Memory Management (Ch. 8)

Compilers generate instructions that make memory references.

Code and data must be stored in the memory locations that the instructions reference.

How does the compiler know where the code and data will be stored?

When code and data are stored, what if the memory locations the instructions reference are not available?

Background Absolute code:

Compiler knows where code will reside in memory.

Creates memory references in instructions based on where code will reside.

Also called Compile time binding. Give assembly-type example showing some

load/add instructions.

Relocatable code: binding of code to real memory locations done at

load time. Also called load time binding.

Execution time binding: binding of code to real memory during execution.

Allows load modules to be moved.

Physical address: Actual memory location in the memory chip that

is referenced. Logical address (sometime called virtual

address): Memory location specified in an instruction.

May not be the same.

Base register approach. See figures 8.1-8.4: physical address = logical address + contents of

base register. Used in IBM 360/370 architectures – a milestone

in computer architecture.

Set of addresses defines the logical address space or the physical address space.

Assumes entire process loaded into memoryconstraints on process size.

Dynamic loading: code sections loaded into memory as needed.

Static linking: object code and libraries combined into one executable to create a large file.

Dynamic linking: Executable contains a stub (small piece of code that locates the real library) for each library routine.

Overlays: Sections of code that occupy the same memory but never at the same time. Discuss using hierarchical structures. Used in older systems

Swapping: All or parts of processes are moved between

memory and disk. Moved into memory when needed Moved to disk when not needed

Backing store: Temporary area (usually disk) for a running

process’ code. Fig. 8.5

CPU Scheduler calls a dispatcher which checks to see if process code is in memory.

If not, swap in and replace another process if necessary.

See example equation for context switch time on page 315.

Contiguous memory allocation Memory protection:

Logical address < contents of a limit register; physical address=logical address + relocation

register. Figure 8.6.

Fixed partitions: Memory divided into fixed length partitions. Process code stored into a partition. Internal fragmentation (unused part of a

partition). Finding the best partition: first fit, best fit,

Variable partitions: OS creates partitions as processes are loaded. External fragmentation: many small unused

partitions compacting: moving all free partitions to adjacent

areas of memory. Coalescing: combing adjacent partitions. More overhead as OS creates, compacts, and

coalesces partitions.

Paging Prior schemes try to configure memory to fit a

process. Why not try to configure a process to fit

memory? Code divided into pages. Memory divided into frames.

Logical address: (p, offset). P is the page number; offset is the relative byte

position on that page Page table: table that associates a frame with each

page p (page number) indexes a page table to get a

frame number, f. Physical address is (f, offset).

Figure 8.7-8.10. Need a page table base register to locate the

page table. Offset is n bits if the page size is 2n

.

Pages need not be stored in contiguous memory.

NO external fragmentation; internal fragmentation possible.

OS maintains a free-frame list/frame table. Size of memory= number of frames * size of

frame (anywhere from ½ K to about 8K.

Translation look-aside buffer (TLB) Page table access would mean double

memory references. TLBs: hardware registers that associate a

page # with a frame #. Instruction cycle checks TLB first; if page not

found, then access the page table. Figure 8.11

Spatial locality: If a memory location is referenced then it is likely

that nearby locations will be referenced. Temporal locality:

If a memory location is referenced then it is likely it will be referenced again.

Means that TLB searches will be successful most of the time.

Hit ratio (H): Frequency with which page# found in TLB.

Effective memory-access time: H*(time to access TLB and memory) +

(1-H)*(time to access page table and memory). See book for numbers (p. 326)

NOTE: OS may run in unmapped mode, making it quicker.

TLB reach: metric defined by amount of memory accessible through the TLB

(# entries*page_size).

Protection bits Bits associated with each frame

stored in page table. Control type of access (read, write, execute,

none). Discuss (not in book).

Valid/Invalid bit. Bit in page table indicating whether page

referenced is valid (in memory). Figure 8.12

tradeoffs: small page size: less fragmentation but more

pages and large page tables. large page size: the reverse.

Shared memory Two (or more) process’ virtual memory can

map to the same real memory. Common examples of sharing code

(compilers, editors, utilities). Example: if everyone is using vi, could use a

lot of memory if vi not shared. Figure 8.13. Program demos show how to do this. (later).

Linux Shared memory demos Shared memory commands: see ftok(),

shmget(), shmat(), shmdt(), and shmctl() in the Linux list of command provided via the course web site.

Compile getout1.c and call it getout. Compile putin1.c and run.

The putin program creates a shared memory segment and writes to it. It also forks and execs the getout program.

The getout program displays the constents of the shared memory segment.

Remove the comments in putin’s loop that writes to the shared memory segment.

Run the program several times. Comment out the shmctl command at the end

of the putout program. What happens?

Bash shell command: ipcs – reports on interprocess communication facilities.

Bash shell command: ipcrm – can remove one of the IPC facilities that may be “orphaned”.

The shell script removeshm makes this easier to do.

Programs putin2.c and getout2.c are an attempt at synchronizing multiple instances of writing to and reading from shared memory.

Sometimes it works!!

Storing (structuring) the page table Page table could have a million entries. Can’t

store contiguously. Two level paging

p1: identifies an outer page table entry, which locates an inner page table

p2: identifies an inner page table entry, which locates the frame number

Used in some Pentium machines; 12-bit offset 2K-byte page size. 10 bits for each of p1 and p2.

Sometimes called forward-mapped page table.

Figs 8.14-15.

VAX architecture 512-byte page sizes. Outer page actually

called a section.

other examples: Use a 3-level hierarchy (SPARC) architecture

or even 4-level (Motorola 68030).

Hashed page table Virtual page number acts as a hash value, not

an index. Hash value identifies a list of (p#, f#) pairs. Fig. 8.16.

Inverted page table Previous approaches assume one page table

for each process and one entry for each virtual page. Over many processes, this could require many entries.

inverted page table: one entry for each frame. Each entry contains a virtual page # and process

ID of the process that has that page. Fig 8.17

Used by PowerPC and 64-bit UltraSPARC. Virtual address looks like

Look for virtual address in TLB. If not found, search (actually use a hash on

(pid, p#) ) inverted page table looking for (pid, p#).

If found, extract f#.

Segmentation Paging requires a view that is simply a

sequence of pages. Segmentation allows a more flexible view:

data segment, stack segment, code segment, heap segment, standard C library segment

Segments vary in size.

Pure segmentation works much like paging (use segment table, not page table and segment #, not page #) except segment sizes vary. Segment table entry contains segment address and its size.

Diagrams are similar. See book. Figs 8.19-8.20

Segmentation/paging Can protect many pages the same way using

protection bits on a segment table entry. Facilitates security – one entry protects an entire segment

Common on Intel chips. Book describes 80x86 address translation but

I won’t cover.

reference [http://en.wikipedia.org/wiki/Virtual_memory#Windows_example]

Pentium and Linux on Pentium Systems (Sections 8.7.2-8.7.3)