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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