Ram usable space is available for use.
Ram usable space is available for use.
It's common to experience limited RAM usage after a fresh Windows 10 install, especially if you're using a discrete GPU. Running a virus can also affect system performance. With 24GB installed and only 23.9GB usable, your system is working within constraints. The 100MB of RAM you mentioned can help, but it depends on your application needs and settings.
The minor variation is typical. It remains exclusive to Windows.
Hardware often sets aside some memory for itself, like video cards and integrated graphics. NVMe drives without DRAM caches are another example. You can usually see a more detailed list by opening Task Manager and checking the right section. A gap of around 100MB is unlikely to cause issues—it's less than half a percent. If you really need extra memory and can't upgrade, consider a 16GB Optane stick (often found for around $7 on eBay). Use it as a page file, and it should be sufficient at a reasonable cost. The remaining RAM isn’t typically used very often.
Windows has stored the missing data. It won’t be noticeable at all. Simply launching any browser will consume more than that.
Your system features a 4690K CPU with an integrated GPU. The motherboard supports it, reserving 128MB of your RAM for its use. The GPU stays active as a secondary dedicated unit regardless of whether you have a display connected. You can only adjust the UEFI settings on your motherboard to disable it, though this may just prevent video outputs from functioning rather than fully turning it off.