First post, by bushnrvn

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Why did the original Tandy 1000 (25-1000) have the option for a 256K ROM? The technical reference describes two ROM configurations. One with U9 populated on the motherboard for a 128K configuration , and another with a different resistor populated and U9 and U10 populated with ROM chips for a 256K configuration.

Here's a reference via the Radio Shack Hardware Manual: Tandy 1000 Computer Service Manual

I am having trouble finding what the extra 128K was used for, and perhaps that might answer the question of why there were two configs available at the time of purchase.

My goal is to clarify the statement made in this FAQ:

**** If you have the two-chip version of the original 1000 BIOS (version 01.00.00), you also need to upgrade the PAL chip when you upgrade the BIOS. The PAL chip goes in the socket labelled U9. The new one is part number MXP-0081. Tandy has been out of the PAL chips for a while. If you have this BIOS version, there are still ways to install a hard drive, but your options are more limited

Is this referring to the presence of the 256K configuration? Or, is there a revision of the original 25-1000 (not A/HD) logic board that does not have two ROM sockets, populated or otherwise? If the ROM is currently in U9, would it have to be put into U10, and the PAL put in its place?

The FAQ goes on to explain that Tandy provided a service where they would upgrade a PAL chip on the Memory Plus Expansion card to resolve a DMA issue:

Tandy once offered to take the Memory Plus expansion adapter back and replace the PAL chip (U14) to correct the problem. Tandy no longer sells either the BIOS upgrade or the PAL chip. If you have the old BIOS version, you can still have a hard drive, it just won't be bootable (you will have to boot from floppy).

Is there a second PAL chip, apart from the one mentioned above, needed to correct the DMA issue?

Is it not possible, in a system with only U9 populated with a 128K ROM to upgrade or modify the ROM to allow the installation of something like an XT to CF adapter?