Warm-start IPL

In this case, the IPL doesn't get control immediately after the reset, but instead gets control from the BIOS or ROM monitor.

The x86 PC BIOS allows extensions, as do various ROM monitors. During the power-up memory scan, the BIOS or ROM monitor attempts to detect extensions in the address space. To be recognized as an extension, the extension ROM must have a well-defined extension signature (e.g. for a PC BIOS, this is the sequence 0x55 and then 0xAA as the first two bytes of the extension ROM). The extension ROM must be prepared to receive control at the extension entry offset (e.g. for a PC BIOS, this is an offset of 0x0003 into the extension ROM).

Note that this method is used by the various PC BOOTP ROMs available. The ROM presents itself as an extension, and then, when control is transferred to it, gets an image from the network and loads it into RAM.