1. The Operating System Memory Manager
Memory Management
The Operating System Memory Manager
This lecture:
Memory hierarchy
Physical vs logical (virtual) address space
Address binding
Relocation and Protection
Reading: Bacon

2. The Memory Hierarchy OS OS Storage Access Capacity Managed Type Timeby
Registers
100 to 1K
compiler 1 ns
Cache
Memory
CPU
8 to 128M
2-20 ns
hardware
Main
100M
100 ns OS
Memory
to 1G
10M
to 1T
Disk
10 ms OS
Storage
19 November 2018
CS2051 Lecture 9

3. Background
Program must be brought into memory and placed within a process so that it can be run.
Simplest scheme: mono-programming:
Memory is shared only between operating system (resident part) and the program.
If the program was too big? …overlay techniques
Multiprogramming:
simple but not efficient: fixed partitions
more dynamic approaches are needed, which introduce interesting problems!
19 November 2018
CS2051 Lecture 9

4. Multiple processes share memory
Transparency
No process needs to be aware of this
Processes should not care about physical memory allocation
Safety
processes should not be able to write to each other’s space
Efficiency
Two fundamental problems:
Relocation and protection
Also: How to allocate/manage memory? What if memory does not suffice? (swapping)
19 November 2018
CS2051 Lecture 9

5. The Address Space Machine instructions: Address width is:
load (r1)
jump (r3)
Address width is:
32 bits in Pentium
64 bits in Alpha, Itanium, SPARC-V9
Addresses need binding to locations before use
Physical address space vs virtual address space
(recall slide 3 from lecture 2)
19 November 2018
CS2051 Lecture 9

6. Address Binding Requirement
Compiler allocates addresses within module
Linker allocates addresses within program
application
libraries
heap
gap
stack
128K
application
libraries
heap
gap
stack
192K
Program b
Program a
19 November 2018
CS2051 Lecture 9

7. Address Binding in Action
Option (a): Static relocation: fix up addresses when program is loaded
But … once a process is assigned a place in memory the OS cannot move it (unless ???)!
Option (b): Dynamic relocation: use Dynamic Address Translation
Translate program addresses into main memory addresses during execution using H/W
Use different translations for different programs
19 November 2018
CS2051 Lecture 9

8. Dynamic Translation: Datum schemeP1 OS P2 P3 D1 D2 D3 P1
NB: Fragmentation P2 P3
19 November 2018
CS2051 Lecture 9

9. Relocation & Protection H/W
Base Register +
Virtual (or Logical)
Address from CPU
Physical Address to Memory
>?
Limit Register
ERROR
MMU
19 November 2018
CS2051 Lecture 9

10. (Contiguous) Memory Allocation
As processes are created, grow(?), terminate, the OS must keep track of the memory available: P1 OS P2 P3
free
T0: 3 processes
T1: P1, P2 finished
P4 arrived P4 OS
free P3 P4 OS
free P3 P5
T2: P5 arrived
How to allocate memory?
How to deal with fragmentation?
How to reclaim memory?
How to know what memory is available?
19 November 2018
CS2051 Lecture 9

11. (Contiguous) Memory Allocation -2
Memory allocation policies:
First-fit: first hole that is big enough
Best-fit: the smallest hole that is big enough
Worst-fit: the largest hole
Fragmentation:
External (as before - reduce by compaction)
Internal (within a process’s space)
Reclaim memory (read about garbage collection)
More dynamic memory allocation?
19 November 2018
CS2051 Lecture 9

12. More to come: Towards Sharing?OS P2 P3 P1 D1 D2 D3
libc
My linux box has ~50 processes running, each with libc (0.6Mb), libX (0.6Mb), libKDE, etc...
19 November 2018
CS2051 Lecture 9