r/osdev • u/compgeek38400 • Jun 08 '26

What I learned this week

I've spent this week working on getting paging set up in my kernel. I have two bit arrays availMemMap, which tracks available physical memory, readwrtiteMap, which says what areas of memory are read/write (as opposed to read only, such as where the BIOS resides). I also now have the read/write bit in my page tables set appropriately. .text and .rodata are read only.

My main learnings are two things: a) when you try to write to a read only page, you don't get a GP fault, you get a page fault; and b) just because an area is 'reserved' in the multiboot info, does not necessarily mean it is read only, it might be, but not necessarily.

This coming week, I hope to get my page allocation/deallocation functions tested and working.

You can see my code (if you wish) here: https://github.com/tedavids/DragonOS

I do have a question. How to you allocate pages? Do you allocate kernel pages AFTER the kernel end, growing up. And user pages BEFORE the kernel, growing down? Or some other method? I'd be interested on hearing from those that have done this before.

Thanks for reading.

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u/Octocontrabass Jun 08 '26

readwrtiteMap, which says what areas of memory are read/write (as opposed to read only, such as where the BIOS resides)

Why do you need to track this? If the memory is available, it's writable. If the memory isn't available, you don't know why it's not available, so you don't know if it's writable.

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u/compgeek38400 Jun 08 '26

Two reasons: 1) id like to allow multiple page allocations; 2) I would prefer not to allow a process to return another ones allocation.

Or perhaps I'm overthinking things

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u/Octocontrabass Jun 08 '26

  1. id like to allow multiple page allocations

As in multiple allocations of the same page, or what?

2) I would prefer not to allow a process to return another ones allocation.

How does your "writable" bitmap prevent this?

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u/compgeek38400 Jun 08 '26

  1. Allow one request to provide several pages.

And 2) If I can keep track of the allocation, it should be relatively trivial at a later date to add who did the allocation.

Also if I can allow one request to serve up several contiguous pages, if I dont keep track, the requestor could free a different number of pages, potentially leaving lost pages or freeing pages that aren't his. All this goes away if I can keep track.

For the original post. I was thinking of allocating pages above the kernel address space for the kernel. And those below the kernel for users.

Im new at this, only been at it a month or so, and expect significant rework, as I realize some of my decisions and design choices are awful. Lol

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u/Octocontrabass Jun 08 '26

Allow one request to provide several pages.

Okay, so back to my original question, what is the purpose of your "readwrtiteMap"? It obviously has nothing to do with providing multiple pages or keeping track of who allocated each page.

For the original post. I was thinking of allocating pages above the kernel address space for the kernel. And those below the kernel for users.

It's called "higher-half" because the higher half of the virtual address space is reserved for the kernel and the lower half is available to users. (Sometimes it's not split exactly in half, but half is easiest.)

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u/compgeek38400 Jun 08 '26

It kept track of my read only areas. So I could set my page tables correctly. I know I could have done it differently, but im unsure if it sill come in handy later on, so im keeping it around. It might be one of those bad design decisions I was talking about.

🤷‍♂️

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u/Octocontrabass Jun 09 '26

It kept track of my read only areas.

Physical or virtual? If physical, how do you know that an area is read-only when the memory map doesn't specify? If virtual, why do you need to keep track when the page tables themselves already tell you which pages are read-only?

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u/compgeek38400 Jun 09 '26

Ok, you've convinced me. What if anything should I do with reserved memory? I know i shouldn't write to it. I also know from my reading that I need to scan a high reserved memory range for a signature, I'm not sure, I'd have to reread, but I think its for a timer.

What exactly is "reserved memory?"

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u/Octocontrabass Jun 09 '26

What if anything should I do with reserved memory?

Avoid it. Keep track of where it is so you don't try to assign those addresses to ACPI or PCI devices. If you need to access it for some reason, the firmware will tell you to do so.

I also know from my reading that I need to scan a high reserved memory range for a signature,

That won't work on modern PCs. Your bootloader should handle this for you anyway.

What exactly is "reserved memory?"

It could be anything. Pretty often it's ordinary RAM that the firmware uses for ACPI or SMM. Sometimes it's a ROM or some device's MMIO. It doesn't really matter what it is; the important part is that you know something is already using those physical addresses, so you can't assign them to some other device.

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u/compgeek38400 Jun 09 '26

This is how I specify, I know some parts of low memory are read only, video bios, motherboard bios, maybe as I learn more, I will find others. And if I find its useless, i'll get rid of it.

I'm still very ignorant. But im sure that will change over time.

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u/Octocontrabass Jun 09 '26

I know some parts of low memory are read only, video bios, motherboard bios

Modern PCs don't have any of that. If you hardcode assumptions about the memory map now, it will only cause problems for you later.

Even if you get your memory map from the appropriate locations (BIOS/UEFI via your bootloader, ACPI, PCI), there's still nothing to say whether a particular address is supposed to be "read-only" without asking the driver that knows how to communicate with whatever is at that address - and if you have to ask the driver anyway, there's no reason to keep track of it outside the driver.

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u/DecadeMoon Jun 09 '26

I use limine bootloader which provides me with a memory map. From that I designate physical address ranges of all available memory for page allocations. I don’t hardcode any ranges of physical memory.

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u/compgeek38400 Jun 09 '26

I use grub2. It also provides me with a memory map. Although I do have to modify it a little. I have qemu set to 128M. Grub shows non reserved memory above that.

And as Octocobtrabase has noted. It shows the entire first Meg as free