> Memory-addressed cards seems different... infact if, in order to change
> the state of the MEMCS16, I must decode the SA17-SA23 address, I deduce
> that my 16 bit memory mapped device  can be addressed only between
> 0x03FFFF and FFFFFF... is this right ?

  Yes, and no.  I think addresses below 0x00FFFF are used for the IVT, and I
suspect that 0x010000-0x03FFFF may be reserved for other uses, too.
  On the other hand, addresses above 0x0FFFFF are available only from
protected mode, as the result of decoding through a descriptor.  So
depending on your target software platform, those may be out of bounds as
well.

David Gillett



On 24 May 2004 at 9:34, Giampaolo Bellini wrote:

> Hi David... I'm sorry but I don't understand...
>
> >>Hi all...
> >>
> >>I have a question regarding the ISA bus: looking at the specs, it seems
> >>that a device can drive the MEMCS16 signal if it want to be serviced
> >>with a 16 bit data bus.
> >>But MEMCS16 is decoded from SA17-SA23 lines address... so it seems that
> >>I cannot have a 16bit device in the low memory (under 0x03FFFF)... is
> >>this right ?
> >>
> >>
> >
> >  No.  It has been a while since I looked, but I'm pretty sure that I/O
> >addresses are a maximum of 16 bits.  SA17-SA23 are available for uses like
> >MEMCS16 *because* they are not used as part of the I/O address.
> >
> >
> you are right, IO address range goes from 0x0000 and 0xFFFF, and I
> cannot have an IO card outside this address... the IOCS16 is decoded
> from SA0-SA15 and so I can serve an IO card with an 8/16 data bus.
>
> Memory-addressed cards seems different... infact if, in order to change
> the state of the MEMCS16, I must decode the SA17-SA23 address, I deduce
> that my 16 bit memory mapped device  can be addressed only between
> 0x03FFFF and FFFFFF... is this right ?
>
> >
> >
> >>also... can I address an IO device and a memory-mapped device in the
> >>same space (for example at 0xd000) ?
> >>
> >>
> >
> >  Yes.  Unlike some other architectures, the x86 family uses a separate
> >address space for memory and for I/O.  (Memory-mapped devices, other than
> >ROM and RAM, are extremely rare in the PC world.)
> >
> >
> ok... I expect that.
>
> >David Gillett
> >
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> >
> >
> >
> thanks a lot for your answer....
>
>
>     Giampaolo Bellini
>
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