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Operating Systems [CS-604] Lecture No.32
Operating Systems
Lecture No. 32
Reading Material
Chapter 9 of the textbook
Lecture 32 on Virtual TV
Summary
MVT
Paging
Logical to physical address translation
Multiprogramming with Variable Tasks (MVT)
This is the generalization of the fixed partition scheme. It is used primarily in a batch
environment. This scheme of memory management was first introduced in IBM OS/MVT
(multiprogramming with a varying number of tasks). Here are the main characteristics of
MVT.
Both the number and size of the partitions change with time.
Job still has only one segment (as with MFT) but now can be of any size up to the
size of the machine and can change with time.
A single ready list.
Job can move (might be swapped back in a different place).
This is dynamic address translation (during run time).
Must perform an addition on every memory reference (i.e. on every address
translation) to add the start address of the partition.
Eliminates internal fragmentation.
¾ Find a region the exact right size (leave a hole for the remainder).
¾ Not quite true, can't get a piece with 10A755 bytes. Would get say 10A760.
But internal fragmentation is much reduced compared to MFT. Indeed, we say
that internal fragmentation has been eliminated.
Introduces external fragmentation, i.e., holes outside any region.
What do you do if no hole is big enough for the request?
¾ Can compact memory
Transition from bar 3 to bar 4 in diagram below.
This is expensive.
Not suitable for real time systems.
¾ Can swap out one process to bring in another
Bars 5-6 and 6-7 in the following diagram
