Analysis of the Principle and Application Scenarios of PQBoot for Multi-System Boot Switching

Analysis of PQBoot’s Multi-OS Boot Switching Principle and Application Scenarios

Download PQBoot tool: Follow the official account and reply with “pq” in the backend to get the download link.

PQBoot does only two things when switching operating systems:

• Sets the partition where the target OS resides as the active primary partition
• Changes the partition type of all other OS partitions in the MBR to 0x17, which marks them as hidden

From the above two operations, we can see:

• Resetting the active primary partition is the key operation that allows PQBoot to successfully boot other systems
• Hiding other OS partitions in the MBR is an additional operation—optional and not strictly necessary

Based on the above, we can derive the prerequisites for using PQBoot:

• Each partition containing an OS must only contain the boot information for that specific OS (i.e., a boot folder and BOOTMGR in the root directory, and a BCD file inside the boot folder)

If the system does not meet the above prerequisites after installation:

• If a single BCD file contains boot entries for multiple OSes, the computer will still pause at the Windows boot manager screen after startup, requiring the user to manually select an OS and press Enter.
  • If the user selects the OS installed on the active primary partition, it will boot normally.
  • ※※ If the user mistakenly selects an OS located on another partition, Windows may initially boot correctly (because winload.exe can be loaded directly by BOOTMGR without needing to mount the partition first), but winload.exe cannot complete its job. Since the selected partition is marked as a forced hidden partition in the MBR, Windows cannot assign it a drive letter (i.e., cannot mount it as C:). As a result, Windows fails to boot and spins endlessly on the logo screen ※※

From the above analysis, we can also conclude: PQBoot is not suitable for modern UEFI firmware or GPT disks.

• Not suitable for UEFI firmware: PQBoot works by changing the active primary partition on the disk. UEFI firmware’s boot process is unrelated to the active primary partition.
• Not suitable for GPT disks: GPT disks have no concept of an active primary partition. Alternatively, all partitions on a GPT disk are considered active primary partitions.

However, if your UEFI firmware has CSM mode enabled by default, it can effectively behave like Legacy BIOS firmware. In this case, concepts like MBR active primary partitions apply, and there is no difference from Legacy BIOS.

Therefore, once you understand the principle behind PQBoot, you can achieve the same boot-switching effect without relying on PQBoot—simply by manually modifying the active primary partition and hidden partition attributes.

A Side Note: Can the BIOS detect whether a partition is marked as hidden on the disk?

The answer is: No.

What does Legacy BIOS actually do during boot?

• BIOS’s job
  1. Initialize hardware
  2. Find a bootable device according to the boot order
  3. Read 512 bytes from the first sector (LBA 0, MBR) of that device
  4. Load those 512 bytes into memory and execute them (jump to 0x7C00)

The BIOS is only responsible for handing the entire MBR code to the CPU for execution. It does not interpret the “hidden” flag in the partition table.

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Who interprets the MBR partition table?

• It is actually the MBR boot code (the 446-byte machine code) that parses the following 64-byte partition table.
• In the partition table, each partition record is 16 bytes, and the 5th byte is the partition type.
• If the type is “hidden,” the OS will recognize it and ignore it—meaning it won’t be mounted.

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Can the BIOS understand “hidden partitions”?

• No.
  The BIOS does not parse the partition table at all, nor does it ignore a partition just because it is marked as 0x17 (hidden NTFS).
• The BIOS simply hands the MBR boot code to the CPU. The interpretation of the partition table and the meaning of the hidden flag are handled by the subsequent MBR code and the operating system.

Please note: All conclusions in this article have been verified through experiments!

Download PQBoot tool: Follow the official account and reply with “pq” in the backend to get the download link.