[Qemu-devel] [Bug 1806114] [NEW] Reading sectors from floppy with BIOS INT 13h is broken

[Jacek Wieczorek]

Public bug reported:

I'm developing a game bootable from a floppy disk, written in i386
assembly. I found out it doesn't work on newer QEMU versions. I managed
to isolate the issue and it seems that there's a problem with handling
of BIOS interrupt 13h when it comes to reading disk sectors (AH=02).

I've written a simple test in assembly. It simply accesses four
different floppy disk sectors and prints out the strings they contain.
The problem is, the two latter strings don't show up at all nor the BIOS
interrupt returns an error (CF set). I've attached the code to this bug
report. I use following commands to compile it and run:

$ nasm disk-test.asm -o disk-test.bin
$ qemu-system-i386 -boot a -fda disk-test.bin

After running, the expected output is:

    I am a test string from boot sector
    C:H:S 0:0:2 - Hello
    C:H:S 0:0:3 - there!
    C:H:S 0:1:4 - In QEMU you can't
    C:H:S 1:0:5 - see these two lines! :(

while the actual output is:

    I am a test string from boot sector
    C:H:S 0:0:2 - Hello
    C:H:S 0:0:3 - there!


So far, I found this problem in the current QEMU version for Ubuntu 18.04 
(Debian 1:2.11+dfsg-1ubuntu7.8), as well as in the 3.1.0-rc3 and 2.12.1 
versions, available on the www.qemu.org website. Thus, the issue doesn't seem 
to be very recent.

To be sure, I ran the program on my other machine with older QEMU
version (Debian 2.0.0+dfsg-2ubuntu1.43) and on my RaspberryPi 2 (Debian
1.1.2+dfsg-6+deb7u25) and everything works as expected there. It also
works well in VirtualBox.

I hope these information are useful to you, however, I feel like this
bug may be more related to Seabios. If so, please let me know and I will
report it somewhere else.

** Affects: qemu
     Importance: Undecided
         Status: New


** Tags: bios disk floppy interrupt

** Attachment added: "disk-test.asm"
   
https://bugs.launchpad.net/bugs/1806114/+attachment/5217760/+files/disk-test.asm

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https://bugs.launchpad.net/bugs/1806114

Title:
  Reading sectors from floppy with BIOS INT 13h is broken

Status in QEMU:
  New

Bug description:
  I'm developing a game bootable from a floppy disk, written in i386
  assembly. I found out it doesn't work on newer QEMU versions. I
  managed to isolate the issue and it seems that there's a problem with
  handling of BIOS interrupt 13h when it comes to reading disk sectors
  (AH=02).

  I've written a simple test in assembly. It simply accesses four
  different floppy disk sectors and prints out the strings they contain.
  The problem is, the two latter strings don't show up at all nor the
  BIOS interrupt returns an error (CF set). I've attached the code to
  this bug report. I use following commands to compile it and run:

  $ nasm disk-test.asm -o disk-test.bin
  $ qemu-system-i386 -boot a -fda disk-test.bin

  After running, the expected output is:

      I am a test string from boot sector
      C:H:S 0:0:2 - Hello
      C:H:S 0:0:3 - there!
      C:H:S 0:1:4 - In QEMU you can't
      C:H:S 1:0:5 - see these two lines! :(

  while the actual output is:

      I am a test string from boot sector
      C:H:S 0:0:2 - Hello
      C:H:S 0:0:3 - there!

  
  So far, I found this problem in the current QEMU version for Ubuntu 18.04 
(Debian 1:2.11+dfsg-1ubuntu7.8), as well as in the 3.1.0-rc3 and 2.12.1 
versions, available on the www.qemu.org website. Thus, the issue doesn't seem 
to be very recent.

  To be sure, I ran the program on my other machine with older QEMU
  version (Debian 2.0.0+dfsg-2ubuntu1.43) and on my RaspberryPi 2
  (Debian 1.1.2+dfsg-6+deb7u25) and everything works as expected there.
  It also works well in VirtualBox.

  I hope these information are useful to you, however, I feel like this
  bug may be more related to Seabios. If so, please let me know and I
  will report it somewhere else.

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