Yes, the BIOS needs to support memory remapping if you're mixing soldered and socketed RAM.

Otherwise the extra memory is invisible or causes instability. Most modern laptops handle this automatically, but older or cheaper ones might not.

The short reason: x86 systems have a “memory hole” below 4GB where reserved addresses live (PCIe, ACPI, etc.). Remapping shifts physical RAM above 4GB to avoid overlapping. When you have soldered RAM fixed at, say, 8GB, and you install a 16GB stick, the total is 24GB. Without remapping, the BIOS might only see the first 4GB plus whatever fits in the remaining address space — often halving your upgrade.

Some laptop platforms restrict the total socketed capacity based on the soldered size even with remapping. For example, older Intel 8th-gen U-series laptops cap socketed RAM at 16GB if soldered is 8GB