Q. Segmentation is alike to paging but uses variable-sized "pages". Describe two segment-replacement algorithms based on FIFO and LRU page replacement schemes. Remember that since segments aren't the same size the segment that is chosen to be replaced mayn't be big enough to leave enough consecutive locations for the needed segment. Consider tactics for systems where segments cannot be relocated and those for systems where they can.

Answer:

a. FIFO. Find the first segment large sufficient to accommodate the incoming segment. If relocation isn't possible and no one segment is large enough select a combination of segments whose memories are contiguous which are closest to the first of the list and which are able to accommodate the new segment. If relocation is probable rearrange the memory so that the first N segments large sufficient for the incoming segment are contiguous in memory. Add any leftover space to the free-space list in both cases.

b. LRU. Select the segment that hasn't been used for the longest period of time and that is large enough adding any leftover space to the free space list. If no one segment is large sufficient select a combination of the oldest segments that are contiguous in memory (if relocation isn't available) as well as that are large enough.

If relocation is obtainable rearrange the oldest N segments to be contiguous in memory as well as replace those with the new segment.