Chapter Three -- Operating System Structures -- Lecture Ideas

• For a personal computer user, what are the advantages of multiprogramming?
  
  
• An OS can be characterized by the services it offers, or the interface, or by its components and their interconnections.
  
  
• We can think of an OS as an entity that allocates resources to processes (and deallocates).
  
  
• How does Unix accomplish these aspects of process control?
  
  
  • process creation and destruction?
    
    
  • suspension/resumption
    
    
  • synchronization
    
    
  • communication
    
    
  • deadlock handling
    
    
• do a "ps" on a Unix system and discuss interesting processes that are running (probably check processes that are vital to the running system first, then other processes owned by root.).
  
  
• Discuss generally how the OS could do the following tasks connected with memory management:
  
  
  • Keep track of what parts of memory are allocated to which processes
    
    
  • Make the decision about which process should next be brought into primary memory and be made runnable.
    
    
• Discuss generally how the OS could do the following tasks connected with file management:
  
  
  • create/delete files
    
    
  • create delete directories
    
    
  • support file access primitives
    
    
  • map files to secondary storage
    
    
  • back-up files in RAM onto stable storage
    
    
• Discuss generally how the OS could do the following tasks connected with disk management:
  
  
  • free-space management
    
    
  • storage allocation
    
    
  • disk scheduling
    
    
  (Management of this bottleneck is critical. One of the big developments of Unix was the re-design of the file system. It greatly improved throughput.)
  
  
• Discuss these basic OS services:
  
  
  • program execution
    
    
  • I/O operations
    
    
  • File-System manipulation
    
    
  • Communication
    
    
  • Error detection
    
    
  • Resource allocation -- something like a request/release paradigm is necessary in a multiprogramming environment.
    
    
  • Accounting
    
    
  • Protection
    
    
• Security Defined: any mechanism for controlling who is allowed entry to the system.
  
  
• Protection Defined: any mechanism for controlling the accesses to resources that are made by users that have cleared security.
  
  
• System Calls
  
  
  • the operating system services mentioned above are furnished by means of OS system calls.
    
    
  • If we think of the OS as a data structure, then system calls are the operations of the data structure.
    
    
  • The "system call instruction" generates a trap (interrupt).
    
    
  • System call parameters may be left in registers, RAM, or program stack. Registers can hold pointers to areas in RAM containing parameters.
    
    
  • Look on page 66 at the list of system call types. Do we understand what all these operations do?
    
    
• Discuss "single-tasking approach" of DOS versus "multi-tasking" approach of Unix.
  
  
• Any questions about the "message passing" and "shared memory" communication paradigms?
  
  
• Discussion of shells? -- internal versus external commands.
  
  
• Operating System Structuring
  
  
  • Some OS's are not very modular
    
    
  • Some are built in layers, but the layers can "bypass" each other.
    
    
  • Some are more strictly layered -- bypassing is not common. Each layer depends on the one underneath.
    
    
  • layered design facilitates testing.
    
    
  • It's not easy to design layers without any "circular" dependencies -- (e.g. backing store driver depends on CPU-scheduler and vice-versa.)
    
    
  • Microkernel design puts just a few essential services (say process & memory management plus process communication) in the kernel and the rest is pushed to "user-level"
    
    
  • With microkernel architecture it becomes fairly easy (if not maximally efficient) to emulate many different operating systems "on top of" the microkernel. The idea is to interpret foreign system calls with microkernel system calls.
    
    
• The virtual machine idea is to implement in software "an interface that is identical to the underlying bare hardware." -- nice way to isolate users -- secure -- each user can run a different operating system. A user can experiment on a new operating system design without risking bringing down the part of the system being used by others. The Java virtual machine is a modern instance of this type of design. The IBM version dates back to the late sixties.
  
  
• OS Implementation Principles
  
  
  • Separate policy from mechanism.
    
    
  • Write most of the OS in a high level language. (one writes code faster; code is more compact; code is easier to understand and debug; the operating system is "portable." -- There is really no downside -- modern compilers generate efficient assembly code. Increases in efficiency of OS usually comes by using better data structures and algorithms -- we can write a few small pieces of the OS in assembly where that is really needed for efficiency or for compatability with the hardware.)
    
    
• System generation -- setting compile parameters for the OS versus table driven systems that are pre-compiled. The trend is toward pre-compiling.