What is the "serial number being found"? Is there a serial number stored in the eeprom... and is it used for something?
Now that you've discovered the SPR-BSA being a Seiko S-2913CR chip... there's also a Seiko datasheet for it
http://datasheet.datasheetarchive.com/o ... 144153.pdf
- so no need to use the Seeq datasheet.
The read/write multiple functions don't do anything with 7E0000h. The source/dest address in RAM is in Y register.
For read_multiple it's using [DB:0000h+Y], with DB needed to be a "lorom" bank, as it's also used for [DB:2198h].
For write_multiple it's using [D+00h+Y], with the direct page... I guess it should be zero (or maybe the related functions survive other values).
If the D, DB, Y registers are all zero then it will use address 000000h, which will actually mirror to RAM at 7E0000h.
So well, there's an EEPROM and the BSX-BIOS has functions for accessing it - but I don't know of any games that would use that functions (nor does the BSX-BIOS itself use that functions (nor do any known games access 2198h directly)). If I didn't miss something: The EEPROM is there, but it was never ever actually used?
For the other serial port at 2199h: That port is actually used: by the BSX-BIOS, by BSX-BIOS patches, and by Itoi, and by BS Dragon Quest 1, and in respect to data in the Town Status packet. Being used that often, I have some doubts that it was used for any kind of yet unreleased expansion hardware.
I would have assumed that it goes to the MN88821 chip (there's a datasheet for a similar chip, which has a serial port). You do have a BSX unit, don't you? What happens when executing the 2199h reading functions? Does it return any data - despite of not having a HDD or whatever attached at the EXT port?
Or if it goes to the EXT port, if you attach an oscilloscope, a logic analyzer, a multimeter, or a bloody LED... can you see a pin getting toggled when toggling the 2199h clock bit by software? Going by DaveyPocket's findings, the five candidates for serial or other unknown features would be EXT pins 29, 30, 31, 33, 34.