Chapter 9: Memory Management

1
Chapter 9 Memory Management

• Background
• Contiguous Memory Allocation
• Paging

2
Binding of Instructions and Data to Memory
Binding of instructions and data to memory
addresses canhappen at three different stages

• Compile time If memory location of process is
  known a priori, absolute code can be generated.
• Must recompile code if starting location changes
• Load time if memory location of process is not
  known at compile time, compiler must generate
  relocatable code.
• Must reload code if starting location changes
• Execution time if the process can be moved from
  one memory segment to another during its
  execution, binding must be delayed until run
  time.
• Need special hardware support
• Use by most modern OS

3
Logical- vs. Physical-Address Space

• Logical address address generated by the CPU.
• Physical address address seen by the memory
  unit.
• Logical-address space the set of all logical
  addresses generated by a program.
• Physical-address space the set of all physical
  addresses corresponding to the logical addresses
• Logical and physical addresses are the same in
  compile-time and load-time address-binding
  schemes
• Logical and physical addresses differ in
  execution-time address-binding scheme.
• Memory Management Unit (MMU) hardware device
  that performs run-time mapping from logical to
  physical addresses.

4
Contiguous Memory Allocation

• A process physical memory is contiguous
• Memory protection
• limit register contains the range of logical
  addresses each logical address must be less
  than the limit register.
• Logical addresses in range 0..limit-1
• MMU maps logical address to physical address by
  adding the value in the relocation register to
  the logical address
• relocation register contains the value of the
  smallest physical address
• Physical addresses in range relocation..relocatio
  nlimit-1
• During context switch, dispatcher loads
  relocation/limit registers from PCB

5
Hardware Support for Relocation and Limit
Registers
6
Memory Allocation

• Hole a block of available memory
• When a process arrives, it is allocated memory
  from a hole large enough to accommodate it
• Allocate only as much memory as is needed
• When a process terminates, its block of memory is
  returned to the set of holes
• Merge adjacent holes when possible

OS
OS
OS
OS
OS
process 5
process 5
process 5
process 5
process 10
process 10
process 6
process 6
process 8
process 8
process 8
process 8
process 8
process 2
7
Dynamic Storage-Allocation Problem

• How to satisfy a request of size n from a list of
  free holes?
• First-fit Allocate the first hole that is big
  enough.
• Best-fit Allocate the smallest hole that is big
  enough
• Must search entire list, unless list is ordered
  by size.
• Produces the smallest leftover hole.
• Worst-fit Allocate the largest hole
• Must also search entire list, unless list is
  ordered by size.
• Produces the largest leftover hole.
• Comparisons
• First-fit usually fastest
• First-fit and best-fit better than worst-fit in
  terms of memory utilization

8
Fragmentation

• External Fragmentation enough total memory
  space exists to satisfy a request, but it is not
  contiguous.
• First-fit 1/3 of memory may be unusable
• Internal Fragmentation allocated memory may be
  slightly larger than requested memory.
• Solutions to external fragmentation
• Compaction shuffle memory contents to place all
  free memory together in one large block.
• Possible only with execution-time address binding
• Paging allow the physical-address space of a
  process to be noncontiguous