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What is an operating system (OS)?
A program that operates as the interface between the user and the hardware
Web Browser
Accting package
etc.
Compiler Editors Command Window Interpreter system
Operating System - file system, memory manager, etc.
Hardware - CPU, memory, I/O devices
Runs in User Mode
Runs in Kernel or Supervisor Mode
system calls
Goals of OS
- Make computer convenient to use by providing a virtual/extended machine that is easier to use and program than the underlying hardware,
CPU
data
addr.
control
data reg
status/ctrl
I/O logic
I/O Controller
data
status
control
Disk
e.g., writing/reading to file on disk
OS provides high-level system calls so programmer does not need to know details of disk
- Use computer resources/hardware efficiently Resources - processor(s), memory, timers, disks, network
Resources are competed for by all running programs
Examples:
- which programs are loaded in limited memory
- restricts access to memory used by other programs and the operating system
- which program can run on the CPU We can view the OS as resource manager that is responsible for resource allocation, tracking resources, accounting, and mediating conflicting requests
OS manages processes (running programs):
A process is the term for a running program. A process’s state consists of the CPU register values, its run-time stack in memory, and it’s other memory content. Many processes maybe executing concurrently, but only one can be executing on a CPU at a time. When the CPU switches to another process, a context switch occurs which involves saving the complete state of the previously executing process before loading the state of the next process to execute into the CPU. Depending on the hardware, this can take up to 100 microseconds (i.e., very slow in computer terms).
Process State Diagram
new
waiting
I/O request or event wait
I/O completion event signaled
Scheduler Dispatched
Interrupt (CPU timer)
Admitted (short-term) Exit ready running terminated
Queues are used to hold process control blocks (PCB) that represent processes internally to the OS.
Process Control Block
Next PCB in queue pointer
Process State
Program Counter Registers
Memory Mgt. Info CPU Scheduling Info.
Accounting Info.
I/O Status Info
CPU Timer - the operating system sets a count-down timer before turning control over to a user program. If the timer expires, it generates an interrupt a user pgm before the user pgm is started. Remember that only one program (in a single CPU system) can be executing at a time so when the OS turns control over to a user program it has “lost control.” Modifications to the CPU timer are privileged
- Restrict a user program to its allocated address space in memory. In a simple computer, a user program might be allocated a single continguous address space in memory. The two special purpose CPU registers: StartMemory and EndMemory can bracket the user program's address space. All memory addresses that the user program performs can be checked by hardware in the CPU to make sure that they fall between the values in these registers. If the user program tries to access memory outside the range of addresses indicated by these registers, an interrupt/exception is raised to return control back to the operating system. On more complex computers, a memory-management unit (MMU) provides a more sophicated address mapping scheme (paging, segmentation, paged segments, none). Modifications to the memory-management registers are privileged.
CPU
Memory-Mgt Unit
StartMemory
EndMemory
Memory
Block of Memory Allocated the Current Process
- Protection to restrict a process from access files of other programs varies depending on whether the computer is using memory-mapped I/O or instruction-based I/O (see section 4.5 of text). If memory-mapped I/O is being used, the memory address associated with the external device I/O registers are outside of the process accessable memory address space. Thus, our solution (3) above is enough to force a process to request I/O through operating system calls.
If instruction-based I/O instructions array being used. I/O has a separate address space from memory, but we can make these I/O instructions privileged so they can only be executed in system mode. Thus, a process could not execute them directly.
