Chapter 8 Operating System Support Focus on Architecture

Chapter 8 Operating System Support • Focus on Architecture

Functions of Operating System • Managing Resources Access to System Utilities Access to files Access to I/O devices Managing Interrupts & Bus Control Error detection and response Accounting • Scheduling Processes (or tasks) Program creation Program execution I/O Processing • Managing Memory Utilization Partitioning, Paging, Virtual memory, Segmentation

Types of Operating Systems • Interactive • Batch • Uni-tasking • Multi-tasking • Real-Time

Batch Operating System Model • Jobs are batched in queue • Monitor handles scheduling • Monitor controls sequence of events to process batch • When one job is finished, control returns to Monitor which loads next job

Desirable Hardware Support for OS • Memory protection — To protect the Monitor & Utilities • Timer — To prevent a job monopolizing the system • Privileged instructions — Only executed by Monitor — e. g. I/O, files • Interrupts — Allows for relinquishing and regaining control • DMA — Allows for optimizing bus usage

The Case for Multi-programmed Batch Systems • I/O devices are very slow Waiting is inefficient use of computer • When one program is waiting for I/O, another can use the CPU

Multi-Programming with Three Programs

Utilization: Uni-programmed vs Multi-programmed

Multiprogramming Resource Utilization

Types of Scheduling

A Five State Process Model

Process Control Block Layout

Scheduling Time Sequence Example:

Key Elements of O/S

Process Scheduling:

Memory Management • Uni-programming —Memory split into two —One for Operating System (monitor) —One for currently executing program • Multi-programming —“User” part is sub-divided and shared among active processes • Note: Memory size implications - 16 bits 64 K memory addresses - 24 bits 16 M memory addresses - 32 bits 4 G memory addresses

Swapping • Problem: I/O is so slow compared with CPU that even in multi-programming system, CPU can be idle most of the time • Solutions: — Increase amount of main memory – Expensive — Swapping

What is Swapping? • Long term queue of processes stored on disk • Processes “swapped” in as space becomes available • As a process completes it is moved out of main memory • If none of the processes in memory are ready (i. e. all I/O blocked) — Swap out a blocked process to intermediate queue — Swap in a ready process or a new process But swapping is an I/O process… Isn’t I/O slow? So why does swapping make sense ?

Implementation of Swapping

Partitioning • Partitioning: May not be equal size: Splitting memory into sections to allocate to processes (including Operating System!) • Fixed-sized partitions —Potentially a lot of wasted memory • Variable-sized partitions —Process is stored into smallest reasonable “hole” • Dynamic partitions — no room for additional memory allocation — memory leak – need periodic coalescing, or – need periodic compaction

Fixed Partitioning

Effect of Dynamic Partitioning – memory leaks

Relocation Challenges • Can’t expect that process will load into the same place in memory as last time • Instructions contain addresses —Locations of data —Addresses for instructions (branching) – Logical address - relative to beginning of program – Physical address - actual location in memory (this time) A Solution: • Use Base Address • Automatic (hardware) Conversion

Paging • Split memory into equal sized, small chunks - page frames • Operating System maintains list of free frames Then: • Split programs (processes) into equal sized small chunks – pages • Allocate the required number page frames to a process - A process does not require contiguous page frames - Each process has its own page table

Allocation of Free Frames

Paging - Logical and Physical Addresses

Paging Implementations • Demand paging —Do not require all pages of a process in memory —Bring in pages as required • Page fault —Required page is not in memory —Operating System must swap in required page —May need to swap out a page to make space —Perhaps select page to throw out based on recent history

Thrashing • Too many processes in too little memory • Operating System spends all its time swapping • Little or no real work is done • Solutions —Good page replacement algorithms —Reduce number of processes running —Add more memory

Virtual Memory • We do not need all of a process in memory for it to run - We can swap in pages as required • So - we can now run processes that are bigger than total memory available! Differentialtions: • Main memory is called real memory • User/programmer can see much bigger memory space - virtual memory Implications: • Tables can become huge that can’t fit into memory – need multiple level tables – yech!

Alternate Inverted Page Table Structure

Translation Lookaside Buffer • Every virtual memory reference causes two physical memory access —Fetch page table entry —Fetch data • Use special cache for page table(s)

TLB and Cache Operation (special Cache for tables)

Segmentation What is it? • Segmentation is visible to the programmer - Paging is not (usually) visible to the programmer • Usually different segments allocated to program and data • May be a number of program and data segments, e. g. to support protection levels, priority levels, organization, flexibility, etc.

Advantages of Segmentation • Simplifies handling of growing data structures • Allows programs to be altered and recompiled independently, without relinking and re-loading • Lends itself to sharing among processes • Lends itself to protection Some systems combine segmentation with paging

OS Review Scheduling: uni-programming multi-programming time-sharing long-term scheduler (queue of all jobs potentially schedulable) short-term scheduler (queue of processes that are ready to execute) medium-term scheduling (queue of jobs that can reside in memory) blocked monitoring (queue of processes blocked for resources) new – ready – running – blocked – exit state machine Memory management: partitioning paging – frames, page fault, page table, logical/physical addr virtual memory – inverted page table, Translation Lookaside Buffer segmentation

Pentium and Power PC • Pentium – Intel • Power PC – Motorola & IBM A 32 bit memory address space is 4 G Bytes A 46 bit memory address space is 64 T Bytes • • • 1. 25 terabytes has been claimed as the capacity of a human being's functional memory (according to Raymond Kurzweil). A Holographic Versatile Disc (HVD) can hold up to 3. 9 terabytes. One hour of uncompressed Ultra High Definition Video (UHDV) consumes approximately 11. 5 terabytes of data. The U. S. Library of Congress has claimed that "as of December 31, 2005, the Library has collected more than 40 terabytes of data. " A Protein-coated disc (PCD) can hold 50 terabytes of data. A 64 bit memory address space is ? (Who cares!) The point is that it is not clear to me why we care for some at least some years to come.

Pentium II (Uses hardware for segmentation & paging) • Unsegmented, unpaged — virtual address = physical address — Used in Low complexity, High performance systems • Unsegmented, paged — Memory viewed as paged linear address space — Protection and management via paging (Ex: Berkeley UNIX) • Segmented, unpaged — Collection of local address spaces — Protection to single byte level, Translation table needed is on chip when segment is in memory, provide predictable access times • Segmented, paged — Segmentation used to define logical memory partitions subject to access control — Paging manages allocation of memory within partitions (Ex: Unix System V)

Pentium II Address Translation Mechanism “Segment” uses 2 bits to provide 4 levels of protection, typically: • 0: OS kernel, 1: OS, 2: apps needing special security, 3: general apps

Pentium II Paging • Segmentation may be disabled —In which case linear address space is used • Two level page table lookup —First, page directory – 1024 entries max – Splits 4 G linear memory into 1024 page groups of 4 Mbyte – Each page table has 1024 entries corresponding to 4 Kbyte pages – Can use one page directory for all processes, one per process or mixture – Page directory for current process always in memory —Use TLB holding 32 page table entries —Two page sizes available 4 k or 4 M

Power. PC 32 -bit Address Translation

Power. PC Memory Management Hardware • 32 bit – paging with simple segmentation — or 64 bit paging with more powerful segmentation • Or, both do block address translation —Map 4 large blocks of instructions & 4 of memory to bypass paging —e. g. OS tables or graphics frame buffers • 32 bit effective address — 12 bit byte selector – =4 kbyte pages — 16 bit page id – 64 k pages per segment — 4 bits indicate one of 16 segment registers – Segment registers under OS control

Power. PC 32 -bit Memory Management Formats