The hardware initialization portion of the ROM contains calls to a series of Power-On Self Test (POST) routines. The principle features are a ROM checksum test, memory testing, detection of the manufacturing test pin, and test manager support. These diagnostics run in native Power PC code.
The hardware initialization code on the Power Mac G4 computer is different from that on earlier Power Macintosh models because of the new ICs (Uni-N and KeyLargo). Additional diagnostics are run out of the ROM in emulation.
Functions of the hardware initialization code include
initialization of the Uni-N bridge and memory controller IC
memory sizing using I
2
C presence detect on the memory module
memory timing setup with support for fast SDRAM devices
L2 cache detection and sizing
initialization of the KeyLargo I/O controller IC
generating diagnostic sounds, including the boot chord that is emitted when hardware initialization has been successfully completed