Computer Architecture part Memory Management Free

1. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
Memory Management
• Uni-program – memory split into two parts
– One for Operating System (monitor)
– One for currently executing program
• Multi-program
– Non-O/S part is sub-divided and shared among active
processes
• Remember segment registers in the 8086
architecture
– Hardware designed to meet needs of O/S
– Base Address = segment address

2. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
Swapping
• Problem: I/O (Printing, Network,
Keyboard, etc.) is so slow compared with
CPU that even in multi-programming
system, CPU can be idle most of the time
• Solutions:
– Increase main memory
• Expensive
• Programmers will eventually use all of this memory
for a single process
– Swapping

3. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
What is Swapping?
• Long term queue of processes stored on disk
• Processes “swapped” in as space becomes available
• As a process completes it is moved out of main
memory
• If none of the processes in memory are ready (i.e. all
I/O blocked)
– Swap out a blocked process to intermediate queue
– Swap in a ready process or a new process
• But swapping is an I/O process!
– It could make the situation worse

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CSCI 4717 – Computer Architecture
Partitioning
• Splitting memory into sections to allocate
to processes (including Operating System)
• Two types
– Fixed-sized partitions
– Variable-sized partitions

5. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
Fixed-Sized Partitions (continued)
• Equal size or Unequal size partitions
• Process is fitted into smallest hole that will
take it (best fit)
• Some wasted memory due to each block
having a hole of unused memory at the
end of its partition
• Leads to variable sized partitions

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CSCI 4717 – Computer Architecture
Fixed-
sized
partitions

7. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
Variable-Sized Partitions
• Allocate exactly the required memory to a process
• This leads to a hole at the end of memory, too small
to use – Only one small hole - less waste
• When all processes are blocked, swap out a
process and bring in another
• New process may be smaller than swapped out
process
• Reloaded process not likely to return to same place
in memory it started in
• Another hole
• Eventually have lots of holes (fragmentation)

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CSCI 4717 – Computer Architecture
Variable-Sized Partitions

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CSCI 4717 – Computer Architecture
Solutions to Holes in Variable-
Sized Partitions
• Coalesce - Join adjacent holes into a
single large hole

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CSCI 4717 – Computer Architecture
Overlays
• In the early days of computing, programmers
had a small amount of memory to squeeze
programs into
• First "stabs" at memory management were
overlays:
– Programmer divided single application into smaller
independent programs called overlays
– When program first loaded, load first overlay into
memory
– When new overlay was required, next overlay is
read from drive and loaded in place of previous one

11. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
Relocation
• No guarantee that process will load into the same
place in memory
• Instructions contain addresses
– Locations of data
– Addresses for instructions (branching)
• Logical address – relative to beginning of program
• Physical address – actual location in memory (this
time)
• Base Address – start of program or block of data
• Automatic conversion using base address

12. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
Paging (continued)
• Split memory into equal sized, small
chunks -page frames
• Split programs (processes) into equal
sized small chunks – pages
• Allocate the required number page frames
to a process
• Operating System maintains list of free
frames
• A process does not require contiguous
page frames

13. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
Paging (continued)
• Use page table to keep track of how the
process is distributed through the pages in
memory
• Now addressing becomes page
number:relative address within page which
is mapped to frame number:relative
address within frame.

14. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
Paging (continued)

15. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
Paging Example – Before
Process A
Page 0
Page 1
Page 2
Page 3
13
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20
21
Free frame list
13
14
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20
In
use
In
use
In
use
In
use

16. Memory Management – Page ‹#› of 49
CSCI 4717 – Computer Architecture
Paging Example – After
Process A
Page 0
Page 1
Page 2
Page 3
Free frame list
20
Process A
page table
13
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18
13
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21
In
use
In
use
In
use
In
use
Page 0
of A
Page 1
of A
Page 2
of A
Page 3
of A