Today, you can't buy a new x86 processor that runs at under 1 GHz, with most mainstream desktop CPUs carrying base clocks that are well over 2 GHz or even in the 3 GHz range and boosting up much higher than that. However, back in the 1980s and 1990s, processors that operated at just a few MHz were common, with the original IBM PC clocking in at just 4.77 MHz itself.
Could one run a modern Windows operating system on a CPU with such a low clock speed? Developer and popular YouTuber NTDEV has proven that you can, booting and using Windows 7, which launched in 2009, on a Pentium-S processor that was downclocked to just 5 MHz. That's a full 995 MHz below the 2009-era OS's 1-GHz minimum requirement. The test system also had just 128MB of RAM, which is way short of Windows 7's 1GB minimum requirement.
In a YouTube video (embedded below), NTDEV shows the system, which is actually a virtual machine running in the 86Box emulator, boot up into Windows 7 Ultimate, launch a program which shows its 5.00 MHz clock speed and even run Notepad. By the way, if you follow the sped-up time counter in the video, you'll note that it takes more than 28 minutes for the Windows 7 desktop to appear!
In the video, you can see NTDEV power on his virtual machine which POSTs as a Pentium-S running at 50 MHz with a 128MB of RAM. However, it has been downclocked to 5 MHz, a low speed which NTDEV told us he achieved by editing 86Box's source code.
NTDEV boots the system and is given a choice to use Normal or Safe modes. He chooses Normal mode, but it actually boots into Safe mode, showing all of the .sys, .dll and .exe files as they load. NTDEV told us that, as part of the hacking process, he edited the BCD (boot configuration data) so even choosing Normal mode results in Safe mode booting.
To get Windows 7 to boot and run with such a slow CPU, NTDEV told us that he had to disable a ton of system resources, which he accomplished by running Safe mode and disabling most drivers and services to the point where only three services are running at boot. He also had to address an issue with the logonUI.
"When going below 50MHz the login screen (logonUI) doesn't want to load," NTDEV told us. "So in order to get to a command prompt, I had to put the OS in a pseudo-OOBE state by modifying the registry and deleting everything in the c:\windows\system32\oobe folder, so that it wouldn't load."
The OOBE (out of the box experience) is the one you get when running a Windows installer and it doesn't have a Start menu or Windows Explorer. However, the command prompt is usually available during OOBE and you can usually get it by hitting Shift + F10.
We noted that the desktop NTDEV shows in the video has no Start menu, there's no wallpaper and the windows themselves have a very simple theme; there's no way Aero transparencies would work here. To launch a program, WCPUID / Real Time Clock Checker, NTDEV has to type its shortcut name into the command prompt. He later does the same to launch Notepad.
Perhaps what's most impressive about this whole video is that NTDEV is able to, at one point, have four different programs running: the command prompt, WCPUID, Winver (showing the Windows 7 version) and Notepad with some text in it. So this is a fairly-stable environment.
NTDEV told us that he has actually run Windows 7 at a clock speed as low as 3 MHz, but that it wasn't functional enough to make an interesting video. He said that he has actually run Windows 7 with as little as 36MB of RAM in the past, but went with 128MB to make the sample system not need the page file (virtual memory), but the system actually used about 70MB during his demo.
The virtual machine also uses a lot less storage space than the 16GB listed in Windows 7's system requirements. NTDEV said that the entire install uses less than 1GB and the .wim file disk image for the OS is less than 350MB.
Now that he's gotten Windows 7 to run at 5 MHz (or even 3 MHz with less to do), NTDEV says he's looking at ways to get Windows 10 or Windows 11 to run on a processor that's slower than 1 GHz. He's already managed to get Windows XP to run at just 1 MHz.
He said that the 28-minute plus boot time for the 5 MHz Windows 7 system is far from the slowest he's experienced.
"It's nothing compared to Windows XP on 1 MHz," he said. "That thing took 3 hours to boot!"
