Windows 10 Minimum Hardware Requirements

The Minimum System HW #Requirements & Compatibility for Windows Platforms", states the screen size, RAM, storage, graphics and #firmware mentioned to run Windows 10. For PCs, the OS would need the UEFI 2.3.1 firmware, a minimum screen resolution of 800x600 pixels, and DirectX 9 graphics. For 64-bit PCs, Microsoft specifies a minimum of 2GB of RAM and 20GB of storage. For 32-bit PCs on the other hand, Microsoft specifies a minimum of 1GB of RAM and 16GB of storage. 

As for #mobile devices, the devices would need the same firmware version and graphics as detailed for desktops. The minimum memory requirement however, depends on the screen resolution of the device. Supporting screen sizes ranging from 3-inch to 7.99-inch, the Windows 10 for phones will need a minimum of 512MB of RAM for FWVGA (854x480 pixels) and WVGA (800x480 pixels) resolution devices; 1GB of RAM for WSVGA (1024x600 pixels) devices, HD (1280x720 pixels), WXGA (1366x768 pixles), and qHD (960x540 pixels) devices; 2GB of RAM for full-HD (1920x1080 pixels), WUXGA (1920x1200 pixels), and WSXGA (1440x900 pixels) devices; 3GB of RAM for WQXGA (2560x1600 pixels) and QWXGA (2048x1152 pixels) devices, and finally 4GB of RAM for QSZGA (2560x2048 pixels) resolution devices.

As per the table, Windows 7 RTM, Windows 8, and Windows 8.1 RTM OSes can be only updated via the media (iso image), while Windows 7 SP1 and Windows 8.1 S14 can be updated to Windows 10 both via media and Windows Update.

Also, a minimum of 4GB of inbuilt storage is required for phones, and for handsets with this base spec an SD card slot will be required to support updates. Windows 10 for phones also requires power and volume up/ down buttons, while for phones with a WVGA display, Start, back, and search buttons are also required.