Complex sentence: During this phase, BIOS first executes Power-on self-test (POST), checks existence of boot disk on A drive, if it is not found in the A drive, the ROM BIOS checks for a hard disk.
Split into simpler sentences: During this phase, BIOS first executes Power-on self-test (POST), then checks the existence of boot disk on A drive. If it is not found in the A drive, the ROM BIOS checks for a hard disk.