Operating Systems Lecture Notes

5 March 2012 • Physical Addressing

Outline

Physical storeage and addressing.
Program addressing.
Process storage management.
- Partitioning, overlaying, swapping.
Storage allocation.
- Representation, management, evaluation.

Physical Store

Physical store is intermediate-level storage in a system.
- Between CPU cache and external bulk store.
Moderate size (0.25 to 8 Gbytes), moderate speeds (20 to 30 nsec).
Physical store is contiguous and undifferentiated.
- Some non-contiguous differentiation may be imposed.

Physical Addresses

Physical store modeled as an array.
- The array element is the smallest hardware addressable unit.
- The array-element index is the physical address of the unit.
Additional structure may be imposed.
- Multi-unit elements, such as four-unit integers.
- Modulo-n-aligned address; addresses divisible by n.

Some Code

int find(a[], n, x)
  for i = 0 to n - 1
    if a[i] == x
      return i
  return -a

    movl 0, (iadr)
    jmp  L2
L5: movl (iadr), %eax
    sall 2, %eax
    addl aadr, %eax
    movl (%eax), %eax
    cmpl (xadr), %eax
    jne  L3
    movl (iadr), %eax
    jmp  L4
L3: addl $1, (iadr)
L2: movl (iadr), %eax
    cmpl (nadr), %eax
    jl   L5
    movl -1, %eax
L4:

Addresses

    movl 0, (iadr)
    jmp  L2
L5: movl (iadr), %eax
    sall 2, %eax
    addl aadr, %eax
    movl (%eax), %eax
    cmpl (xadr), %eax
    jne  L3
    movl (iadr), %eax
    jmp  L4
L3: addl $1, (iadr)
L2: movl (iadr), %eax
    cmpl (nadr), %eax
    jl   L5
    movl -1, %eax
L4:

Various values (L2 and iadr for example) have to resolve to numbers representing addresses.

This is known as relocating or binding the program.

Binding Example

example program relocation

    movl 0, (iadr)
    jmp  L2
L5: movl (iadr), %eax
    sall 2, %eax
    addl aadr, %eax
    movl (%eax), %eax
    cmpl (xadr), %eax
    jne  L3
    movl (iadr), %eax
    jmp  L4
L3: addl $1, (iadr)
L2: movl (iadr), %eax
    cmpl (nadr), %eax
    jl   L5
    movl -1, %eax
L4:

Partitioned Allocation

Each process gets a segment of the physical address space.
Segments (or partitions) can be fixed or allocated.
- Fixed segments are relatively static
- Allocated segments are dynamic.
A new process is allocated a segment and relocated into the segment.

Summary

Only the simplest systems use pure physical addressing.
- But the basic ideas, if not the mechanisms, carry over to more advanced systems.

This page last modified on 2011 October 9.