https://wiki.osdev.org/Entering_Long_Mode_Directly

i've had a bootloader for a small 64-bit kernel based on this that fit comfortably into the bootsector, including loading the kernel from disk and setting up vesa modes, no stage2 required.

How in the world do you fit all that in 512 bytes? I'm guessing you don't have a real-world filesystem (that allows the kernel to be anywhere on the disk just as a normal file)? Because just dealing with file fragmentation should bump you way over 512 bytes I would imagine.

Historically, many filesystems have had a special file type or attribute for contiguous files - a file guaranteed to not be fragmented on disk. Most commonly used for boot loaders, OS kernels, or other essential system files - although historically some people used them for database files for a performance boost (which is likely rather marginal with contemporary systems, but decades ago could be much more significant).

Some systems required certain system files to be contiguous without having any special file metadata to mark them as such - for example, MS-DOS required IO.SYS and MSDOS.SYS to be contiguous on disk, but didn’t have any file attribute to mark them as contiguous. Unlike an operating system with proper support for contiguous files, DOS won’t do anything to stop you fragmenting IO.SYS or MSDOS.SYS, it is just the system will fail to start if you do. (Some might interpret the System attribute as implying Contiguous, but officially speaking it doesn’t.)

Somewhere in the first sector (aligned to a 64 bit boundary), there would be a small structure that just contains a unique signature (not ASCII but some random value; 64 bits seems like more than enough as opposed to a GUID), and a pointer to the "FS info block". Leaving all remaining space in the sector available for boot code or possibly another "overlayed" filesystem.

That info block in turn would point to the MFT entry for the stage 2 boot code. An MFT entry would contain at a minimum an easy to locate list of (start sector,count) extents. Maybe there could be a flag that tells the OS that a certain file should never be fragmented?

File names would be entirely optional and for human consumption; for direct links within a filesystem, sector numbers would be used, and some kind of unique numeric identifier for anything "higher-level".

I'm genuinely wondering if some expert here sees any problem with this scheme, other than it not conforming to how mainstream OSes do things?

Each file has index number ("Inode"?) in the MFT. The first 24 (0-23) are reserved, and 11 of them contain metadata about the volume . https://flatcap.github.io/linux-ntfs/ntfs/files/ - somewhere in the Windows API is something that allows opening a file by "Inode" number. This link and info may be really old so it could be more of the reserved inodes are used now.

So, if 23 isn't being used yet, you could use that to put your 2BL - create a file there and name it $2BL or something. Would be funny to see what future Windows update that does use it does to it, if that ever happens (and of course maybe it is used).

> Maybe there could be a flag that tells the OS that a certain file should never be fragmented?

Haven't looked but I recall from an old book I read that small files are stored right in the MFT and I think existing data + the cluster size is the limit there.

not fragmenting files.... well. U don't fragment files until you do.. u know. If u write an FS it will only fragment what u tell it to. So if u want all contigious files then u can simply do it that way. store them as (file_id,sector_count,start). or even more simple (fname_len,fname,sector_count,start_sector) so u dont need to keep nameblock around for filenames (fat?)

If u look at Ext2/4, FAT16/32 and others u will see that it all started quite basically, keepin a list of files and offsets. But due to things like disk reliability, user errors, system stability issues etc., you need a lot of extra stuff.

Also, questions like : how long is a maximum for a filename length? This kind of stuff can really impact what kind of features u need to implement.

How long is a file allowed to be? How many files are maximum for the FS?

This might sound silly, but there's already datacenters out there (a lot actually) who cannot use most filesystems because either files are too huge, partitions are too huge, too many files are present for index structures etc.

If you want dead simple for a starting OS: (sector_count,start_sector,fname_len,fname,0)

If you want more, try looking at ext2 or FAT32 or if your system specifications require it, look even beyond. (ext4, ZFS, NTFS, etc.) - A lot of these are subtely different, trying to solve different problems, or similar problems in different ways.

the whole boot disk image was generated during build, which is common for small systems.

If you do this all within 1 sector, equally you do not do any error checking. just ram stuff into memory and yolojump into it.

the basic would be: load kernel from disk using bios interrupt get memory map using bios interrupt parse kernel ELF header / program headers and relocate it - elf header to find ph_off and ph_num and entry_point - program headers to find all pt_load and rep movb them into tgt phys addr.

Also with 510 bytes generally u will not make nice 'page tables' though this is actually possible with only few bytes. - i did not manage it yet in 510 :D but i am sure there's someone who can do it... it can be done really efficiently.

the disk would be formed by assemblding the mbr. then doing something like

cat mbr.bin kernel.bin > disk.bin (maybe here use truncate to padd the disk.bin to a certain size like 10+MB or so - helps some controllers recognize it better)

all that said it's not useful to do this. you will find it an interesting excersize at best. like trying to make 'tiny ELF' file or so. fun to learn, useless in practice.

If you want to use an actual modern harddisk, you might want to look at GPT rather than MBR, as it won't overflow partition table stuff and allow for very large disks (2TB+?) (uefi gets rid of all of that and allows u to use a proper disk layout without any additional difficulty!)

there is no need for protected mode if you want to launch into 64-bit mode. I would say though, DO NOT USE BIOS. It's a piece of rubbish which will just make things more tedious.

Using UEFI via EDK2 or GnuEFI is the way to go, and both methods are really easy and a blessin to implement. It's a bit hard to get around the initial idea of UEFI, but if you view some other people's example projects on github you can find easily how it works. EDK is a bit shitty with .dec and .inf files etc, and GnuEFI is just reading headerfiles to discover what is there, but it's inifnitely better than the unspecified bios interface. You can litterally not even assume the int 0x10, int 0x15 etc. are there properly if you start running on real hardware. On UEFI systems, you can assume a stable minimal basis, and easily enumerate other things (hardware/platform capabilities etc.) in a sane way.

Also UEFI sets up the platform a long way already, so you don't need to do any initialization for your os-loader component (stage2 or 3). You can simply start loading your os right away. or drivers or whatever kind of design your kernel is. get memory map, get some access to efi file system, start pulling in and loading stuff.

Additionally, UEFI's reliability is.. sketchy, as far as I know (using the classic logic of "if Windows doesn't use it does it really matter?"). And GNU-EFI suffers from build portability troubles, AFAIK.

[bootelf]: https://github.com/n00byEdge/bootelf

Thanks for this comment. It makes me realize this is likely not an AHCI thing ,but how the seaBIOS(? qemu's flavor?) handles enumerating disks via AHCI rather than IDE.

If it uses the IDE controller then it will recognize the boot disk. If i pick AHCI, i need to add the partition table.

UEFI reliability is sketch, but really BIOS is incredibly crap, so much more than UEFI.

Windows DOES use EFI/UEFI, how else will it boot on a system that has EFI/UEFI firmware inside of it? It can let you do secureboot, edit efi variables... - where do you get this classic logic from? (maybe i am totally misisng something, but they interface with it, and should thus use the spec? even tho they might not use gnuefi or EDK2 ofc :P (edk2 is still likley...))

https://wiki.osdev.org/CPU_Registers_x86-64#CR0.

Sign : Zero : always '0' : AuxCarry : always '0' : Parity : always '1' : Carry

(the parity flag came all the way from the 8008 / Datapoint 2200[1], and is the inverted XOR of the result's lower 8 bits; aux carry is the carry out of bit 3, used for BCD arithmetic)

Flag bit 15 has also stayed reserved, except at one time it was used by the NEC Vxx chips for their 8080 compatibility mode. That feature had to be first unlocked by executing a special instruction, because there is code out there that loads the entire (16 bit) flag register with 0000 or FFFF. With the mode bit unlocked, that would inadvertently switch the CPU to running a completely different set of opcodes!

[1] https://www.righto.com/2023/08/datapoint-to-8086.html

Ditto for why CR1 and 5-7 are still "reserved" and CR8 came into existence.

Booting is the process of going from mini-me mode/single user/recovery mode to flying.

I have been running Unix along side a Microsoft product since Xenix/dos. ( Looks like 40 years...) How much have we advanced?

I also have been using Linux since the swedish version came out ( first release ) and GNU 0.1.

My apologies about calling Xenix, Unix, It is a has-been wanna-be me-too square-excrament from shortly after release until it's languishing demise.

Microsoft does not release products, they empty their cat boxes onto customers. ( The most recent example is both co-pilot And 22H2. )

If you look at how F1 cars have evolved, and pencils as well as pocket calculators - how close are we to the usable ideal?

Why isn't the bootloader a static kernel mode? It used to be. Someone recently suggested it should be, and I agreed.

U can use a linker script to create file layout. If you want a flat binary file... u dont want a file layout. So linkerscript is really useless for a flat binary blob. Even if you make it with multiple files. It'd be just the same as saying: cat blob1 blob2 blob2 > finalyblob.

If you'd say have multiple blobs, and use linker directives to align them, the position dependent code within the assembled files will be wrong, unless you specifically define that using for example ORG directive in NASM. If you use the ORG directive in NASM, u will need to keep that synchronized with the linker script in order for all the labels etc. to keep the right offsets calculated.

So essentially.. this linker script might even add complexity and issues when working with multiple binary blobs. u can't align them or use nice linker features....

If you use more structured files, which allow for example for relocation... then ur already using ELF or PE and can simple produce those files. They can be more masterfuly linked with nice features, and linker scripts are then essential.

You can add your binary blob in the right location in the output file of a linking run, using a linker script. This is useful to add data into your files but for an MBR it's not particularly useful. People do it, but it adds no benefit over just sticking it on the front if your disk using 'dd' for example. ------

That being my views, I am wondering what you see the benefit here? Are there some linker features i am unaware of that are particularly useful here? (I really know only alignment stuff in there... and include blobs or put stuff into /discard/ and some basic define sections/segments etc.). I am not familiar with perhaps more advanced linking features.

Later on, make files and linker scripts are an important headache. but when generating flat binary, just generate flat binary! No need to bloat it.

My OS used to have a file called make.sh to tease at this :D Now i am using a 'fileformat' and other fancy things and alas -fbin and --oformat=binary are but fleeting memories. I tried a long time to write separate data c files and code c files, dump it out to binary, and then building some monstrosity from there but it gets really difficult to link&load etc. xD --- better to just use the ELFs or PEs! I suppose that is litteraly what they do for u ;P

They all seem to be steps needed for backwards compatibility.

Could Intel just provide a flag, command, to start in the right mode from the beginning.

Or just remove all the backwards compatibility.

I think I remember doing some research and ARM64 has some of the same issues.

Are there any CPUs that are designed from scratch as 64 bit it will not have any need for backwards compatibility and would enter the required state by default?

I guess sthat was the goal / design of Itanium?

are made to start in the desired 64 bit state from th

> X86S is a legacy-reduced-OS ISA that removes outdated execution modes and operating system ISA.

> The presence of the X86S ISA is enumerated by a single, main CPUID feature LEGACY_REDUCED_ISA in CPUID 7.1.ECX[2] which implies all the ISA removals described in this document. A new, 64-bit “start-up” interprocessor interrupt (SIPI) has a separate CPUID feature flag.

[0] https://cdrdv2.intel.com/v1/dl/getContent/776648 [pdf warning]

Yes, I see the argument that if you go to that point you might as well just use emulation. However running mixed 32bit/16bit code (Windows 98/95) becomes problematic just because of performance reasons. DosBox does well, but good luck supporting games for the Pentium 3 era that still used 16bit libraries because of course they did. (16bit Installshield was so common going into 64bit that MS just has code to replace it so 32bit applications can still be installed despite having a 16bit installer)

[1] https://uefi.org/specs/UEFI/2.10/index.html

Seems like a bad tradeoff, and part of a slippery slope towards a completely locked down system, where writing your own code and getting it to run on the 'bare metal' is flat out impossible.

Sometimes I wonder: if UEFI instead used ELF format, ext2 filesystem, and somewhat less complicated API (let's be honest: UEFI API is pretty straightforward if tedious), would people still complain about such dependencies? Or would it be deemed to be "fine" since it's not Microsoft technology even though it still would require one to use a multi-megabyte toolchain?

I could easily read NTFS with hexeditor, calculator and pen&paper. Trivial even (most annoying part was the run length packing in 4-bit nibbles).

I gave up trying to read ext's cylinder-oriented mappings.

And FWIW, everyone has a read/write driver for FAT with LFN (and UEFI generally avoids names outside 8.3 just in case), using a different filesystem would lead to issues with OS support.

PE itself isn't really that much of legacy cruft, especially as far as it is used in UEFI (sure, you have the "program not for MS-DOS header", but that's useful for the rare case someone tries to open it under FreeDOS).

Unless you're targeting S/360, but even there XA and newer made it a bit more complex.

> Cool if you actually came along this far.

Cool indeed.

Deprecated doesn't mean deleted, it just means "no longer updated/developed with a goal towards removal".

Not sure why would you want a single-process OS on modern hardware, but there are some alternatives that run much less things on the background than regular Linux: Haiku, FreeBSD, NetBSD, OpenBSD, or some lightweight non-glibc, non-systemd Linux-based like Adelie or Alpine.

It won't fork random processes you don't explicitly tell it to. I thought it was obvious that if you don't want unsolicited processes, then don't specify /bin/init as /bin/foo. The practical example is /bin/sh, but it could be any other executable.

Up to you to specify a binary that does what you want, and doesn't require a bunch of other processes like gdbus to function itself.

init=/bin/sh is more or less like ms-dos loading command.com

Give me a date, and I will tell you what existed.

Nothing mainstream existed until windows 3.1. at least on x86/32

There also was OLEC on windows 2, that did run in real mode, but the only thing that took advantage of it was the demos, and samples: no commercial product used it.

My point was that running your own chosen process as PID1 (or, to be more practical, a minimal init or maybe system v init with single entry for process to run) matches the requirements of running small set of processes just as well as DOS (if you need more, it's IMO more worthwhile to start getting a build of some RTOS for your platform)

How's that working?

RISC-V at least can have a formal spec that companies can choose to follow. The profile mentioned in another comment is one way. Companies can say they're compliant with this or that profile and software can target it.