Secure design 1565

Anne & Lynn Wheeler ... snip ...

I had a system which buttigned code blocks in segments, and the segment count could not exceed 127 (usually no more than 31, this was in an 8080 system). A used segment was marked with a bit, as usual. However, whenever an intersegment transfer occurred (and there was insufficient free space, and the segment was not present) all 'used' fields were shifted right in a field of 5 or 6 bits. The resultant binary number was used to select the segment to discard. In practice this seemed to do a good job of uncovering a suitable working set. In particular I could watch it get rid of initialization code very quickly.

When a segment was discarded the space was also compacted, which meant frequently used segments tended to migrate to the head and no longer needed moving, while rarely used segments tended to be at the tail and adjacent to any free space, thus minimizing the compaction overhead.

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