CIT 118 - Chapter 3 - Memory Management Includes Virtual Memory

Operating Systems

Evolution of memory management

• Paged, demand paging, segmented, segmented/demand paging

• Foundation of current virtual memory method

Areas of improvement from the need for

• Continuous program storage

• Placement of the entire program in memory during execution

Incoming job

divided into pages of equal size

Internal fragmentation

job’s last page frame only

Entire program

required in memory during its execution

Three tables for tracking pages:

Job Table (JT), Page Map Table (PMT), and Memory Map Table (MMT)

• Stored in main memory: operating system area

Job Table: information for each active job

• Job size

• Memory Location: job’s PMT

Page Map Table: information of each page

• Page number: beginning with page 0

• Memory address

Memory Map Table: entry for each page frame

• Location

• Free/busy status

Displacement (offset)

o Distance: from beginning of its page
o Location: within its page frame
o Relative value

Determining page number and displacement

o Divide byte location of the data we want to retrieve by the page size
o Page number: integer quotient
o Displacement: remainder

Two-dimensional addressing scheme

o Segment number and displacement

• Disadvantage: External fragmentation

Major difference between paging and segmentation

o Pages: physical units; invisible to the program
o Segments: logical units; visible to the program; variable sizes

Subdivides segments: equal-sized pages

• Smaller than most segments

• More easily manipulated than whole segments

• Segmentation’s logical benefits

• Paging’s physical benefits

Segmentation problems removed

o Compaction, external fragmentation, secondary storage handling

Three-dimensional addressing scheme

o Segment number, page number (within segment), and displacement (within page)