A retro computer enthusiast managed to run Windows XP on a computer with an Intel i486 processor. Now, for many people, the time of release of both seems like antiquity, but in fact, ten years have passed between the release of the processor and the OS — they are not compatible, that is, they were until now. Author set out ISO image of a modified Windows XP SP3 (German).
Windows XP was introduced in 2001. It survived for quite a long time and was popular among PC users even after the end of support in 2014. The Intel i486 processor was released in 1989 and was Intel’s fastest processor until the Pentium was released in 1993. Thus, the processor and OS eras do not overlap.
The minimum system requirements of Windows XP also do not allow you to install it on the i486 from the OS. It is stated that the OS requires a minimum of a Pentium (586) or compatible chip that runs at 233 MHz or higher, 64 MB of RAM, and at least 1.5 GB of storage. The i486 processor is 133 MHz, and some instances have been successfully overclocked. In addition, i486 systems typically had less memory.
The enthusiast Dietmar used the disassembly and debugging tools to view the operation codes. With this detailed information, he replaced the operation codes that were shutting down the CPU with those that the i486 understood. After this change and adjusting for other incompatibilities, he was successful. The computer works, but the system information is not displayed very adequately, as you can see in the screenshot.
Source: Tom’s Hardware