Setting guest cpu profile as 8086 results in only blank screen?

[Virgil Ming]

Background: I am learning operating system course and I have this assignment of writing a toy bootloader from scratch. It is stored on a virtual floppy, and only displays a string for now.

Today I stumbled upon the paragraph in VBox documentation, stating that I can set guest CPU profile other than my host (Intel Core i5 4285). I attempted to set my VM to 80x86 series, and found that although on 80286 and 80386 things still worked fine as previously, with 8086 option the VM only gives a black screen with a blinking cursor at upper left corner, and nothing else. It won't even show the VBox title screen. When I remove the virtual floppy image, nothing changes, and I also didn't see error like "no OS found". Just a blank screen.

I'm running VirtualBox 5.2.8 on macOS 10.12.6, the machine is Macbook Pro 13' (Late 2013).

I suppose something other than my code is wrong, but am now clueless.

Attached are vbox.log files as Andy have said.

[andyp73]

Without having looked into the code my guess would be that either the 8086 code paths haven't been well tested (not a common use case I would suspect) or that the VirtualBox BIOS runs some instructions that aren't supported by the 8086 and it causes an exception and hangs. If you post your VBox.log file (zipped of course) then there may be some clues in there.

Given your assignment, I would expect that you could do it all with the default setting or with the 80286 setting that you have shown works.

-Andy.

[Virgil Ming]

Andy,

I have added vbox.log. There are two of them. Hope these could be helpful.

For the time being I guess I'll just stick with 80286.