit storage address and a 10-bit size, as do most I/O descriptors. Program descriptors and External Result descriptors have a 15-bit address but no size field.
B5x00 Program Descriptors
Program descriptors are used to define program segments. Either an Operand Call or a Descriptor call that refers to a Program Descriptor will cause a subroutine call if the presence bit is 1, otherwise it will cause a presence interrupt.
Data Descriptors
Data Descriptors refer either to a block of data present in memory (P=1) or to a block of data that must be read into memory (P=0), Depending on the value of the Presence bit. Either an Operand Call or a Descriptor call that refers to a Data Descriptor will check the presence bit and the size field; if the presence bit is 0 then a presence interrupt will occur; if the size field is nonzero then the second word on the stack must be within range or an index interrupt will occur.
I/O Descriptors
External Result Descriptors
An External Result Descriptor contains the I/O Descriptor used to initiate the operation with some fields replaced.
B6500, B7500 and successors
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 B6500. 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 segmented bit Bit 43 – The read only bit
In later implementations these status bits evolved to keep up with growing memory sizes and gained insights.