On 19 May 2004 at 9:57, Giampaolo Bellini wrote:

> 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.

> 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.)

David Gillett

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