Intel ARCHITECTURE IA-32 User Manual download pdf (Page 380)

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9-6 Vol. 3A

PROCESSOR MANAGEMENT AND INITIALIZATION

9.1.4 First Instruction Executed

The first instruction that is fetched and executed following a hardware reset is located at physical

address FFFFFFF0H. This address is 16 bytes below the processor’s uppermost physical

address. The EPROM containing the software-initialization code must be located at this address.

The address FFFFFFF0H is beyond the 1-MByte addressable range of the processor while in

real-address mode. The processor is initialized to this starting address as follows. The CS

address mode, the base address is normally formed by shifting the 16-bit segment selector value

4 bits to the left to produce a 20-bit base address. However, during a hardware reset, the segment

selector in the CS register is loaded with F000H and the base address is loaded with

FFFF0000H. The starting address is thus formed by adding the base address to the value in the

EIP register (that is, FFFF0000 + FFF0H = FFFFFFF0H).

The first time the CS register is loaded with a new value after a hardware reset, the processor

will follow the normal rule for address translation in real-address mode (that is, [CS base

address = CS segment selector * 16]). To insure that the base address in the CS register remains

unchanged until the EPROM based software-initialization code is completed, the code must not

contain a far jump or far call or allow an interrupt to occur (which would cause the CS selector

value to be changed).

9.2 X87 FPU INITIALIZATION

Software-initialization code can determine the whether the processor contains an x87 FPU by

using the CPUID instruction. The code must then initialize the x87 FPU and set flags in control

A hardware reset places the x87 FPU in the state shown in Table 9-1. This state is different from

the state the x87 FPU is placed in following the execution of an FINIT or FNINIT instruction

(also shown in Table 9-1). If the x87 FPU is to be used, the software-initialization code should

execute an FINIT/FNINIT instruction following a hardware reset. These instructions, tag all

data registers as empty, clear all the exception masks, set the TOP-of-stack value to 0, and select

the default rounding and precision controls setting (round to nearest and 64-bit precision).

If the processor is reset by asserting the INIT# pin, the x87 FPU state is not changed.

9.2.1 Configuring the x87 FPU Environment

Initialization code must load the appropriate values into the MP, EM, and NE flags of control

suggested settings for these flags, depending on the IA-32 processor being initialized. Initializa-

tion code can test for the type of processor present before setting or clearing these flags.

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Intel ARCHITECTURE IA-32 User Manual Page 380

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