problem with rom

[Steve Gehlbach]

Gregg C Levine wrote:
> Hello from Gregg C Levine
> Steve, now I am completely confused. Can you elaborate on the term
> "gdt"? I am familiar with its use, on the PC, during the time period
> that covers the pre-Linux era. Namely working in assembler for the
> IBM-PC, itself, during the 1980s to the 1990s.


The gdt is one of a number of registers in the IA-32 model used for 
protected mode memory management (ie, for 32-bit mode).  Typically, it 
is loaded to point to a data structure that defines a flat (ie, 4 GB) 
data segment, and an identical code segment, although you can in reality 
have many segments defined. These segments are referenced by their 
offset into the gdt data structure, for example in linux startup, 0x18 
defines the data segment, and you would put this into the %ds register. 
  This means the segment is defined 24 bytes into the gdt table.  This 
is very different, of course, from how you use the %ds in real or 16-bit 
mode.

Originally in linuxbios, the gdt was defined in flash, and not changed 
until jumping to linux.  However, linux will hang if the gdt is located 
above 1M (not sure why, but I have tested this many times).  So if you 
setup biosbase=0xffff0000, this would put the gdt high, and it has to be 
loaded low so linux will work.  However, recently, Eric setup a gdt 
reload in c_start.S, which is relocated into ram below 1M, and so this 
will work fine for linux.  This makes some gdt reload code I put into 
the sis630 area unnecessary.

But the bottom line is you have to setup the gdt to point to a 
compatible (read that identical) gdt table to linux, before jumping to 
linux.  Linux expects to be in 32-bit mode with the gdt data structure a 
certain way.  You can see it at the bottom of 
linux/arch/i386/boot/setup.S in the kernel source tree.

Anytime you go into protected mode, you have to setup the gdt, and 
usually also the idt, which handles interrupts.

You can find more that you would ever want to know about it in the Intel 
reference manuals, but most folks just copy the table data from someone 
else.  It is very painful code to debug, many bytes of the entries in 
the table requiring bit by bit settings.

-Steve