Part four - MS-DOS debugging
After a week I’ve stared to reverse the MS-DOS boot process to find the bug that is keeping this WYSE tin client from booting DOS 6 from the DOM and is sitting there only with a blinking cursor. So I decided to build a new image, only with DOS 6 on it, extract the MBR from there and disassembly this code in IDA. The new DOS 6 image was booting well on Bochs virtual machine so I know that the image is good. The MBR: MBR structure On the left image you can see the MBR structure. From the 512 bytes that it has, 446 bytes are just code that is executed after the BIOS passes control to the MBR. The last 66 bytes represent the Partition Table and the Magic Word: 55AAh. MS-DOS 6 MBR code does not print anything on the screen by default, but it doesn’t mean that is not working on the WYSE machine. I have found that it has a function to print an error if it encounters one. This was good, because I have used thi...