ESSENTIALS: Subroutines & The Wheeler Jump - Computerphile

A true essential, the subroutine saves time, effort and helps avoid bugs. Dr Bagley explains why he has two essentials!

Many thanks to Microsoft Research UK for their support with the 'Essentials' mini-series.

This video was filmed and edited by Sean Riley.

[kennypu]

It makes me happy that Brady is really learning and understanding how this works

[vaer-k]

An ESSENTIALS playlist would be really useful to catch up on fundamentals

[PvblivsAelivs]

The interesting thing is that the Wheeler Jump will actually still work on modern processors. It won't be very efficient. But the processor needs to be able to support self-modifying code. (It always amazes me when people condemn self-modifying code and turn around and extol the virtues of "just-in-time" compilers. How, exactly do they think "just-in-time" compilers work?

The processor guarantee is that after a jump instruction any code modifications made before the jump instruction will take effect. This may require discarding the instruction prefetch queue and is not generally recommended.

[stevencotton]

Dr Bagley's office is the best office ever.

[talideon]

Now, the reason why ARM chips use a link register, as described in the video, is because they have LDM and STM instructions, which allow you to load/save load multiple registers from/to a stack at once. When you have instructions like that, using a register makes a tonne of sense.

[Kr-nvfo]

So store "subroutines" in a text file and copypaste them into the code as needed, got it!

[IllusionzZBxD]

Just one thing on why subroutines are useful is also that bugs only exist once, not in every copy/paste of the same slice of code so you need only fix the subroutine, not all the versions of it.

[jsbarretto]

Can you make a video on how the x86 MOV instruction is Turing-complete?

[veloctyTV]

"Self modifying code" is some pretty nice playground for a non study guy (in germany). Pretty fascinating ^^

[nab-rkob]

Can you imagine code without subroutines? Miles and miles of lines.

[indexoverflow]

Love the essentials series, keep it up!

[AstAMoore]

Long story short—use SMC. I do it all the time when writing in Z80 assembly. Not just for jumps, but for saving memory and speeding up execution in many parts of the code (e.g. storying and modifying variables).

Can’t do it on modern systems, though—the protected memory paradigm will not allow for SMC to execute.

[thomasreichert]

It's quite common for subroutibes to have multuple return points. Wheeler jumps would have had to modify the address at each point individually.
+1 for pushing return addresses to the stack

[jamieoglethorpe]

Our second year class project was to build an assembler for a simulator designed by our lecturer. He did not have "save program pointer in register before jumping" and I invented the Wheeler Jump. I was really annoyed when he changed his simulator. I felt really hard done by.

[JaapVersteegh]

... and then there was "inlining", getting rid of the whole thing again :)

[linguica]

I see Dr Bagley is living the #donglelife with his late-model Macbook Pro

[russellthompson]

Ah yes, I remember COBOL's ALTER verb! It was a fireable offense to even think of using it!

[xybersurfer]

can you do recursion with the wheeler jump?

[littleal]

Could you please make a video to covering how you would make a complete but simple computer.

[Lostpanda]

I have exam in medicinal chemistry tomorrow, and I'm watching this now 😐 great video though!