Documentation

[Steve Gehlbach]

steven james wrote:
> All x86
> machines start at 0xf000:fff0 in real mode.
> 

To be strictly correct, x86 machines start at 0xfffffff0 in real mode. 
( IA-32 Intel Architecture Software Developer’s Manual Volume 3, System 
Programming Guide, sect. 9.1.4.).

Quoting the manual, for those that don't have it handy:

"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 register has two parts: the 
visible segment selector part and the hidden base address part. In 
real-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)."

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If the biosbase option is not set, almost immediately, linuxbios does a 
far jump to 0xf0000:0004, and so reloads the segment register such that 
the aliasing of this address to the top 4G becomes important, as Steven 
James points out. But setting biosbase=0xffff0000 will use a relative 
jump and actually execute physically in the top 4G.

-Steve