On Fri, 28 May 1999, Bjorn Eriksson wrote:
> .org 0xfff0 ; Power-up Entry Point
> JMP_AP(0xf000, power_onA)
[...]
> .org 0xf200
> power_onA:
> cli
> cld
>
> mov ax, cs
> mov ds, ax
Uhh, I don't think this is kosher. On x86, the instruction pointer at
reset is always at the very top of the physical address space (minus 0xf).
That means on a 32-bit bus, it's up at 0xfffffff0 (some chips, like the
386EX, only have 64MB of address space). So even though in 16-bit mode,
you can only see CS holding 0xf000, the selector that's loaded into it on
power-up is _actually_ pointing up at 0xffff0000. It's not very clear from
the reference manuals, I know, but that's what it's doing.
This means you can't copy CS to DS and get what you want - you'll be
pointing in the low megabyte, but your boot ROM will be in the upper meg.
Instead you have to access all your data relative to cs with segment
overrides, use immediate data, or copy your bootstrap to some part of the
lower meg. Of course, if you do that, you have to do it after you've setup
the chipset to configure the RAM. And if your boot device is FLASH, you'll
have to copy your code to lower memory if you want to add a
FLASH-programming routine - the bus cycles of programming the part
generally can't be interleaved with reading (and executing).
--
"Love the dolphins," she advised him. "Write by W.A.S.T.E.."
