The first megabyte of DRAM is tested by the BIOS before the BIOS code is copied from ROM to DRAM.
After copying ROM to DRAM, the BIOS performs a simple memory test (where a write/read of every location with the pattern 55aa55aa is performed).
Note - The simple memory test is performed only if Quick Boot is not enabled from the Boot Settings Configuration screen. Enabling Quick Boot causes the BIOS to skip the memory test.
The BIOS polls the memory controllers for both correctable and non-correctable memory errors and logs those errors into the SP.