I recently upgraded my ST5112 to 4GB of RAM. No surprise the upper 800 MB were not available due to PCI memory window and 32-bit OS.
I upgraded to Windows 7 Home Premium x64. I was stunned to see that 800 MD are not available yet. I searched and searched to discover that BIOS needs to support memory remapping to free the PCI MMIO 800 MB to the addresses above 4GB boundary.
You claim support of 4GB RAM in the specs for ST5112. How come your BIOS doesn't support remapping? When the update can be expected?