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Hardware Initialization Code

The hardware initialization portion of the boot 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 new iMac family computers is different from that on earlier iMac 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 I² 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

The firmware in the boot ROM sets up and sizes memory, then stores the information in the device tree where it is available to the OS. The firmware obtains information about the memory by way of the Serial Presence Detect mechanism of the DIMM that is used for memory expansion. The JEDEC standard for the DIMM mandates that all DIMMs include a ROM with information about the memory. The Uni-N IC reads that information from that ROM by way of the I² C bus.