Boot sector code which can boot both MS-DOS and PC DOS

Question

Has there been boot sector code (either as binary or assembly source) released, which can boot MS-DOS and PC DOS, by autodetecting the files found in the root directory of the FAT12 or FAT16 filesystem? (It loads io.sys and msdos.sys, or ibmbio.com and ibmdos.com, whichever it finds. If it finds both pairs, then I don't care which one it loads.)

I know that I can install GRUB4DOS to the MBR (+ ~1 MiB), and configure it for such autodetection, but I'd prefer something more lightweight (i.e. less than 512 bytes of machine code).

Answer 1

I don't know that one has been written, but it looks like it should be pretty easy to write one. If we look at the code for DOS 4.0 on GitHub, the boot sector has code like this:

    MOV DI,BX
    MOV CX,11
    MOV SI,OFFSET BIO       ; point to "ibmbio  com"
    REPZ    CMPSB           ; see if the same
    JNZ CKERR           ; if not there advise the user
;
; Found the BIOS.  Check the second directory entry.
;
    LEA DI,[BX+20h]
    MOV SI,OFFSET DOS       ; point to "ibmdos  com"
    MOV CX,11
    REPZ    CMPSB
    JZ  DoLoad

To make it work for either, we'd basically duplicate this code, but in the duplicate modify the data to search for "IO SYS" and "MSDOS SYS" (which are already present in the code, and the PC-DOS/MS-DOS version selected with an ifdef). At the jnz CKERR, you'd jump to the second set of code to search for the second set of entries. And if either pair matches, do the jz DoLoad. That adds around 30 bytes of code and 22 bytes of data for the directory entries. I haven't assembled the existing boot sector to see whether there's ~55 bytes free to do this the easy way, or whether you'd have to get tricky to make it fit. There is a comment further down in the code saying there's not room for a full 32-bit division though, so fitting this change in could be non-trivial.

Reference

https://github.com/microsoft/MS-DOS/blob/main/v4.0/src/BOOT/MSBOOT.ASM

[lines 256-268]