EfiCompatibility consists of the Legacy BIOS Driver with various protocols:

• Legacy BIOS Protocol

• Legacy BIOS Platform Protocol

• Legacy Region Protocol

• Legacy 8259 Protocol

• Legacy Interrupt Protocol

Legacy BIOS Protocol is the primary protocol of the CSM and is hardware-independent. Legacy BIOS Platform Protocol is platform-specific and differentiates a platform from other platforms using the same chipset. Legacy Region Protocol is chipset-specific and controls the read-write capability of the memory region 0xC0000-0xFFFFF. Legacy Region Protocol may also optionally manage the hardware to prevent a write to the region from propagating to any aliased memory region. Legacy 8259 Protocol is independent of platform and chipset. Legacy 8259 Protocol controls the Programmable Interrupt Controller 8259 in 32-bit protected mode as well as in 16-bit real mode and manages the interrupt masks and edge/level mode programming. Legacy Interrupt Protocol is chipset-specific and controls the assignment of IRQs to PCI devices.

EfiCompatibility also contains drivers to emulate various traditional software interrupts such as UGA emulation of INT10, Keyboard emulation of INT16, and block I/O emulation.

The driver for UGA emulation of INT10 is needed when traditional Option ROMs are invoked. The UGA controller is in VGA emulation mode and a VGA Option ROM is invoked. This driver translates EFI console-out data into their VGA equivalent. This driver assumes that all INT10 functions are supported, that the Option ROM may access directly the VGA registers and video memory buffers, and that UGA hardware supports a VGA mode that can be switched between UGA and...