SHADED AREAS INDICATE UNUSED CIRCUITS. DOTTED LINE SHOWS INTERRUPT PATH.
The IMCR is supported by two read/writable or write-only I/O ports, 22h and 23h, which receive
address and data respectively. To access the IMCR, write a value of 70h to I/O port 22h, which
selects the IMCR. Then write the data to I/O port 23h. The power-on default value is zero, which
connects the NMI and 8259 INTR lines directly to the BSP. Writing a value of 01h forces the
NMI and 8259 INTR signals to pass through the APIC.
The IMCR must be cleared after a system-wide INIT or RESET to enable the PIC Mode as default.
(Refer to Section 3.7 for information on the INIT and RESET signals.)
The IMCR is optional if PIC Mode is not implemented. The IMCRP bit of the MP feature
information bytes (refer to Chapter 4) enables the operating system to detect whether the IMCR is
Figure 3-2. PIC Mode