Specifications

Design Discussion
3
thunk
The code to transition from native execution mode to 16-bit real mode and
back.
traditional OpROM
16-bit OpROMs that are executed in real mode.
2.2 CSM-Specific References
The following reference is useful for implementing CSM code. See References in the
master help system for additional related specifications.
• IBM Personal System/2 and Personal Computer BIOS Interface Technical
Reference. Second edition. IBM Corporation, IBM No. S68X-2341-00, 1988.
2.3 CSM Overview
Legacy Overview
This document describes the additional EfiCompatibility functionality (over the standard
EFI) that is provided to support traditional BIOS (non-EFI) OSs and/or traditional
OpROMs. This functionality along with the associated IBV Compatibility16 and
CompatibilitySmm code is called the Compatibility Support Module (CSM).
It is expected that traditional OpROM support will be required longer than traditional OS
support. The figure below presents a block-diagram-level overview of how a legacy
system operates using the CSM.