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MBR88 OS Compatibility Notes

This document surveys operating systems that a person might reasonably install on IBM XT-class (8088/8086) hardware, with notes on how each interacts with the MBR88 boot record and the standard MBR partition table structure.

The focus is on coexistence: can multiple operating systems share a single partitioned hard disk and be independently selected from the mbr88 boot menu?

The Bootable Flag

Each partition table entry contains a one-byte status field at offset 0 of the 16-byte entry. The conventional values are:

Value Meaning
0x80 Bootable / active
0x00 Not bootable / inactive

MBR88 only displays and boots partitions marked 0x80. This is intentional: it prevents accidentally handing control to a data partition whose VBR contains only a "not a bootable disk" stub message.

Unlike conventional MBR loaders, MBR88 permits more than one partition to be marked 0x80 at the same time. All bootable partitions appear in the menu and can be selected freely. The user sets which partitions are bootable using mbrpatch's b command.

Conventional tools such as DOS FDISK enforce a single active partition; they will clear the flag on all other partitions when they set one active. After running such tools you will need to re-run mbrpatch -p to restore the flags on any additional partitions you want visible in the MBR88 menu.

When to Re-run mbrpatch

Several events will require running mbrpatch again after the fact:

  • Installing any OS that writes its own MBR boot code — Xenix, some versions of CP/M-86, and any modern OS installer (Linux, BSD) will overwrite the MBR88 boot record. Run mbrpatch -u to reinstall MBR88, then mbrpatch -p to restore labels and bootable flags.

  • Running DOS FDISK — FDISK clears the active flag on all partitions except the one it makes active. Run mbrpatch -p afterward to re-set 0x80 on any additional partitions you want selectable.

  • Running DOS FDISK /MBR — This command rewrites the MBR boot code with the standard DOS single-boot loader, erasing MBR88. Run mbrpatch -u to restore.

  • Adding or repartitioning — Any time the partition table changes you should verify labels and bootable flags with mbrpatch -p.

Operating System Survey

FreeDOS (ongoing, open source)

FreeDOS is a free, open-source DOS-compatible OS actively developed and tested on real 8088/8086 hardware. It is fully compatible with MBR88 and has been verified working on the Leading Edge Model D test platform used to develop MBR88.

Partition types: 0x01, 0x04, 0x06

VBR 55 AA signature: Present.

MBR overwrite risk: FDISK /MBR rewrites the MBR. Same procedure as MS-DOS.

Bootable-flag behavior: Same as MS-DOS. FreeDOS FDISK enforces one active partition; use mbrpatch -p to restore multi-boot flags afterward.

ELKS — Embeddable Linux Kernel Subset (ongoing, open source)

ELKS is a modern open-source project that ports a minimal Linux-like kernel to 8086/8088 hardware. It is designed with current best practices and full awareness of the MBR boot convention.

ELKS has been verified working on the Leading Edge Model D test platform used to develop MBR88, including multi-partition coexistence with FreeDOS.

Partition type: 0x80 (MINIX) is conventional for ELKS.

VBR 55 AA signature: Present.

MBR overwrite risk: ELKS installation tools may write boot code to the MBR. Reinstall with mbrpatch -u if needed.

Bootable-flag behavior: Standard. Uses 0x80 for the bootable partition. Multi-boot with DOS and other systems works correctly with MBR88.

MS-DOS / PC-DOS 2.0 – 3.3 (1983 – 1987)

The MBR and the 0x80 active-partition convention were introduced with PC DOS 2.0 specifically for the IBM PC XT. All versions of MS-DOS and PC-DOS in this range use a standard FAT VBR with a 55 AA boot signature and are fully compatible with MBR88.

Multiple DOS versions (e.g. PC-DOS 3.1 and MS-DOS 3.3) can coexist on separate FAT partitions. Each can be marked 0x80 and selected independently from the MBR88 menu.

Partition types: 0x01 (FAT12), 0x04 (FAT16 <32 MB), 0x06 (FAT16 ≥32 MB)

VBR 55 AA signature: Present in all versions from 2.0 onward.

MBR overwrite risk: FDISK /MBR rewrites the MBR. Reinstall with mbrpatch -u afterward.

Bootable-flag behavior: DOS FDISK enforces exactly one active partition. After using FDISK, run mbrpatch -p to re-flag any additional boot partitions.

DR-DOS / Novell DOS (1988 onward)

Digital Research DR-DOS (later Novell DOS, then Caldera OpenDOS) is a DOS-compatible OS notable for squeezing LBA, FAT32, and other extensions into single-sector VBR code while maintaining full 8088 compatibility. It is structurally identical to MS-DOS from an MBR perspective.

Partition types: 0x01, 0x04, 0x06

VBR 55 AA signature: Present.

MBR overwrite risk: FDISK /MBR rewrites the MBR.

Bootable-flag behavior: Same as MS-DOS.

Windows 1.x / 2.x (1985 – 1987)

Windows 1.x and 2.x are graphical shells that run on top of MS-DOS; they are not standalone operating systems. Booting means booting DOS first and then launching Windows from within DOS. There are no independent partition or boot record requirements beyond those of the underlying DOS version.

Partition types: Same as the underlying MS-DOS.

VBR 55 AA signature: Present (inherited from DOS).

MBR overwrite risk: None specific to Windows. The DOS installer carries the same risk as noted above.

Bootable-flag behavior: Same as MS-DOS. No special considerations.

CP/M-86 (Digital Research, 1982 – 1983)

CP/M-86 is the 8086/8088 port of the classic CP/M operating system. It predates the standardized MBR partition scheme and has its own partitioning conventions.

Partition type: 0xDB (CP/M, Concurrent CP/M, Concurrent DOS)

VBR 55 AA signature: Not reliably present. Early CP/M-86 boot sectors predate the 55 AA convention and may not carry the signature. MBR88 checks for 55 AA before jumping to the VBR; a CP/M-86 partition without the signature will be reported as "No boot record" rather than booting. Whether a given CP/M-86 installation has the signature depends on the specific version and how it was installed.

MBR overwrite risk: CP/M-86's HDMAINT.CMD disk setup tool may write its own boot code to the MBR. Reinstall MBR88 with mbrpatch -w afterward.

Bootable-flag behavior: CP/M-86's installer may set the status byte to a non-standard value. Verify with mbrpatch -r after installation.

Coexistence with DOS: Structurally possible — CP/M-86 uses its own partition type and the partition table is shared. In practice, run CP/M-86 installation first, then reinstall MBR88, then add the DOS partitions.

SCO Xenix (Microsoft/SCO, 1984 for XT)

SCO Xenix was probably the most widely deployed Unix variant of the mid-1980s. The 8086/8088 version shipped for the PC XT starting around 1984. Xenix uses standard MBR partition table entries and in principle coexists with DOS on the same disk.

Partition types: 0x02 (Xenix root), 0x03 (Xenix /usr)

VBR 55 AA signature: Version-dependent. Unix systems of this era did not always carry the 55 AA signature in the partition boot sector. As with CP/M-86, a missing signature will cause MBR88 to report "No boot record." Testing with your specific Xenix version and install media is necessary.

MBR overwrite risk: High. Xenix installs its own boot loader into the MBR. Reinstall MBR88 with mbrpatch -w after completing the Xenix installation.

Bootable-flag behavior: Xenix may set the status byte differently from the DOS convention. Verify and correct with mbrpatch -p after installation.

Coexistence with DOS: Possible but requires care. Install Xenix first (it will partition and write its MBR), then install DOS into a separate partition, then reinstall MBR88 and set bootable flags on the desired partitions.

Compatibility Summary Table

Operating System VBR 55 AA MBR overwrite risk Bootable flag Multi-boot with DOS
FreeDOS ✓ Yes FDISK /MBR Standard ✓ Yes (tested)
ELKS ✓ Yes Unsure Standard ✓ Yes (tested)
MS-DOS / PC-DOS 2–3 ✓ Yes FDISK /MBR Standard ✓ Yes
DR-DOS / Novell DOS ✓ Yes FDISK /MBR Standard ✓ Yes
Windows 1.x / 2.x ✓ Yes Via DOS installer Standard ✓ Yes (runs on DOS)
CP/M-86 ⚠ Maybe HDMAINT.CMD Non-standard ⚠ Possible
SCO Xenix ⚠ Maybe Installer Non-standard ⚠ Possible

Key:

  • ✓ Yes — works reliably, may have been tested on real hardware
  • ⚠ Maybe — possible but requires verification and careful installation order

Recommended Installation Order for Multi-Boot Setups

When mixing OS types, install in this order to minimize re-work:

  1. Install any OS that writes its own MBR code last, or be prepared to run mbrpatch -u after it (Xenix, CP/M-86).
  2. Install DOS-family OSes in their desired partitions.
  3. Install ELKS in its partition.
  4. Run mbrpatch -u on the disk to install or restore mbr88 boot code.
  5. Run mbrpatch -p to set partition labels and mark all desired boot partitions with status 0x80.
  6. Test each partition from the mbr88 boot menu.

If a partition shows "No boot record," that OS either does not place 55 AA at offset 0x1FE of its VBR, or the VBR was not installed correctly. mbr88 will not chain-load to it in that case — this is a safety measure, not a bug.