Integration in Memory Architecture
The address field in the B5000 was only 20 bits, which meant that only 1 Meg words (6MB) of memory could be addressed by descriptors. This was a significant restriction of the architecture. To overcome this, two solutions were implemented:
1. Swapper – this solution actually implemented another layer on top of memory management, moving large clusters of related data in and out of memory at once.
2. ASN – this solution allowed physically more memory to be configured in a system, divided into separately addressable chunks. This architecture became known as ASN (Address Space Number) memory. Memory was logically divided into two areas, allocating low memory addresses to a Global address space for the operating system and support software and high memory addresses to several parallel Local address spaces for individual programs. Address spaces were numbered, zero indicating Global, 1..n indicating the local address spaces. Programs sharing data were automatically placed in the same address space.
No program code modifications were necessary for these features to be utilized. Both solutions could even be combined, but eventually the MCP memory requirements and program data sharing requirements outgrew the maximum size of the address spaces itself.
With the advent of the A Series in the early 1980s, the meaning of this field was changed to contain the address of a master descriptor, which meant that 1 Meg data blocks could be allocated, but that the machine memory could be greatly expanded to gigabytes or perhaps terabytes. This architecture was named ASD (Advanced Segment Descriptors) memory. This required a new common microcode specification, referred to as Beta. The main visionary behind ASD memory is John McClintock. Later the 3-bit memory tag was increased to a 4-bit specification, allowing the segment descriptor to grow from 20 to 23 bits in size, allowing even more memory to be addressed simultaneously. This microcode specification became known as level Gamma.
Read more about this topic: Burroughs Large Systems Descriptors
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