Today you can’t buy a new x86 processor running at less than 1GHz, and most mainstream desktop CPUs have a base clock frequency of more than 2GHz. However, back in the 1980s and 1990s, processors running at just a few MHz were common, with the original IBM PC itself clocked at just 4.77MHz.
Can a modern Windows operating system run on such a low clock rate CPU? Developer and YouTuber NTDEV has proven it can, successfully booting and using Windows 7 Ultimate from 2009 on a Pentium-S processor downclocked to just 5MHz. That’s a full 995MHz short of Windows 7’s minimum requirement of 1GHz. And the test system has only 128MB of memory, which is far below Windows 7’s minimum requirement of 1GB of memory.
The video shows that the Windows 7 system is actually booted from a virtual machine running in the 86Box emulator, showing its 5.00 MHz clock speed after starting a program, and even running Notepad. It is worth mentioning that the Windows 7 system takes up to 28 minutes to start.
To get Windows 7 to boot and run with such a slow CPU, NTDEV said he had to disable a lot of system resources, which he did by running Safe Mode and disabling most drivers and services so that only three services were running on boot To run, he also had to fix a problem with logonUI.
“The logon screen (logonUI) didn’t want to load when the frequency was below 50MHz,” NTDEV said, “so in order to get to a command prompt, I had to modify the registry and delete the c:\windows\system32\oobe folder everything, putting the OS in a pseudo-OOBE state so that it doesn’t load.”
The system desktop shown by NTDEV in the video has no start menu, no wallpaper, and the theme of the window itself is very simple. In order to start the WCPUID / Real Time Clock Checker application, NTDEV must enter its shortcut in the command prompt name, and so does launching Notepad.
Most impressively, NTDEV was able to run four different programs simultaneously at one point: Command Prompt, WCPUID, Winver (shows Windows 7 version), and Notepad with some text, so it’s a pretty stable system environment.
NTDEV also claims that he has actually run Windows 7 at clock speeds as low as 3MHz, but it wasn’t powerful enough to make interesting videos. He said that in the past he actually used only 36MB of memory running Windows 7, but used 128MB to make the sample system not need a paging file (virtual memory), but in his demo, the system actually used about 70MB.
The storage space used by the virtual machine is also much less than the 16GB listed in the system requirements for Windows 7. NTDEV says the entire installation process used less than 1GB of space, and the operating system’s .wim file disk image was less than 350MB.
NTDEV says he is researching how to get Windows 10 or Windows 11 to run on a processor slower than 1GHz, and he has managed to get Windows XP to run at just 1MHz.
He also said that the 28-plus-minute boot time of a Windows 7 system running on a 5MHz processor was far from the slowest he had ever experienced. “That’s nothing compared to Windows XP running on a 1MHz processor,” he said. “That thing takes 3 hours to boot!”
