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Using a network card for providing BIOS extensions

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First post, by carlostex

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I'm still working and gathering parts to finish my Turbo XT build. One of the things i absolutely want to have is the ability to boot from High Density floppy disks.

I'm perfectly aware of 2M, but i need HD floppy boot support, as i plan to use GOTEK Floppy emulators on the machine. These look like ass for a vintage build but are extremely useful.

Here's my idea, grab a couple of ISA network cards with a BIOS extension socket, burn an EEPROM with the needed BIOS extensions and get glorious HD floppy support on an XT. I wonder what addresses will the network card will place the BIOS extensions. Has anyone tried this?

Also my Juko ST-12 board has an empty socket for a 27C256 EPROM. I'm wondering if that extra socket is there for the case of ODD/EVEN BIOS, but since the board is using only one socket for the JUKO BIOS i wonder slaso if i could place the EEPROM directly there to support extensions. Probably not.

There are also other options, like James Pearce ROM extension board, which has DIP switches for setting the address space, but i do not have the necessary equipment to assemble and solder the stuff. I've asked James but i'm afraid he might be too busy for the job.

Anyway the network card seems to be the cheapest option, i would like to use EEPROMS instead of EPROMS just for the convienience, i'm also wondering if i can use a bigger chip like a Winbond W27C512 and the card will still read the first 8192 bytes only.

Reply 1 of 4, by Jepael

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I've succesfully been involved in a project where we burned a piece of code to an (E?)EPROM and put it on a network card for booting it. I recall we had to burn the same binary image many times to fill the memory chip as we had no idea which part of the memory chip was mapped to ISA memory space. This was maybe because the memory chip was larger than what the network card originally expected so maybe some address line of the memory chip was held high, or even worse, floating at unknown state.

So, in your case you have 64k memory chip and a 8k memory window, so it might not be the first 8k page, but one of the eight 8k pages, unless you measure or make sure in what state the rest of the memory chip address bits are. Bend the memory chip bits and put some DIP switches there, so you can select one of 8 different pages with different test images?

Reply 2 of 4, by Zup

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Have you checked that your multi I/O can use 1.44 floppy drives? Older floppy controllers don't support HD floppies (although they can use DD disks on HD drives) because of transfer speeds, and that's something you can not fix only with BIOS.

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Reply 3 of 4, by carlostex

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Zup wrote:

Have you checked that your multi I/O can use 1.44 floppy drives? Older floppy controllers don't support HD floppies (although they can use DD disks on HD drives) because of transfer speeds, and that's something you can not fix only with BIOS.

If it can't no problem, i can just as easily disable the floppy and I/O controller, and use it only as an RTC card. Then just as easily i can use one of those 16bit controllers. The BIOS extension will do the rest.

Reply 4 of 4, by carlostex

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Jepael wrote:

I've succesfully been involved in a project where we burned a piece of code to an (E?)EPROM and put it on a network card for booting it. I recall we had to burn the same binary image many times to fill the memory chip as we had no idea which part of the memory chip was mapped to ISA memory space. This was maybe because the memory chip was larger than what the network card originally expected so maybe some address line of the memory chip was held high, or even worse, floating at unknown state.

So, in your case you have 64k memory chip and a 8k memory window, so it might not be the first 8k page, but one of the eight 8k pages, unless you measure or make sure in what state the rest of the memory chip address bits are. Bend the memory chip bits and put some DIP switches there, so you can select one of 8 different pages with different test images?

That is very much a concern of mine, what i thought of is to burn the first 8192 bytes and then go from there. If it works awesome! If not try the next 8192 bytes and so on. Painful to do, but you gotta do what you gotta do.

The simplest solution is to use James Pearce Lo Tech ISA ROM board, which includes DIP switches to set the address of the BIOS extensions. Saves a lot of work and eliminates the possibility of the 16bit connector being in the way of components of the motherboard, although i could also cut the 16bit connector on the network card.

I should also order 2764 type EPROM's and try those. I've seen some network cards that have 2764 silkscreened on their boot rom sockets.

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