Memory Management

Norman WhiteStern School of Business

ProblemHow does operating system handle memory

Not a huge problem for non-multiprogramming OS like DOS All you need to do is separate OS

from Application program

Much bigger problem for a multiprogramming system

Address Space Considerations

Early computers often had 16 bit or 18 bit address spaces (limited by the pointer to a location in memory)As programs became larger, this was a problemAs OS starts to support multiprogramming, memory needs increase Can’t fit all programs into a single address

space.

Some solutions to address space limitations

“SWAP” programs into memory to run Have more “real” memory than the address space can hold. I.e. 16 bit address implies 2**16 locations

(65536) Increase real memory to 256K Use special addressing registers to point to

memory to be used by the current program Means the operating system now has to

move programs around to compact memory

Simple solution – Fixed Partitions

Divide real memory into Fixed Size Chunks (partitions)Run programs in partitions that are large enough for prog to fit.Problems- Much wasted memory Large programs can’t run Very difficult to schedule

Better – relocation and bounds registers

Assume MR0 reg points to the “REAL” memory location of location “0” of a an address space and MR1 points to the last location (or the length)OS can now tell hardware to “run in relocation mode” Every address reference now has contents of MR0

added to it Hardware will not allow an address beyond MR1 Memory mgt (OS) responsible for locating programs

in memory. PROBLEM – What happens when programs finish?

Compaction

As programs finish, memory becomes fragmentedOS needs to move things around to free up contiguous chunks of memory.Program size limited by real memory

More flexible – Memory mapping

Divide memory into fixed size chunks (pages)Assign a register for each page (memory map register)All instructions references use the mapping registers to figure out where the pages is in real memoryAddress references to locations viewed as references to a page and a location with the page (in decimal, location 32,357 is viewed as page 32 displacement 357) (1,000 locations per page)Mapping register 32 points to the “real” memory location of page 32.

Memory mapping example

As instructions are decoded, each address reference is “mapped”, I.e. the page in the reference is used to indicate which memory map register to use to find where the page is in physical memory. (I.e., 32,567 will be changed to 78,567 if map register 32 contains 78)Advantage - No longer need to move programs around to solve the fragmentation problem, just update mapping registersProblem – registers are expensive, as memory needs grow, memory mapping needs to many registers

Solution – Virtual Memory

Memory mapping divided memory into a table Page no Real location 00 78 01 87 … 99 21

Segmentation and paging

Divide address space into segments and pagesNow 32567 becomes

Segment 3 page 25 displacement 67

Have a set of segment tables and page tables (1 page table for each segment) to describe where the physical memory locations are for pages in an address spaceA single register (Segment Table Origin Register, STOR) points to the segment table, entries in the segment table point to the page tableAll address references use the tables to translate addresses.

Segment tables and paging (continued)

Seg number Page table location0 100,0001 80,0002 …3 60,000…9

Page tables -1 per segment

Segment 3 examplePageno Location00 30,00001 10,00002..25 40,00099 78,000

So what happens during address translation

Instruction references location 32567Hardware uses STOR to find address of segment tableRelative offset in segment table points to page table for segment 3 (60,000)Relative offset 25 points to page location for segment 3, page 25So, 32567 reference is translated to 40067

Ouch! That is really complex

Yes, complex and involves lot’s of overheadFirst, every address reference now generate 2 extra memory references (to segment table and page table)Will run verrrry slowly…Second, lot’s of table maintenance

But look what we can do!

Every address space is completely defined by a segment table and page tablesChanging an address space is accomplished by pointing STOR to a new segment table.Easy to add on virtual memory!

Virtual Memory

Allow programs to run with some of their pages missing.Hardware generates an interrupt if page table entry is missing (add column(s)to page table for missing entry)External page table used to locate page on diskPage paged in, and instruction re-executed

Ok, that’s cool but isn’t it very slow?

Need some tricks to make it go fastAdd some extra memory logic to help translation (translation look-aside buffer, TLB)TLB is very high speed associative memory that looks up recently referenced segno,pageno combinations and replaces them with their real locations. If the the info is in TLB, segment table and page table not necessary.TLB has a limited number of entries. Only recent references remembered.

So what happens?

Instruction references are translated in the processor with minor overheadIf TLB is fast and large overhead is minimal (< 1% typically)But OS is much more complex due to interaction between memory management and processor management

Virtual Memory Advantages

Can now run programs larger than physical memory (but still limited by address space)Performance is limited by amount of real memory (thrashing if REAL < WORKSET)OS can run in separate address space, better for securityFile I/O can be combined with virtual memory (connect an address space to a segment and page table). File can be manipulated as if it were in memoryProgrammers (usually) do not need to know size of real memory.

Conclusion

Virtual Memory is now the standard on almost all systems (except cell phones and PDAs)