Memory Management

1. Memory Management • 1. Single contiguous allocation • 2. Partitioned organization: • Static, Dynamic • 3. (Pure) Paging

2. Memory Management Memory management developed over time (Historical Overview) • 1. Single Contiguous Allocation • Early, uni-programmed machines • Memory divided into 3 areas: OS (32K) User Job (64K) Memory (256K) Wasted (160K)

3. Single Contiguous Allocation • Hardware : Bounds register, supervisor mode • Software : User job available memory? • If so, allocate and run to completion • If not, error • Advantage : simplicity • Disadvantages : poor M, P utilitization • job must fit into M

4. Memory Management • 2. Partitioned Organization • Early multiprogramming environment • Memory divided into partitions. Each holds 1 job. • E.g.

5. Partitioned Organization • Software : • 1. Keep track of status of partitions • Size, Free or Allocated • 2. Which job gets M [Job Scheduler] • 3. Alloc : Free partition of sufficient size assigned • 4. Dealloc : Freed when job terminates • Hardware : • 2 bounds registers to bracket partition • Locks and keys [IBM] • Each partition has own protection key • When processor assigned, “key” in PSW = Key of partition

6. Partitioned Organization • Two types of partitions : • Static Partitions • Memory divided into partitions according to OS or Operator specification • Each job specifies max memory req’d; memory management assigns partition of sufficient size • Appropriate if : size, frequency of jobs known • OW : wasteful of M • Estimated 75% wasted

7. Partitioned Organization • Two types : • Dynamic Partitions • Created during job processing to match partition to job size • Need tables for allocating, deallocating partitions : Size, Location, Status. • E.g.

9. Partitioned Organization • Dynamic Partitions • Jobs terminate, new partitions created from freed areas, subject to : • 1. Free area  Job size • 2. If >, fragmentation • 3. On deallocation, merge adjacent free areas • Implementation : • Table size varies  • Append info to each partition and chain • Lists of free, alloc partitions

10. Memory Management • 3. (Simple or Pure) Paging • Each job’s address space  Equal pieces  frames • Memory  pieces of same size  Frames (blocks) • User : • Sees logically continuous address space • But pages loaded into frames which are not contiguous • HW maps any page into any frame • Mapping : PMT (Separate registers, 1/Page) • Page size is critical (1-4K, in powers of 2) • Solves fragmentation problem without moving partitions

11. (Pure paging)

12. Paging • Hardware : • 1 register/page needed for memory mapping • High speed registers or memory locations (cache) • Dynamic Address Translation (DAT) • Divides EA into : 0 …. 7 8 …. 19 20 …. 31 Page number Byte offset

13. Paging • Consider IBM example : • Page Size = 212 or 4096 bytes • Effective address (EA) = 24 bits • DAT : Divides EA into page index, byte displacement • PMT : Maps page number to frame number •  Physical address of page • Add byte offset Address of operand • Software : Need 3 tables • 1 PMT/job, with 1 entry per page • A memory block (frame) table (MBT), with status info for each frame (if used, by whom) • A job table (JT), with 1 entry/job (Size, status, pointer to PMT)

18. Paging • Advantages : • Facilitates multiprogramming by eliminating fragmentation (more jobs in memory, no compacting) • Facilitates sharing of info (memory) among jobs by incorporating pages in more than 1 job’s address space

19. Paging • Disadvantages : • Page address mapping - HW costly, time-consuming • Tables : Memory required for PMTs, time to update • Internal fragmentation (page breakage) • Half page/job wasted on average • Tradeoff : • Small pages (min page wastage) vs • Large pages (reducing # entries in PMT) • Know average job size • Some memory may not be used (if #frames < address space) • Memory still contains infrequently used info • Job size must be  Memory size