4.8: Segmentation

Up to now, the virtual address space has been contiguous.

Homework: 37.

** Two Segments

Late PDP-10s and TOPS-10

** Three Segments

Traditional (early) Unix shown at right.

  1. Shared text marked execute only.
  2. Data segment (global and static variables).
  3. Stack segment (automatic variables).
  4. (In reality, since the text doesn't grow, this was sometimes treated as 2 segments.)

** Four Segments

Just kidding.


================ Start Lecture #12 ================

** General (not necessarily demand) Segmentation

** Demand Segmentation

Same idea as demand paging applied to segments.

The following table mostly from Tanenbaum compares demand paging with demand segmentation.

Consideration Demand
Paging
Demand
Segmentation
Programmer aware NoYes
How many addr spaces 1Many
VA size > PA size YesYes
Protect individual
procedures separately
NoYes
Accommodate elements
with changing sizes
NoYes
Ease user sharing NoYes
Why invented let the VA size
exceed the PA size
Sharing, Protection,
independent addr spaces

Internal fragmentation YesNo, in principle
External fragmentation NoYes
Placement question NoYes
Replacement question YesYes

** 4.8.2 and 4.8.3: Segmentation With Paging

(Tanenbaum gives two sections to explain the differences between Multics and the Intel Pentium. These notes cover what is common to all segmentation).

Combines both segmentation and paging to get advantages of both at a cost in complexity. This is very common now.

Homework: 38.

4.9: Research on Memory Management

Skipped

4.10: Summary

Read

Some Last Words on Memory Management

Chapter 5: Input/Output

5.1: Principles of I/O Hardware

5.1.1: I/O Devices

5.1.2: Device Controllers

These are the ``devices'' as far as the OS is concerned. That is, the OS code is written with the controller spec in hand not with the device spec.

5.1.3: Memory-Mapped I/O

Think of a disk controller and a read request. The goal is to copy data from the disk to some portion of the central memory. How do we do this?

5.1.4: Direct Memory Access (DMA)

Homework: 12

5.1.5: Interrupts Revisited

Skipped.

5.2: Principles of I/O Software

As with any large software system, good design and layering is important.

5.2.1: Goals of the I/O Software

Device independence

We want to have most of the OS, unaware of the characteristics of the specific devices attached to the system. Indeed we also want the OS to be largely unaware of the CPU type itself.

Due to this device independence, programs are written to read and write generic devices and then at run time specific devices are assigned. Writing to a disk has differences from writing to a terminal, but Unix cp and DOS copy do not see these differences. Indeed, most of the OS, including the file system code, is unaware of whether the device is a floppy or hard disk.

Homework: 5.9

Uniform naming

Recall that we discussed the value of the name space implemented by file systems. There is no dependence between the name of the file and the device on which it is stored. So a file called IAmStoredOnAHardDisk might well be stored on a floppy disk.

Error handling

There are several aspects to error handling including: detection, correction (if possible) and reporting.
  1. Detection should be done as close to where the error occurred as possible before more damage is done (fault containment). This is not trivial.

  2. Correction is sometimes easy, for example ECC memory does this automatically (but the OS wants to know about the error so that it can schedule replacement of the faulty chips before unrecoverable double errors occur).

    Other easy cases include successful retries for failed ethernet transmissions. In this example, while logging is appropriate, it is quite reasonable for no action to be taken.

  3. Error reporting tends to be awful. The trouble is that the error occurs at a low level but by the time it is reported the context is lost. Unix/Linux in particular is horrible in this area.

Creating the illusion of synchronous I/O

Buffering

Sharable vs dedicated devices

For devices like printers and tape drives, only one user at a time is permitted. These are called serially reusable devices, and are studied next chapter. Devices like disks and Ethernet ports can be shared by processes running concurrently.