Date : Thu, 02 Mar 2006 18:21:03 +0000 (GMT)
From : BBC Micro <b_b_c_m_i_c_r_o@...>
Subject: Re: SHELIA and the VIA's
Thanks Jonathan and others, a useful list in that
link...
Its quite interesting dissasembling, a bit like a
crossword, to begin with, I'll see how far I get
before it gets too difficult. I put up the page of the
main body of code:-
http://whitebody.tripod.com/castlequest/quest1_1528.htm
and created a quick and dirty page with links to other
fragments:-
http://whitebody.tripod.com/castlequest/
Mark
--- Jonathan Graham Harston <jgh@...>
wrote:
> > Message-ID:
>
<20060227185954.6030.qmail@...>
>
> BBC Micro <b_b_c_m_i_c_r_o@...> wrote:
> > confused...Page 397 tells us that the VIAs are
> > &FE40-&FE7F. The next chapter tells us that VIA-B
> is
> > &FE60-&FE6F.
>
> Correct. The VIAs are at &FE40-&FE7F. The System VIA
> is at
> &FE40-&FE5F and the User VIA is at &FE60-&FE7F.
> There are sixteen
> registers in each VIA, so their defined locations at
> &FE40+x and
> &FE60+x
>
> > I'm having trouble tying up the memory locations
> with
> > the registers/ports on the VIA can anyone give a
> list
> > from &FE40-&FE7F? I only found the Electron one on
> the
> > internet search.
>
> http://mdfs.net/Docs/Comp/BBC/Hardware/SHEILAddrs
>
> > I've dissasembled Castle Quest (with 6502dis) and
> > looking at the VIA calls. &FE79? Joystick/Timer?
>
> &FE79 is a reflection of &FE69. It happens because
> only address
> lines A5-A7 are decoded, so &FE60 is repeated at
> &FE70. Similarly,
> the CRTC is repeated all along &FE00-&FE07.
>
> You should never use or depend on accessing anything
> at a
> reflection address.
>
> > This was the one of the pieces of code:-
> >
> > $2cfc: a5 32 LDA $32
> > $2cfe: 38 SEC
> > $2cff: ed 79 fe SBC $fe79
>
> They are being naughty. They should have used $fe69.
> That code has
> no guarrantee of working on any hardware that may
> fully decode the
> address of the VIA.
>
> --
> J.G.Harston - jgh@... -
> mdfs.net/User/JGH
> RISC OS Internationalisation -
> http://mdfs.net/Software/RISCOS
>
>
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