AW: AW: [OpenBIOS] OT: Bios access speeds

Qiwei Xiao qiwei at
Mon May 3 11:42:14 CEST 1999

James Oakley wrote:
>  ...
> Glue is, however, required for POST and GPIO. There are two ways to go
> with this, and this has been discussed to some degree. Using a GAL is
> definitely the cleanest solution. However, is there anybody who wants
> one of these who doesn't have access to a programmer? Traditional logic
> chips are more accessible to these people. It's still really cheap even
> though chip count will increase.
> Who agrees/disagrees?

I don't see any advantage of using GAL for the simple purpose of decoding 
the address. I don't have a GAL programmer and I don't think most people do. 

There are two senarios for address decoding: memory (eprom, etc) and device
(POST card, printer port, etc). For memory type, there are 20 address lines 
(SA0-SA19). I am attching a schematic for an ISA address decoder circuit 
using only two chips: one 138 and one 139 (might need high speed HCT type). 
You can set jumpers to select the base address. This is a very typical
decoder ciucuit. (I cut it out from a larger application so I am not 
claiming credit for it). 

The other type is device. I believe only 64k can be addressed (16 lines). 
An address decoder can easily be done using similar scheme as the above.
I have the schematic of an original IBM PC printer card if anybody is 

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