The amount of hardware reserved memory either defined in the BIOS/EUFI settings and/or in the system configuration (msconfig).
If you can, update your BIOS/EUFI to the latest version and check how much memory is allocated for the "AGP video apperture" (this could be called something different, depending on your system's BIOS). Try to lower this value, you can always change it back if you have issues.
In the "msconfig" window (run it as admin), check the advance setting under the "Boot" tab. If the "Maximum memory" is checked, uncheck it. Windows will allocate memory as the hardware needed, but you can also put back the check mark if you experience issues.
Both Intel and AMD CPUs with integrated GPUs reserve memory in the BIOS/EUFI and possibly in Windows, if the drivers/software package is installed.