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The DOS 3.3 sys.com bug hunt

YS.COM corrupted a NetDrive image. But why? Posted: 2025-02-22 Tags: DOS, NetDrive, ForgotToCheckReturnCode In ye olden days to make a diskette bootable you had to format it using the /s option of the FORMAT command.

(Note that the segment registers changed causing the instruction pointer to shift ... we are still in the same code though, just using aliased memory locations. So the failure to check the Carry Flag wound up causing bad BPB data to be written to the volume boot record. When in doubt about what it is working with (which is always true of a hard drive) DOS 3.3 SYS.COM will try to use the Generic IOCTL call to get the BPB of the device instead of just reading it directly from the first sector.

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