Intel 64 and IA-32 Architectures Software Developers Manual Volume 3B, System Programming Guide Part 2

Table Of Contents
25-18 Vol. 3
SYSTEM MANAGEMENT
The upper word of the SMM revision identifier refers to the extensions available. If
the I/O instruction restart flag (bit 16) is set, the processor supports the I/O instruc-
tion restart (see Section 25.12); if the SMBASE relocation flag (bit 17) is set, SMRAM
base address relocation is supported (see Section 25.11).
25.10 AUTO HALT RESTART
If the processor is in a HALT state (due to the prior execution of a HLT instruction)
when it receives an SMI, the processor records the fact in the auto HALT restart flag
in the saved processor state (see Figure 25-3). (This flag is located at offset 7F02H
and bit 0 in the state save area of the SMRAM.)
If the processor sets the auto HALT restart flag upon entering SMM (indicating that
the SMI occurred when the processor was in the HALT state), the SMI handler has
two options:
It can leave the auto HALT restart flag set, which instructs the RSM instruction to
return program control to the HLT instruction. This option in effect causes the
processor to re-enter the HALT state after handling the SMI. (This is the default
operation.)
It can clear the auto HALT restart flag, with instructs the RSM instruction to
return program control to the instruction following the HLT instruction.
Figure 25-2. SMM Revision Identifier
Figure 25-3. Auto HALT Restart Field
SMM Revision Identifier
I/O Instruction Restart
SMBASE Relocation
Register Offset
7EFCH
31
0
Reserved
18 17 16 15
Auto HALT Restart
015
Reserved
Register Offset
7F02H
1