COMP2300/6300
Computer Organisation and Program Execution
Operating Systems
Dr Charles 1, 2022
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Week 10: Operating Systems
what is an OS? privilege levels
processes & scheduling
What is an OS?
what is an operating system (OS)?
…itʼs a virtual machine
o ering a more familiar, comfortable and safer environment for your programs to run in
memory management
hardware abstraction
process management inter-process communication (IPC)
…itʼs a resource manager
co-ordinating access to hardware resources
processors
mass storage
communication channels
devices (timers, GPUs, DSPs, other peripherals…)
multiple tasks/processes/programs may be applying for access to these resources!
A brief history of operating systems…
1950s-60s: system monitors
IBM 704 mainframe at NACA in 1957 NASA / Public Domain
1960s – multi-programming system
University of Manchester Atlas, January 1963
Iain MacCallum / CC BY (https://creativecommons.org/licenses/by/3.0)
1970s – multi-tasking systems
DEC PDP-11
Stefan_Kögl / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)
1970s – early workstations…
Xerox Alto
Joho345 / Public domain
1970s-80s: Consumer O / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
NeXTStation
Blake Patterson / CC BY (https://creativecommons.org/licenses/by/2.0)
A brief history of operating systems (1)
in the beginning: single user, single program, single task, serial processing—no OS 50s: system monitors/batch processing
the monitor ordered the sequence of jobs and triggered their sequential execution
50s-60s: advanced system monitors/batch processing: the monitor handles interrupts and timers
first support for memory protection
first implementations of privileged instructions (accessible by the monitor only)
early 60s: multi-programming systems:
use the long device I/O delays for switches to other runnable programs
early 60s: multi-programming, time-sharing systems: assign time-slices to each program and switch regularly
A brief history of operating systems (2)
early 70s: multi-tasking systems – multiple developments resulting in UNIX (and others)
early 80s: single user, single tasking systems, with emphasis on user interface or APIs. MS-DOS, CP/M, MacOS and others first employed ʻsmall scaleʼ CPUs (personal computers).
mid-80s: Distributed/multiprocessor operating systems – modern UNIX systems (SYSV, BSD) late 70s: Workstations starting by porting UNIX or VMS to ʻsmallerʼ computers.
80s: PCs starting with almost none of the classical OS-features and services, but with an user-interface (MacOS) and simple device drivers (MS-DOS)
A brief history of operating systems (3)
last 20 years: evolving and expanding into current general purpose OSes, like for instance:
Solaris (based on SVR4, BSD, and SunOS, and pretty much dead now) Linux (open source UNIX re-implementation for x86 processors and others)
current Windows (used to be partly based on Windows NT, which is ʻrelatedʼ to VMS)
MacOS (Mach kernel with BSD Unix and a proprietary user-interface)
multi-processing is supported by all these OSes to some extent but not (really) suitable for embedded, or real-time systems
Distributed operating systems
all CPUs carry a small kernel operating system for communication services all other OS services are distributed over available CPUs
services may migrate
services can be multiplied in order to guarantee availability (hot stand-by), or to increase throughput (heavy duty servers)
Real-time OS for music?
Real-time OS for robots?
Real-time OS for cars?
Real-time operating systems
fast context switches? small size?
quick response to external interrupts? multitasking?
‘low level’ programming interfaces? interprocess communication tools? high processor utilisation?
should be fast anyway should be small anyway
not quick, but predictable o en, not always needed in many operating systems needed in almost all operating systems fault tolerance builds on redundancy
Real-time operating systems need to provide…
the logical correctness of the results as well as the correctness of the time: what and when
are both important
all results are to be delivered just-in-time—not too early, not too late.
timing constraints are specified in many di erent ways… o en as a response to external events (reactive systems)
predictability, not performance!
Embedded operating systems
usually real-time systems, o en hard real-time systems very small footprint (o en a few kBytes)
none or limited user-interaction
90-95% of all the processors in the world are in embedded systems
How many OSes?
Standard features?
is there a standard set of features for operating systems?
the term operating system covers everything from 4 kB microkernels, to > 1 GB installations of desktop general purpose operating systems
Minimal set of features?
is there a minimal set of features?
almost: memory management, process management and inter-process communication/synchronisation would be considered essential in most systems
is there always an explicit operating system?
no: some languages and development systems operate with standalone runtime environments
Process management
(weʼll talk more about this in a moment)
basically, this is the task of keeping multiple things going all at once… …while tricking them all into thinking theyʼre the main game
whatʼs a task?
Memory management
remember memory? the OS is responsible for sharing it around
allocation / deallocation
virtual memory: logical vs. physical addresses, segments, paging, swapping, etc. memory protection (privilege levels, separate virtual memory segments, …) shared memory (for performance, communication, …)
Synchronisation/inter-process communication
remember all the asynchronism stu ? the OS is responsible for managing that as well
semaphores, mutexes, condition variables, channels, mailboxes, MPI, etc. this is tightly coupled to scheduling / task switching!
Hardware abstraction
remember all the specific load-twiddle-store addresses in the labs? no?
good news everyone! the OS does so you donʼt have to
device drivers
protocols, file systems, networking, everything else…
all through a consistent API
Kernel: definition
the kernel is the program (functions, data structures in memory, etc.) which performs the
core role(s) of the OS
access to the CPU, memory, peripherals all happens through the kernel through a system call
if you want to look at some real system call APIs on Linux,
syscalls.h header file how to add a new system call
on Windows,
Windows API Index
writing an OS seems complicated how is it done in practice?
Monolithic OS
(or ʻthe big mess…ʼ) non-portable/hard to maintain
lacks reliability
all services are in the kernel (on the same privilege level) but: may reach high e iciency
e.g.: most early UNIX systems, MS-DOS (80s), Windows (all non-NT based versions) MacOS (until version 9), etc…
Monolithic & Modular OS
modules can be platform independent easier to maintain and to develop reliability is increased
all services are still in the kernel (on the same privilege level) may reach high e iciency
e.g., current Linux versions
μKernels & client-server models
μkernel implements essential process, memory, and message handling all ʻhigherʼ services are user level servers
kernel ensures reliable message passing between clients and servers
highly modular, flexible & maintainable
servers can be redundant and easily replaced
(possibly) reduced e iciency through increased communications
e.g., current research projects, μ, L4, Minix 3, etc.
Example: UNIX
hierarchical file-system (maintained via mount and unmount ) universal file-interface applied to files, devices (I/O), as well as IPC
dynamic process creation via duplication
choice of shells
internal structure as well as all APIs are based on C relatively high degree of portability
many versions/flavours: UNICS, UNIX, BSD, XENIX, System V, QNX, IRIX, SunOS, Ultrix, Sinix, Mach, Plan 9, NeXTSTEP, AIX, HP-UX, Solaris, NetBSD, FreeBSD, Linux, OPENSTEP, OpenBSD, Darwin, QNX/Neutrino, OS X, QNX ROTS, …
Privilege levels
what do you think privilege means?
how does it a ect your code running on the microbit?
Privilege levels
certain instructions can only be executed in “privileged” mode—this is enforced in hardware
di erent architectures enforce this in di erent ways
check the manual (e.g. Section A2.3.4 on p32 or Table B1-1 Mode on p568 of the ARMv7-M
reference manual
Fun video for the nostalgic: What is DOS protected mode?
ARMv7-M execution levels
thread mode privileged regular code unprivileged regular code
priviliges may control:
code execution
memory read/write access
register access (e.g., for peripherals)
handler mode
all exceptions (including interrupts) n/a
“Supervisor call” instruction
have you noticed these entries in the vector table in labs?
.word SVC_Handler
.word PendSV_Handler
the svc instruction (A7.7.175 in the reference manual) runs the SVC_Handler immediately
Deferred supervisor call (PendSV)
thereʼs a PENDSVSET bit (bit 28) in the Interrupt Control and State Register ( ICSR )
if set, the PendSV_Handler will be called according to the usual interrupt/exception priority rules
both SVC_Handler and PendSV_Handler run in privileged mode, like all interrupts
System calls with
How might we implement a system call on a microbit? How do system calls work in linux?
Processes & scheduling
Trust the Process
(if youʼve got your laptop here) how many processes are running on your machine right now?
how about on your phone?
Process: definition
basically: a running program
includes the code (instructions) for the program, and the current state/context:
registers/flags
memory (stack and heap) permissions/privileges
other resources (e.g. global variables, open files & network connections, address space mappings)
processes as far as the eye can see
exact definition of process depends on the OS
so how do we manage them?
1 CPU per control-flow
specific configurations only, e.g.:
distributed microcontrollers physical process control systems
1 cpu per task, connected via a bus system
Process management (scheduling) not required Shared memory access need to be coordinated
1 CPU for all control-flows
the OS may “emulate” one CPU for every control-flow this is a multi-tasking operating system
support for memory protection essential process management (scheduling) required shared memory access need to be coordinated
Symmetric multiprocessing (SMP)
all CPUs share the same physical address space (and have access to the same resources) so any process can be executed on any available CPU
Processes vs threads
processes (as discussed earlier) have their own registers, stack, resources, etc.
threads have their own registers & stack, but share the other process resources one process can create/manage many threads
Torvalds vs Threads
“Depends on the implementation”…
Process Control Blocks (PCBs)
process ID
process state: {created, ready, executing, blocked, suspended, bored …} scheduling attributes: priorities, deadlines
CPU state: (e.g. registers, stack pointer)
memory attributes/privileges: permissions, limits, shared areas allocated resources: open/requested devices and files, etc.
a data structure for processes
Process states
created: the task is ready to run, but not yet considered by any dispatcher
ready: ready to run (waiting for a free CPU)
running: holds a CPU and executes
blocked: not ready to run (waiting for a resource)
suspended: swapped out of main memory (e.g. waiting for main memory space)
First come, first served (FCFS) scheduling
Waiting time: 0..11, average: 5.9 Turnaround time: 3..12, average: 8.4
FCFS (again)
Waiting time: 0..11, average: 5.4 (was 5.9 before) Turnaround time: 3..12, average: 8.0 (was 8.4 before)
Round-robin (RR) scheduling
Waiting time: 0..5, average: 1.2 Turnaround time: 1..20, average: 5.8
optimised for swi initial responses, but “stretches out” long tasks
when might you want to use FCFS scheduling? how about RR?
Again, a whirlwind tour of OSes
remember the concepts
go build your own in labs.
Fun With Operating Systems
Kernel writing 101 Linux on an 8bit AVR
How to make an operating system (WikiHow)
Further Reading
Essentials of Computer Organisation and Architecture – Chapter 8.2: Operating Systems
ittʼs $7 History of Unix
LGR Tech Tales – How Digital Research Almost Ruled PCs
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