Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

4-360 Vol. 2B
INSTRUCTION SET REFERENCE, N-Z
SYSCALL—Fast System Call
Description
SYSCALL saves the RIP of the instruction following SYSCALL to RCX and loads a new
RIP from the IA32_LSTAR (64-bit mode). Upon return, SYSRET copies the value
saved in RCX to the RIP.
SYSCALL saves RFLAGS (lower 32 bit only) in R11. It then masks RFLAGS with an
OS-defined value using the IA32_FMASK (MSR C000_0084). The actual mask value
used by the OS is the complement of the value written to the IA32_FMASK MSR.
None of the bits in RFLAGS are automatically cleared (except for RF). SYSRET
restores RFLAGS from R11 (the lower 32 bits only).
Software should not alter the CS or SS descriptors in a manner that violates the
following assumptions made by SYSCALL/SYSRET:
• The CS and SS base and limit remain the same for all processes, including the
operating system (the base is 0H and the limit is 0FFFFFFFFH).
• The CS of the SYSCALL target has a privilege level of 0.
• The CS of the SYSRET target has a privilege level of 3.
SYSCALL/SYSRET do not check for violations of these assumptions.
Operation
IF (CS.L ≠ 1 ) or (IA32_EFER.LMA ≠ 1) or (IA32_EFER.SCE ≠ 1)
(* Not in 64-Bit Mode or SYSCALL/SYSRET not enabled in IA32_EFER *)
THEN #UD; FI;
RCX ← RIP;
RIP ← LSTAR_MSR;
R11 ← EFLAGS;
EFLAGS ← (EFLAGS MASKED BY IA32_FMASK);
CPL ← 0;
CS(SEL) ← IA32_STAR_MSR[47:32];
CS(DPL) ← 0;
CS(BASE) ← 0;
CS(LIMIT) ← 0xFFFFF;
CS(GRANULAR) ← 1;
SS(SEL) ← IA32_STAR_MSR[47:32] + 8;
SS(DPL) ← 0;
Opcode Instruction
64-Bit
Mode
Compat/
Leg Mode Description
0F 05 SYSCALL Valid Invalid Fast call to privilege level 0
system procedures.