During the Sys_Critical_Init message, the virtual block device initializes and prepares for fast disk devices to register themselves. Each fast disk device identifies itself by calling the BlockDev_Register_Device service during initialization. This is usually done during the Sys_Critical_Init message, but can be carried out during the Device_Init message, if necessary.
A fast disk device must initialize before any client attempts to locate it. If used by the standard swap file device, a fast disk device must either initialize during Sys_Critical_Init, or must initialize during Device_Init and have an initialization order that is less than the swap file device. A fast disk device does not need to initialize before the Interrupt 13h device, because the virtual block device notifies the Interrupt 13 device whenever a fast disk device that supports an Interrupt 13h drive initializes.