Get PC BIOS Back on UEFI-Only Systems with CSMWrap

Citing the rise of UEFI-only systems, a group of tech-savvy enthusiasts has created a clever hack known as CSMWrap. This ingenious tool utilizes the Compatibility Support Module (CSM) and VESA VBIOS from the SeaBIOS project to emulate a legacy BIOS environment on these modern systems.

The process is relatively straightforward. Users simply need to employ csmwarp.efi as their bootloader, which can be placed in the EFI partition. From there, they can boot into this custom environment. However, it's crucial to note that Secure Boot must be disabled, and Above 4G Decoding in BIOS/UEFI settings should also be turned off.

Enabling above 4G decoding is almost a necessity when using CSMWrap, as UEFI firmwares tend to place the GPU's Video Random Access Memory (VRAM) Barrier above the 4GB mark. Legacy BIOS, on the other hand, are limited to accessing only the first 4GB of memory – a 32-bit limitation that can pose challenges for some systems.

CSMWrap relies on the EFI_LEGACY_REGION2_PROTOCOL to enable writing to the legacy region, but this may not be supported by all systems. As an alternative solution, it will attempt to utilize PAM registers in the chipset for decoding – a process that is not guaranteed to succeed.

Furthermore, Windows XP/7's video modesetting logic remains somewhat enigmatic. In some cases, CSMWrap might try to set an incompatible mode using the int10h function, resulting in flickering or even a black screen after transferring control to the legacy OS. This is a known issue that may be resolved in future updates.

Fortunately, users can attempt to inject the GPU driver directly into the OS image to avoid relying on VESA BIOS altogether. While this workaround may not address all the issues, it offers an alternative solution for those who are experiencing problems with CSMWrap.

Getting Started with CSMWrap

To begin using CSMWrap, users should first ensure that their system meets the necessary requirements. This includes disabling Secure Boot and Above 4G Decoding in BIOS/UEFI settings, as well as placing csmwarp.efi in the EFI partition.

Additionally, it's recommended to verify that the system supports the EFI_LEGACY_REGION2_PROTOCOL, which is a crucial component of CSMWrap's functionality. For systems that do not support this protocol, users will need to explore alternative solutions or workarounds.

While CSMWrap presents an exciting opportunity for UEFI-only system enthusiasts to experience the benefits of legacy BIOS, it's essential to acknowledge its limitations and potential issues. As with any custom solution, patience and persistence may be required to resolve problems and optimize performance.