Re: [hatari-devel] TOS 3.06 does not boot from ACSI

[Christian Zietz <czietz@xxxxxxx>]

Thomas Huth schrieb:

> OK, thanks, that's good to know ... but apparently, TOS 3.06 also reads
> the value from the VME bus GPIO register ... maybe as a fall back if the
> NVRAM is not containing a sane value?

A small clarification: $ffff8e09 is *not* a VME bus register as such,
but it is a GPIO register of the TT's SCU. So the VME bus is not
involved in any way.

Also, here is part of the code that you omitted:

00E00B14                 move.b  ($FFFF8E09).w,d0
00E00B18                 and.w   #$F8,d0
00E00B1C                 move.w  d0,($A00).w
00E00B20                 bne.s   loc_E00B3E
00E00B22                 pea     ($A00).w
00E00B26                 move.w  #2,-(sp)
00E00B2A                 clr.l   -(sp)
00E00B2C                 jsr     sub_E0208C

.... where the last JSR jumps into the NVRAM reading subroutine that then
overwrites the boot selection in $A00. Hence, one can see that if the
upper 5 bits of this GPIO register (note the AND.W #$F8,D0) are
non-zero, this takes preference over the NVRAM.

Now, sadly I don't own a TT to test this theory, but I suppose that the
GPIO register is preserved across a reset and thus allows a one-time
(not permanently stored in NVRAM) boot selection.

Also note that the least significant bit of $ffff8e09 is used for a
different purpose -- that I didn't further investigate. I'm not sure if
it's correct to simply set this to 0x01 as Hatari now does.

Regards
Christian
-- 
Christian Zietz  -  CHZ-Soft  -  czietz@xxxxxxx
WWW: http://www.chzsoft.de/
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