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Vol. 3 9-19
PROCESSOR MANAGEMENT AND INITIALIZATION
4. Load segment registers SS, DS, ES, FS, and GS with a selector for a descriptor
containing the following values, which are appropriate for real-address mode:
— Limit = 64 KBytes (0FFFFH)
— Byte granular (G = 0)
—Expand up (E = 0)
— Writable (W = 1)
—Present (P = 1)
— Base = any value
5. The segment registers must be loaded with non-null segment selectors or the
segment registers will be unusable in real-address mode. Note that if the
segment registers are not reloaded, execution continues using the descriptor
attributes loaded during protected mode.
6. Execute an LIDT instruction to point to a real-address mode interrupt table that is
within the 1-MByte real-address mode address range.
7. Clear the PE flag in the CR0 register to switch to real-address mode.
8. Execute a far JMP instruction to jump to a real-address mode program. This
operation flushes the instruction queue and loads the appropriate base and
access rights values in the CS register.
9. Load the SS, DS, ES, FS, and GS registers as needed by the real-address mode
code. If any of the registers are not going to be used in real-address mode, write
0s to them.
10. Execute the STI instruction to enable maskable hardware interrupts and perform
the necessary hardware operation to enable NMI interrupts.
NOTE
All the code that is executed in steps 1 through 9 must be in a single
page and the linear addresses in that page must be identity mapped
to physical addresses.
9.10 INITIALIZATION AND MODE SWITCHING EXAMPLE
This section provides an initialization and mode switching example that can be incor-
porated into an application. This code was originally written to initialize the Intel386
processor, but it will execute successfully on the Pentium 4, Intel Xeon, P6 family,
Pentium, and Intel486 processors. The code in this example is intended to reside in
EPROM and to run following a hardware reset of the processor. The function of the
code is to do the following:
• Establish a basic real-address mode operating environment.
• Load the necessary protected-mode system data structures into RAM.