Let’s write a Kernel with keyboard and screen support (2014)
arjunsreedharan.org
arjunsreedharan.org
Sounds enticing!
I know what I'm gonna do this winter :P
Finding the kernel is not as easy as it used to be. You need to read the partition table on the disk. You need to be able to understand the filesystem that the kernel is stored on. This adds a fair bit of complexity.
If your boot partition is FAT32, UEFI makes things much much simpler.
I recommend the OSDev wiki for a deeper dive but ultimately, implementing processes involves a lot of different moving pieces that need to work.
The most basic process manager would be cooperative, otherwise you'd have to write drivers for some external driver to regularly drive an interrupt.
Cooperative means the program will either print A or B and then call an interrupt into the kernel. The kernel would save the registers and instruction pointer, restore the one of the other program (which prints the opposite of what the first printed; A or B) and do an interrupt return.
This doesn't scale beyond a few processes that you have written yourself very carefully.
Any more competent process management will need to rely on regular timer interrupts, will have to keep track of processes and how much CPU time they got, have a thread to run if nothing else can run, manage processes when they die and has to be able to switch in and out of ring3 for running the actual process. On top of that it has to be performant and efficient, you have only a few hundred opcodes before the latency of your process management becomes too large.
For RT you can write a scheduler that at any point knows if all processes will meet their deadlines and you can easily design an algorithm to get to the most efficient scheduling order.
In non-RT it gets harder because a process might not need to run at all and is just eternally paused on reading some dead socket, it might have higher priority than other processes, you might have 2000 processes to run but only 1000 timeslots left for the current timeslice.
A process might begin to eat up CPU time and you have to somehow preserve the interactivity of the system, ie prevent other processes from starving without starving the big process in turn.
And once you enter multi-CPU it gets even harder; coordinating multiple concurrent schedulers to run from the same task queue, which cannot ever take a simple spinlock without the system suddenly becoming dead.
The parent comment simply noted that setting up the timer interrupt is fairly easy (though if you want something precise and faster than 1ms and multicore it might get more complicated), it's the stuff after that were you are forced to solve NP-hard problems in a fast way.
I've been trying to figure out how a powerpc machine is being booted so that I can insert my own 'hello world' OS. Sure there is a BIOS equivalent somewhere in there, but how to figure out how and when to talk to it?
Booting x86 might seem like nothing, but booting some old SPARC for instance seems like black magic to me.
Which sort of PPC machine is it? Have you already managed to get e.g. NetBSD running on it?
http://pages.cs.wisc.edu/~remzi/OSTEP/
It's fantastically well-written, yet detailed without being overwhelming.
The correct approach is to use mmap with the MAP_ANON flag (Which interestingly isn't doesn't seem to be fully documented in the Linux documentation...)
Disclaimer: I'm definitely not an expert, I'm reading the book to learn.
Does anyone know these days if the 16-bit boot environment is still "bare metal", or is it inside an UEFI or ACPI hypervisor of some sort?
Real BIOS that boots from 16bit is quite a rarity these days.
My understanding was that the CPU still starts in 16 bit real mode (with an evil hack to sign-extend IP) but that by the time UEFI hands off to code on a hard drive it is in long mode.
Not sure how UEFI's "Compatibility Service Module" works exactly but if it acts like the original BIOS did, it's just a chunk of code that can be called, either by a program or be set as the destination for interrupts.
But there is no "VM exit" mechanism like there is in hypervisors. One thing a hypervisor does is intercept IO address accesses and does them on behalf of the client code (without the code knowing), don't think the CSM is doing this.
Looks very accessible :)
EFI will kill your program if it runs too long unless it disables a lot of the drives by exiting the boot environment.
Additionally, a lot of the EFI structures aren't intuitive or straightforward since they cover a lot of functionality, you'd still write a lot of code, probably more than the non-EFI variant.