Four Fundamental Operating System Concepts Sam Kumar CS 162: Operating Systems and System Programming Lecture 2 https://inst.eecs.berkeley.edu/~cs162/su20 Read: A&D, 2.1-7 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 1 Recall: What is an Operating System? • Special layer of software that provides application software access to hardware resources • Convenient abstraction of complex hardware devices • Protected access to shared resources • Security and authentication appln appln • Communication appln OS Hardware 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 2 Recall: OS Abstracts the Underlying Hardware Compiled Program System Libs Process: Execution environment with restricted rights provided by OS Threads Address Spaces Files Sockets Operating System ISA Networks Processor PgTbl Memory & TLB Storage Hardware OS Mem I/O Ctrlr 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 3 Recall: OS Protects Processes and the Kernel Segmentation fault Compiled (core dumped) Compiled Program 1 Program 2 System Libs System Libs Process 1 Process 2 Compiler Threads Address Spaces Files Sockets Threads Address Spaces Files Sockets Operating System ISA Networks Processor PgTbl Memory & TLB Storage Hardware OS Mem I/O Ctrlr 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 4 Recall: What is an Operating System? • Referee • Manage protection, isolation, and sharing of resources • Resource allocation and communication • Illusionist • Provide clean, easy-to-use abstractions of physical resources • Infinite memory, dedicated machine • Higher level objects: files, users, messages • Masking limitations, virtualization • Glue • Common services • Storage, Window system, Networking • Sharing, Authorization • Look and feel 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 5 Today: Four Fundamental OS Concepts • Thread: Execution Context • Program Counter, Registers, Execution Flags, Stack • Address Space (with Translation) • Program’s view of memory is distinct from physical machine • Process: Instance of a Running Program • Address space + one or more threads + … • Dual-Mode Operation and Protection • Only the “system” can access certain resources • Combined with translation, isolates programs from each other 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 6 OS Bottom Line: Run Programs 0xFFF… Program Source Executable OS int main() { … ; Compiler Memory Editor } data stack and Linker OS Loader instructions heap foo.c a.out data • Create OS “PCB”, address space, stack and heap instructions • Load instruction and data segments of executable file into memory 0x000… • “Transfer control to program” PC: • Provide services to program registers • While protecting OS and program Processor 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 7 OS Bottom Line: Run Programs 0xFFF… Program Source Executable OS int main() { … ; Compiler Memory Editor } data stack and Linker OS Loader instructions heap foo.c a.out data • Create OS “PCB”, address space, stack and heap instructions • Load instruction and data segments of executable file into memory Creates a process 0x000… • “Transfer control to program” from a program PC: • Provide services to program registers • While protecting OS and program Processor 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 8 Review (61C): How Programs Execute Processor next Memory PC: Instruction fetch instruction Decode decode Registers Execute ALU data 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 9 Review (61C): How Programs Execute Addr 232-1 • Execution sequence: … • Fetch Instruction at PC Data1 • Decode R0 Data0 • Execute (possibly using registers) … Inst237 • Write results to registers/mem R31 Fetch Inst236 F0 • PC = Next Instruction(PC) Exec … … • Repeat F30 Inst4 PC Inst3 PC Inst2 PC Inst1 PC Inst0 PC Addr 0 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 10 Today: Four Fundamental OS Concepts • Thread: Execution Context • Program Counter, Registers, Execution Flags, Stack • Address Space (with Translation) • Program’s view of memory is distinct from physical machine • Process: Instance of a Running Program • Address space + one or more threads + … • Dual-Mode Operation and Protection • Only the “system” can access certain resources • Combined with translation, isolates programs from each other 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 11 Key OS Concept: Thread • Definition: A single, unique execution context • Program counter, registers, stack • A thread is the OS abstraction for a CPU core • A “virtual CPU” of sorts • Registers hold the root state of the thread: • Including program counter – pointer to the currently executing instruction • The rest is “in memory” • Registers point to thread state in memory: • Stack pointer to the top of the thread’s (own) stack 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 12 Illusion of Multiple Processors vCPU1 vCPU2 vCPU3 • Threads are virtual cores • Multiple threads: Multiplex hardware in time • A thread is executing on a processor when it Shared Memory is resident in that processor's registers On a single physical CPU: • Each virtual core (thread) has PC, SP, Registers • vCPU1 vCPU2 vCPU3 vCPU1 vCPU2 Where is it? • On the real (physical) core, or Time • Saved in memory – called the Thread Control Block (TCB) 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 13 OS Object Representing a Thread • Traditional term: Thread Control Block (TCB) • Holds contents of registers when thread is not running… • … And other information the kernel needs to keep track of the thread and its state. 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 14 Registers: RISC-V → x86 Load/Store Arch with software conventions Complex mem-mem arch with specialized registers and “segments” • In CS 61C you learned RISC-V • In section tomorrow you’ll learn x86 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 15 Illusion of Multiple Processors vCPU1 vCPU2 vCPU3 • At T1: vCPU1 on real core • At T2: vCPU2 on real core Shared Memory • What happened? On a single physical CPU: • OS ran [how?] T1T2 • Saved PC, SP, … in vCPU1’s thread control block • Loaded PC, SP, … from vCPU2’s thread control block vCPU1 vCPU2 vCPU3 vCPU1 vCPU2 Time • This is called context switch 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 16 Very Simple Multiprogramming • All vCPUs share non-CPU resources • Memory, I/O Devices • Each thread can read/write memory • Including data of others • And the OS! • Unusable? • This approach is used in: • Very early days of computing • Embedded applications • MacOS 1-9/Windows 3.1 (switch only with voluntary yield) • Windows 95-ME 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 17 Today: Four Fundamental OS Concepts • Thread: Execution Context • Program Counter, Registers, Execution Flags, Stack • Address Space (with Translation) • Program’s view of memory is distinct from physical machine • Process: Instance of a Running Program • Address space + one or more threads + … • Dual-Mode Operation and Protection • Only the “system” can access certain resources • Combined with translation, isolates programs from each other 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 18 Key OS Concept: Address Space • Program operates in an address space that is distinct from the physical memory space of the machine 0x000… Processor Memory Translator Registers 0xFFF… 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 19 Address Space • Definition: Set of accessible addresses and 0x000… the state associated with them • 232 = ~4 billion on a 32-bit machine Code • What happens when you read or write to an Static Data address? Heap • Perhaps acts like regular memory • Perhaps causes I/O operation • (Memory-mapped I/O) Stack • Causes program to abort (segfault)? • Communicate with another program 0xFFF… • … 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 20 Typical Address Space Structure 0x000… PC: Code SP: Static Data Heap Processor registers Stack 0xFFF… 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 21 What can hardware do to help the OS protect itself from programs? And programs from each other? 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 22 Base and Bound (no Translation) • Requires relocation 0000… Code • Can the program touch OS? Static Data • Can it touch other programs? Heap Base Address Original Program Stack 1000… 0000… code ≥ 1000… 0010… Code Static Data Program 1010… Static Data heap address Heap stack Bound < 0100… Stack 1100… 0100… FFFF… 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 23 Base and Bound (with Translation) 0000… • Can the program touch OS? Code • Can it touch other programs? Static Data Heap • Fragmentation still an issue! Original Program Stack 0000… Base Address code 1000… 0010… 1000… Code Static Data Program 0010… Static Data heap 1010… address Heap stack Bound < 0100… Stack 1100… 0100… FFFF… 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 24 Paged Virtual Address Space • What if we break the entire virtual address space into equal-size chunks (i.e., pages) and have a base and bound for each? • All pages same size, so easy to place each page in memory! • Hardware translates address using a page table • Each page has a separate base • The “bound” is the page size • Special hardware register stores pointer to page table 6/23/2020 Kumar CS 162 at UC Berkeley, Summer 2020 25 Paged Virtual Address Space Memory Processor Registers <Page #> Page Table <Frame Addr> instruction <Page Offset> Page (eg, 4 kb) <Virtual Address> = <Page #> <Page Offset> PT Addr • Instructions operate on virtual addresses • Translated at runtime to physical addresses