stosw fails with std in real mode => Fixed in SVN

[Joao Trindade]

At least in real mode, the string instruction "stosw" presents an incorrect behaviour when the direction flag is set.

I have observed this behaviour while testing boot code of a personal OS. The same code works fine in Bochs 2.3.7 for Linux, QEMU 0.12.2 for Windows and VMware Player 3.1.0 for Windows.

Attached: 1) Binary image of an 1.44M floppy with said behaviour in its boot sector 2) Source code for the boot sector

[Joao Trindade]

1) Image of 1.44M floppy with the failing behaviour

[Joao Trindade]

2) Source code for the boot sector

[Joao Trindade]

I forgot to say that the floppy should show the following string in the bottom of the screen:

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>><<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

However, in VirtualBox (3.2.6) it shows only:

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

This happens with or without the use of the prefix "rep". Similar tests with "movsw" don't seem to present this problem.

[Frank Mehnert]

Thanks for this report and the testcase! Fixed in SVN, the fix will be part of the next maintenance release.

[Frank Mehnert]

Fixed as of VBox 3.2.8.