Today s topics Virtual Environment for Computing Operating Systems

Today’s topics Virtual Environment for Computing Operating Systems Upcoming Program Execution Times (Great Ideas, Chapter 13) Reading Great Ideas, Chapter 10 CPS 001 34. 1

The Problem v The Raw Machine Provides a Hostile Environment q Imagine program in machine language. Machine Language Program for MIPS Machine [00400000] [00400004] argv [00400008] envp [0040000 c] [00400010] [00400014] [00400018] [0040001 c] v 8 fa 40000 lw $4, 0($29) 27 a 50004 addiu $5, $29, 4 ; lw $a 0, 0($sp) ; addiu $a 1, $sp, 4 # 24 a 60004 addiu $6, $5, 4 ; addiu $a 2, $a 1, 4 # 00041080 00 c 23021 0 c 000000 3402000 a 0000000 c sll $2, $4, 2 ; addu $6, $2 ; jal 0 x 00400020 [main]; ori $2, $0, 10 ; syscall ; sll $v 0, $a 0, 2 addu $a 2, $v 0 jal main li $v 0 10 syscall Imagine doing disk I/O directly: q q CPS 001 disk description controlling Heads; timing keeping track of where things are stored dealing with Errors 34. 2

The Problem v Other I/O q q q v Keeping track of memory (RAM) q q v Multiple tasks Multiple users Sharing the CPU q q v Keyboard Screen Communications Multiple tasks Multiple users The User Interface Problem q q q For the computer professional only? For the lay person The Graphical User Interface o Computation to support this? CPS 001 34. 3

Historical Perspective v Early Years q q q v Early 1960's machines: Almost Bare Mid 1960's Machines: Early Batch Operating Systems Multiprogramming Systems Time Sharing Lab Computers Had Major Theme: CPU Time Precious q q q CPS 001 Ease of use: very low priority Graphical User Interface too costly (and not yet invented) This perspective faded with time and began to disappear with advent of the microprocessor: Cheap CPU time. 34. 4

Historical Perspective v Later Years q q v Microprocessor in late 70’s PC's in early 80’s: Operating Systems for PC Apple: Macintosh (Xerox PARC) Workstations o UNIX -- AT&T: License Wars -- LINUX Major Change: Lower Costs q q Whole new audience User Interface o Essential for non pros o Affordable (cpu cycles to burn) q q CPS 001 Whole new competitive environment Volume! 34. 5

Role of the Operating System 1. Processor Management (Multiprogramming) 1. 2. I/O Systems 1. 2. 3. Windowing Systems / GUIs File Systems (use of your hard disk) Communications/Networking Memory (RAM) Management 1. 2. 4. Several virtual machines Sharing Memory Simulating Additional Memory (Virtual Memory) Software Environments q q q CPS 001 Administration/Accounting Compilers Tools 34. 6

Memory Management v Virtual Memory q v Cache Memory q q v Slow and fast memory Library, bookcase, desktop analogies Memory Hierarchies q q v Simulate memory using disks Registers x 1 Cache x 10 - x 100 Main Memory x 100 - x 1000 Disks x 1, 000 Overhead q q CPS 001 Card Catalog analogies Finding stuff on your desk or bookcase 34. 7

Memory Management. 2 v Historically q q v Swapping in Time Sharing Systems Whole user image involved Paging q Page is conveniently sized block of memory (RAM) o (power of 2) q v Physical swapping done page at a time Protection q q q CPS 001 Security (write protect) Confidentiality (read protect) (Early machines) 34. 8

I/O Systems v Communications/Networking q q v Graphical User Interfaces (GUI) q q v Extremely important in modern systems (Dealt with that before) X-Windows Macintosh Desktop MS Windows Xerox PARC; Legal Fun Files Systems q q q CPS 001 Flat Hierarchical (Directories) Distributed Files Systems o Andrew File System (AFS) o Network File System (NFS) 34. 9

Processor Management v Virtual Machines q q v Process Management q q v True Parallel Processes vs. Simulated o Note that the “interleaving” is unpredictable Interrupts o Contrast with “busy waiting” Fairness Responsiveness Synchronization Problems q Danger of shared resources o Data: Race conditions o Any Exclusive Resource: Deadlock CPS 001 34. 10

Synchronization Problems v Race Conditions q q Two Processes (A and B) A manages (updates, etc. ) “clock” B uses “clock” Example: clock at 8: 59 A: add one to minutes – 8: 00 – note carry! B: reads clock 8: 00! q v v A: add carry to hours 9: 00 Due to bad timing, B gets a time almost 1 hour off ! How can we avoid Race Conditions? Deadlocks q User A needs printer P and modem M q User B needs modem M and printer P q Both are competing for same resources q 3 scenarios possible CPS 001 34. 11

Synchronization Problems Scenario 1 Scenario 2 Scenario 3 A: get P A: get M B: get M (wait!) A: process A: release M, P B: get M B: get P B: process B: get M B: get P A: get P (wait!) B: process B: release P, M A: get P A: get M A: process A: get P B: get M A: get M (wait!) B: get P (wait!) …keep waiting… …forever… DEADLOCK! v How can we avoid Deadlocks? CPS 001 34. 12