IA32SYSENTERCS

IA32_SYSENTER_CS (often referred to as IA32SYSENTERCS in shorthand) is a model-specific register in the IA-32 architecture that stores the kernel code segment selector used by the SYSENTER/SYSEXIT fast system call mechanism. The value programmed into this MSR is loaded into the CS register when the SYSENTER instruction executes, switching the processor from user mode to the kernel code segment.

For correct operation, the selector must refer to a valid, present descriptor in the Global Descriptor Table (GDT) or Local Descriptor Table (LDT) with privilege level 0. The SYSENTER sequence is complemented by two other MSRs that supply the remaining state needed for the transition: IA32_SYSENTER_ESP provides the kernel stack pointer (to load into ESP/RSP) and IA32_SYSENTER_EIP provides the address of the kernel entry point to jump to.

Access to IA32_SYSENTER_CS is restricted to privileged code; writes to this MSR require ring-0 access via the WRMSR instruction, and user-space processes cannot modify it directly. Operating systems typically configure these MSRs during boot or process initialization to establish a fast path into the kernel.

In 64-bit environments, SYSENTER remains supported primarily for compatibility with 32-bit code, but modern x86-64 systems more commonly use the SYSCALL/SYSRET mechanism for system calls. Nevertheless, the IA32_SYSENTER_CS register remains part of the legacy fast system call interface and is documented in Intel manuals for completeness.

See also: SYSENTER, IA32_SYSENTER_ESP, IA32_SYSENTER_EIP, SYSRET, and Intel's Software Developer Manuals.