The BIOS POST memory testing is performed as follows:
The first megabyte of DRAM is tested by the BIOS before the BIOS code is shadowed (that is, copied from ROM to DRAM).
Once executing out of DRAM, the BIOS performs a simple memory test (a write/read of every location with the pattern 55aa55aa).
The BIOS polls the memory controllers for both correctable and uncorrectable memory errors and logs those errors into the service processor.