Re: A86: Hooking interrupt idea
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Re: A86: Hooking interrupt idea
In a message dated 97-10-26 13:22:20 EST, you write:
>
> Steve86Asm@aol.com wrote:
> >
> > When the chip is off, aside from refreshing the RAM, it does nothing.
The
> ON
> > key, I believe, is connected to something that, when pressed (at least
if
> the
> > calc is off), will trigger the Non-Maskable Interrupt (NMI) lead. This
> takes
> > priority on the cpu, so it restarts at 66H and then starts up.
> >
> > At least, this is what I've inferred from the Z80 manuals. Anyone
> > disassembled the contents of 66H?
>
> There are no NMI on the TI-8x calcs. First, 66h is in the middle of the
> default
> 38h interrupt handler. And an NMI handler should end with the RETN
> instruction,
> which doesn't exist.
>
> I've checked an old computer with a Z80 cpu, and at 66h there is a jump
> to
> a routine somewhere which ends with a RETN.
>
> A nice thing about old Z80 computers (at least the one I have) is that a
> LOT
> of ROM routines starts with a call to the end of the RAM memory, so the
> user
> can easily hook rom routines. The default interrupt handler at 38h has
> such a call for example, making interrupt programming VERY easy - just
> change
> a pointer in the RAM :)
>
> When disassembling the 86 ROM, I found some interesting stuff though.
> The interrupt handler at 38h does actually call the RAM, $D2FE if
> bit 2 at (IY+$23) is set AND if
> ($D2FD)=($D2FE)+($D325)+($D34D)+($D39D)+($D3C5)
> (byte pointers). $D2FE contains a RET instruction as default.
> I haven't tried to hook interrupts that way, but it should be
> possible...
>
> I wish we he had a documentation over the RAM, but I guess that's just
> too
> much to ask for :) There are an awful lot of interesting system
> variables...
>
So the TI8x's don't even have anything hooked up to the NMI lead on the vhip?
Strange... I would think that that would be the most efficient way of
staying off, wouldn't it be? If not, then what actuallt does happen when the
calc is off?
Also, what is this interrupt handler at 38H? When does it get called?
Oh, and what is RETN?
~Steve
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