Segmentation

1
Segmentation

• Motivation so far, memory is 1D with starting
  address at 0 and goes to max address
• Different tables in memory can grow and run into
  each other
• Segmentation
• Multiple variable-size addressable regions
  (called segments)
• Each segment is very large, so filling up segment
  and growing it is rare
• Segments are defined by natural relation
  between code and data separate into code, data,
  and stack segments
• Each segment can have a different protection
  policy

2
Segmentation

• Segmentation
• Segment addressing is visible to programmer
  memory address is segment number plus offset
  within segment. 2D address of (segment , offset)
• Segments may increase and decrease in size
• Address space concept separate from process
  concept
• any process can potentially access any segment
  depending on segment protection policy
• Process does not reside in 1 segment. Many
  processes share segments
• Side effects
• Segments may be evicted, resulting in holes in
  memory external fragmentation
• Need to compact segments

3
Segmentation Advantages

• natural model for protection and sharing
  associated with code and data in segment rather
  than process
• A shared library can be loaded into a segment and
  used by many processes
• Example
• Module A references module B
• When B is recompiled, no need to re-link A and B
  if
• A and B are separately compiled
• Each occupies its own segment
• Table at head of segment indicates where within
  segment each externally-visible name (symbol) can
  be found (called dynamic linking)
• Similar to partitioned memory, except that one
  program can access many segments
• Similar to files, but without need to read/write
  before access

4
Segmentation Table

• Since all processes can access segments, each
  process needs a segment table
• Process address space defined by pairs of base
  and limit addresses (descriptors) and protection
  for each segment
• Multics example
• Process consists of program counter and segment
  table
• Segment table is often a segment itself located
  by hardwares segment table base and limit
  register which is part of process state

5
Sharing Memory

• Problem
• Which processes share memory?
• Which sections of memory are shared and how to
  name them?
• What is the sharing arrangment?
• 1 writer, N readers read-only all read-write
• When and how is sharing arrangement started and
  ended?
• Solutions
• Can have separate Instruction and Data spaces.
  Share Instruction but not Data.
• Copy on write When forking off child process,
  provide pointer to parents data rather than
  copying. Data is now read-only. When child or
  parent writes, then do copy. Save on calling
  copy if write is never issued.

6
Sharing Memory

• Function calls
• shmget get id for shared memory
• shmat attaches shared memory segment associated
  with id to data segment of calling process
• shmctl other control options (locking,
  privileges, removal)
• Locking pages prevents page from being swapped
  out
• Pages mapped to I/O buffer should be locked
• Locking should be done when I/O device is mounted
  and mapped to memory
• mlock (addr, len) locks pages from addr to
  addrlen
• Super-user can lock as many as needed others
  have a limit
• munlock unlocks pages