ISA: x86
Mode: Real, Protected, Compatibility, Long
Bitness: 16-bit, 32-bit, 64-bit
CPL: 0, 3
OS: Windows
Language: C
Sample code that demonstrates the usage of the SMSW instruction to fetch the Machine Status Word (bits 0 through 15 of control register CR0) and check the processor's Current Privilege Level (CPL) if the processor is running in Protected mode

Usage

There aren't a lot of useful bits in the MSW
One notable exception: PE (Protection Enable) bit (0)
This flag enables Segmentation
The processor maintains an internal state called CPL which is defined as the privilege level of the currently executing code
CPL is used to determine whether certain operations are permitted or not
CPL is given by CS.RPL (bits 0 and 1)

C:\pub\bld\DriverRun>GetCPL.exe
[*] GetCPL Built Jun 16 2025 18:13:39
Press any key to continue . . .
[DBG]: user-mode
Press any key to continue . . .

[DBG]: +++ GetCPL.sys Loaded +++
[DBG]: GetCPL.sys Built Jun 17 2025 07:12:51
[DBG]: GetCPL: DriverObject = FFFFC486721BDAF0
[DBG]: GetCPL: RegistryPath = \REGISTRY\MACHINE\SYSTEM\ControlSet001\Services\GetCPL
[DBG]: supervisor-mode
[DBG]: --- GetCPL.sys Unloaded ---

Tested OS Versions

Windows 11 21H2 Build 22000 Revision 675 64-bit

README.md

EVENSTAR - GetCPL

Version

Brief

Usage

Tested OS Versions

References