https://homeostasis.scs.carleton.ca/wiki/index.php?action=history&feed=atom&title=Operating_Systems_2020W_Lecture_3 2026-06-02T22:28:29Z Revision history for this page on the wiki MediaWiki 1.42.1 https://homeostasis.scs.carleton.ca/wiki/index.php?title=Operating_Systems_2020W_Lecture_3&diff=22568&oldid=prev 2020-03-20T02:30:00Z

<p>Created page with &quot;==Video== Video from the lecture given on January 15, 2020 [https://homeostasis.scs.carleton.ca/~soma/os-2020w/lectures/comp3000-2020w-lec03-20200115.m4v is now available]. =...&quot;</p> <p><b>New page</b></p><div>==Video==<br /> <br /> Video from the lecture given on January 15, 2020 [https://homeostasis.scs.carleton.ca/~soma/os-2020w/lectures/comp3000-2020w-lec03-20200115.m4v is now available].<br /> ==Notes==<br /> <br /> &lt;pre&gt;<br /> Lecture 3<br /> ---------<br /> <br /> Structure of a UNIX system<br /> <br /> ls<br /> emacs &lt;--- regular programs, running in user mode<br /> firefox<br /> vi /etc/hostname<br /> ------------- user/supervisor mode boundary<br /> <br /> linux kernel &lt;--- in between the hardware and regular programs<br /> runs in supervisor mode<br /> hardware<br /> <br /> user and supervisor mode are CPU modes<br /> - on x86 chips, they are actually ring 3 and ring 0, but ring 0 and 1 aren&#039;t<br /> used so much<br /> <br /> ring 0 = supervisor mode<br /> ring 3 = user mode<br /> <br /> in supervisor mode, programs have access to all hardware features<br /> in user mode, programs have limited access to hardware<br /> <br /> it is much safer to run in user mode<br /> - can&#039;t crash the system<br /> <br /> drivers: code added to the kernel to help it understand specific hardware<br /> <br /> <br /> on UNIX, the root user can do &quot;anything&quot;, while regular users have limited privileges<br /> - root on UNIX is the same as Administrator on Windows<br /> <br /> a root user has to ask the kernel to do things. The kernel will generally<br /> say yes, but it can say no to anything<br /> - and there are ways to configure systems where root is very limited<br /> <br /> programs running as root are still running in user mode on the CPU<br /> only the kernel runs in supervisor mode<br /> <br /> Focus in first half of class is on the userspace/kernel space boundary and user programs. Second half, we talk about the kernel.<br /> <br /> please don&#039;t stream the lecture over the Carleton VPN.<br /> - I&#039;ll show you ways to access the class VMs without the VPN<br /> <br /> <br /> programs running in userspace can&#039;t do I/O on their own, or access much hardware<br /> - they get their CPU allotment and some RAM, that&#039;s it<br /> - if they want want more RAM or want to talk to the disk, keyboard, screen, etc<br /> they have to ask the kernel for access<br /> <br /> running programs ask the kernel for things via system calls<br /> <br /> you can run many copies of the same program<br /> each copy of a running program is referred to as a process<br /> <br /> processes run in user mode<br /> the kernel runs in supervisor mode<br /> <br /> a &quot;user&quot; is just a label for one or more process<br /> - really a number (uid)<br /> <br /> <br /> different processes can use the same memory addresses<br /> - address 2000 means different things for different processes<br /> - each process has its own addresses<br /> <br /> a system call has to<br /> - switch the CPU mode to supervisor mode<br /> - jump to a standard entry point, so the kernel can decide what to do<br /> <br /> CPU mode: can be user mode or supervisor mode<br /> <br /> a function call can&#039;t on its own invoke kernel code<br /> - (the kernel of course is divided into functions like any C program)<br /> <br /> a kernel is just a program...but with special privileges and special responsibilities<br /> <br /> what is a system call, actually?<br /> - a special CPU instruction<br /> <br /> <br /> what is a CPU?<br /> what is an instruction?<br /> &lt;/pre&gt;</div>

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