OS Context Switching - Computerphile

How does an operating system juggle different programs? Dr Steve Bagley explains Context Switching.

This video was filmed and edited by Sean Riley.

[MichaSpielt]

Strange how you always upload these videos just after I wrote my exam about the topics

[proudsnowtiger]

Once did a pre-emptive (interrupt driven) multitasking teeny tiny kernel in Z80 that could do a complete process switch in < 400 T-states. A thing of beauty. This was about four years after anyone would want a multitasking Z80 system, so the project got canned, but just for a moment is was deeply satisfying.

[lucidmoses]

Back in the DOS - 8088 days I did my own cooperative multi tasking inside a C program by allocating memory for another stack and doing some fancy fiddling with the BP pointer and such. It was a supper fast context switch. Basically just push the BP to the appropriate stack and then return. The return would return you to the other thread.

[nico-s]

Props to the guy animating the drawings

[MrMalchore]

These videos make it sound so simplistic. But if you tried to read Linux source code...grab the asprin.

[mikefochtman]

Learned a lot about this topic on SEL's MPX OS. Combined real-time 'hard' priority switching and 'round-robin' priorities. Was pretty interesting setup and could 'peek' in various OS structures.

[artiem]

In the DEC VAX instruction set, there are (privileged) instructions for saving process context and loading process context. At least for the first few releases of DEC's VAX/VMS operating system, there were two instances of save process context, and one instance of load process context in the OS core. Yes, VAX is probably the epitome of complex instruction sets!

[SupaKoopaTroopa]

This is the Computerphile video I've always been waiting for!

[marlandkennedy]

Where were you when I was taking operating system class. Well you might not have been born when I took it.

[joelvanin]

Excellent time I am just learning it this week, love this channel

[trevinbeattie]

Will you be doing a video on virtual memory management? This was a big step in processor evolution from the MC68000 to MC68030 (and probably other CPU's ;) ) which has a direct relationship with context switching, as the OS not only needs to swap the register set but also the page translation tables.

[phoenix]

Dr Steve, always making making us hungry for more. literally.

[ailuros_]

I would love even more if this video had english subtitles :| btw, great video! thx

[perschistence]

I did not understand some things. What is the difference between "SWI 3" and "SWI 4"?

[lawrencedoliveiro]

9:09 You’ve already said that the registers seen by the OS kernel are not the same as those seen in user mode, even though they have the same names. So presumably the OS has access to special instructions for accessing the user-mode registers, in order to do this save/restore (and no doubt other purposes like obtaining arguments and returning results for SWI calls)?

[dsedchenko]

Thank you for the video! Can you make please a video about how protection rings and how process isolation is working?

[peppybocan]

I wonder how is the context switching done on the real-time OS.

[franchello]

I built something like this for my final project in Operating Systems in college.

[acommenter]

what is the video being referred to at 8:50-9:00 ?

[JerehmiaBoaz]

"APR RO, iString" should be
ADR R0, iString