FAT32 C: boot on 16-bit PC systems

[sparky4]

I found a bug on booting a C: drive in my 16-bit systems with FAT32 C: drives

it is a presistant one i know of it for years. I was wondering if anyone sees it.
I first found it in 2014-2015 with my XT attempting to boot FreeDOS

my 286s cannot load it either

only 32 bit systems can load the kernel from FAT32

Is there any way to fix this? :D?

the version i am using is the git version from October 4th, 2025

[diekleineHexe]

there is no such thing as "the version i am using is the git version from October 4th, 2025"

there are kernels compiled for and requiring 32 bit CPUs.
there are kernels supporting FAT32.

usually, kernels supporting FAT32 require 80386, but this is an option.

[ecm-pushbx]

FreeDOS SYS ships with two FAT32 boot sector loaders: boot32.asm uses CHS (old int 13h) access and is 8086-clean, boot32lb.asm uses LBA (int 13h extensions) access and expects to run on a 386 or higher machine.

You can install my FAT32 boot sector loader instead. It needs an FSINFO sector for an additional loader stage. It will be able to load kernel.sys files from any FAT32 partition, using LBA access or falling back to CHS access automatically, and always remain 8086-clean. I prepared an installer executable in https://pushbx.org/ecm/test/20251009.3/

This shows the two NASM commands I used to create this:

test$ nasm -I ~/proj/lmacros/ ~/proj/ldosboot/boot32.asm -o boot32fd.bin -D_COMPAT_FREEDOS
/home/[user]/proj/ldosboot/boot32.asm:452: warning: Possibly crossing 64 KiB boundary while reading file (sector size >= 1024) [-w+user]
/home/[user]/proj/ldosboot/boot32.asm:1492: warning: FAT32: 19 bytes still available. [-w+user]
/home/[user]/proj/ldosboot/boot32.asm:1937: warning: FSINFO: 1 bytes still available. [-w+user]
test$ nasm -I ~/proj/lmacros/ -I ~/proj/instsect/ ~/proj/instsect/instsect.asm -o instfd32.com -D_PAYLOAD_FAT32=boot32fd.bin -D_FAT12=0 -D_FAT16=0
test$ ls
boot32fd.bin  instfd32.com
test$

The instsect.com programs installs boot sectors to a drive specified on its command line. The command help for lDOS instsect is given in the lDebug manual at https://pushbx.org/ecm/doc/ldebug.htm#cmdhelp-instsect The version in the linked directory will install a FAT32 loader that's compatible to the FreeDOS load protocol.

The files I used are from https://hg.pushbx.org/ecm/instsect/file/b780f030905d and https://hg.pushbx.org/ecm/ldosboot.exp/file/bc38292b5665 and https://hg.pushbx.org/ecm/lmacros/file/c0a6f4cfe69d

[ecm-pushbx]

there is no such thing as "the version i am using is the git version from October 4th, 2025"

there are kernels compiled for and requiring 32 bit CPUs. there are kernels supporting FAT32.

usually, kernels supporting FAT32 require 80386, but this is an option.

If this was the root of the problem, and not what I suspect (FAT32 386 LBA loader installed), then she should have been presented with the error message that I added in

kernel/kernel/kernel.asm

Lines 366 to 375 in e3bfd6a

	%define LOADNAME "FreeDOS"
	db 13,10 ; (to emit a blank line after the tracecodes)
	db 13,10
	db LOADNAME, " load error: An 80", '0'+(XCPU / 100)
	db "86 processor or higher is required by this build.",13,10
	db "To use ", LOADNAME, " on this processor please"
	db " obtain a compatible build.",13,10
	db 13,10
	db "Press any key to reboot.",13,10
	db 0

It should read as follows on a 386+ kernel build:

FreeDOS load error: An 80386 processor or higher is required by this build.
To use FreeDOS on this processor please obtain a compatible build.

Press any key to reboot.

Due to lack of space in the early boot loaders, boot32lb.asm will instead hang or crash when run on 16-bit x86 machines.

[ecm-pushbx]

Even if the problem is the kernel, you can get my gcc/8086/FAT32 build from https://pushbx.org/ecm/download/old/fdkernel/20251008.zip

[PerditionC]

The FAT32 enabled kernel should run on 8086 systems if compiled accordingly - FAT32 enablement and 386+ code generation are distinct options for the kernel, so it can be built FAT12/16 as 8086 or 386+ or it can be built FAT12/16/32 as 8086 or 386+.

ISSUE STATUS:

Currently the FAT12/16 LBA/CHS and FAT32 CHS boot sectors should work correctly on 8086+ systems but the FAT32 LBA boot sector assumes 386+.

TODO: fix/enhance SYS and add boot sector to properly support 8086 FAT32 on LBA compatible systems

SYS needs to be updated and corresponding updated/new boot sector to handle FAT32 on systems with LBA that is 8086 code clean. SYS needs updated so it either only has 8086 compatible boot sectors or recognizes (with manual selection/override) if on 386+ or not and install appropriate boot sector.

[sparky4]

OMG THANK YOU!!!! <3

[sparky4]

Even if the problem is the kernel, you can get my gcc/8086/FAT32 build from https://pushbx.org/ecm/download/old/fdkernel/20251008.zip

let me try it! :D

[sparky4]

This has been bothering me since 2013-2015

there is no such thing as "the version i am using is the git version from October 4th, 2025"
there are kernels compiled for and requiring 32 bit CPUs. there are kernels supporting FAT32.
usually, kernels supporting FAT32 require 80386, but this is an option.

If this was the root of the problem, and not what I suspect (FAT32 386 LBA loader installed), then she should have been presented with the error message that I added in

kernel/kernel/kernel.asm

Lines 366 to 375 in e3bfd6a
%define LOADNAME "FreeDOS"
db 13,10 ; (to emit a blank line after the tracecodes)
db 13,10
db LOADNAME, " load error: An 80", '0'+(XCPU / 100)
db "86 processor or higher is required by this build.",13,10
db "To use ", LOADNAME, " on this processor please"
db " obtain a compatible build.",13,10
db 13,10
db "Press any key to reboot.",13,10
db 0

It should read as follows on a 386+ kernel build:

FreeDOS load error: An 80386 processor or higher is required by this build.
To use FreeDOS on this processor please obtain a compatible build.

Press any key to reboot.

Due to lack of space in the early boot loaders, boot32lb.asm will instead hang or crash when run on 16-bit x86 machines.

I dont see any error the system still hangs
it just says

Loading FreeDOS

[sparky4]

I should get a debug build to see whats going on there

give me one with any fixes So I can test (if u got a 16 bit machine with XTIDE and a massive SSD/HDD please test it with me! :D )

[sparky4]

FreeDOS SYS ships with two FAT32 boot sector loaders: boot32.asm uses CHS (old int 13h) access and is 8086-clean, boot32lb.asm uses LBA (int 13h extensions) access and expects to run on a 386 or higher machine.

You can install my FAT32 boot sector loader instead. It needs an FSINFO sector for an additional loader stage. It will be able to load kernel.sys files from any FAT32 partition, using LBA access or falling back to CHS access automatically, and always remain 8086-clean. I prepared an installer executable in https://pushbx.org/ecm/test/20251009.3/

This shows the two NASM commands I used to create this:

test$ nasm -I ~/proj/lmacros/ ~/proj/ldosboot/boot32.asm -o boot32fd.bin -D_COMPAT_FREEDOS
/home/[user]/proj/ldosboot/boot32.asm:452: warning: Possibly crossing 64 KiB boundary while reading file (sector size >= 1024) [-w+user]
/home/[user]/proj/ldosboot/boot32.asm:1492: warning: FAT32: 19 bytes still available. [-w+user]
/home/[user]/proj/ldosboot/boot32.asm:1937: warning: FSINFO: 1 bytes still available. [-w+user]
test$ nasm -I ~/proj/lmacros/ -I ~/proj/instsect/ ~/proj/instsect/instsect.asm -o instfd32.com -D_PAYLOAD_FAT32=boot32fd.bin -D_FAT12=0 -D_FAT16=0
test$ ls
boot32fd.bin  instfd32.com
test$

The instsect.com programs installs boot sectors to a drive specified on its command line. The command help for lDOS instsect is given in the lDebug manual at https://pushbx.org/ecm/doc/ldebug.htm#cmdhelp-instsect The version in the linked directory will install a FAT32 loader that's compatible to the FreeDOS load protocol.

The files I used are from https://hg.pushbx.org/ecm/instsect/file/b780f030905d and https://hg.pushbx.org/ecm/ldosboot.exp/file/bc38292b5665 and https://hg.pushbx.org/ecm/lmacros/file/c0a6f4cfe69d

THIS WORKS!

I WAS ABLE TO BOOT WITH NO FLOPPY! on the fat32 C: drive on my 80286! <3

[jvprat]

I've just had a similar issue. I installed:

• FreeDOS 1.4 from floppies
• on a Pentium MMX PC
• with a 2.1GB HDD with a single partition formatted with FAT32 (I didn't want to repartition or format as I had some data there)

After a successful install from the floppies, the boot process got stuck after the message Verifying DMI Pool Data, right when the bootloader should run. I played a lot with sys from the boot floppy disk and I tried installing the 3 kernels I could find (386, 86, 86N?), and I also tried manually downloading the kernel files, in case it could make a difference. I think I read somewhere that the floppy distribution was limited and it may not have support for FAT32, but I don't remember where it was. It would be useful if it was documented in the tools themselves when running with /?, if that's the situation.

The only progress I could make was when calling sys with the undocumented option /FORCE:LBA (I don't remember where I read about it). After that, I got the bootloader to show Loading FreeDOS, but it wouldn't progress any further. I tried again with all 3 kernels, with the same results.

In the end I used instsect linked from this ticket and it worked like a charm. I didn't have to specify any option. I didn't reinstall the kernel or anything else, so it feels like the issue was isolated on the bootloader. (Thank you @ecm-pushbx!)

After this I'm a bit confused about one of the previous comments:

ISSUE STATUS:

Currently the FAT12/16 LBA/CHS and FAT32 CHS boot sectors should work correctly on 8086+ systems but the FAT32 LBA boot sector assumes 386+.

If I understand correctly, using sys /FORCE:LBA would use the latter boot sector, for 386+ with support for FAT32+LBA, right?

So from my point of view (and limited understanding), I think there's 2 related issues here:

1. sys doesn't detect correctly which boot sector to install:
   • the HDD is addressed using LBA
   • the primary partition, where the kernel is located, is formatted using FAT32
   • it's running on a 386+ machine (Pentium)
   • but it's still installing the 8086 boot sector
2. The bootloader, even when using /FORCE:LBA couldn't progress.

In order to mitigate this issue, I think:

• Having a verbose (/V?) option for sys would be very useful to do troubleshooting. instsect showed a lot of details by default.
• /FORCE:LBA should be documented in sys /?.
• Having a way to know where the bootloader gets stuck would be nice. I know at this point every byte counts, but it could be something like showing the Loading FreeDOS message in stages (Load/ing/Free/DOS, or single characters...)