I had this same question a few years back. I have not been tinkering with my 8201s or 8300 for a while, but stored away. But I found some notes. I had found out that the NEC-8300 system ROM is an 831000 128k x 8bit Mask ROM 28 Pin (the single chip labeled 831000-438 , N83A Basic ...) in socket ROM 0 I have an 8300 with the correct original system ROM. I remember figuring out how to read this ROM and verified it is the same image you do find in VirtualT. I also aquired 1 of those 8300s with a custom operating system for some kind of factory or Lab testing? It had a common 27C256 for a system ROM 0 which led me to believe it is safe to use a 27C256 in ROM 0 socket obviously. I found that an original 8201 system ROM or a 27C256 burned with the 8201 system ROM seemed to work fine in this 8300. No extensive testing but I never saw an issue. It just functions like an 8201 obviously.
For a 8300 ROM replacement I had found that an ST M27C1000 128K x 8 bit 32 Pin UV EPROM would be the best/easiest replacement for the original. No real adapter needed if I figured correctly. No cutting pins or rerouting pins necessary. Somewhere I'm sure I found and confirmed the correct pinout of an 831000 mask ROM as a pinout guide. A 27C1000 plugs right into the ROM 0 28 Pin socket, I believe I just straight burned the VirtualT 8300 system ROM image to the 27C1000. No tricks. Insert correct orientation in ROM 0 leaving Pins 1&2 and 31&32 of the 27C1000 hanging out in mid air. (bottom edge of the computer) I soldered a small wire connecting overhanging not connected pins 1&2 (VPP and /OE I believe) together and to Pin 16 (GND) On the other side I soldered overhanging not connected Pins 31&32 (/PGM & VCC) together and to pin 30 (NC) to pick up VCC from the socket below (ie 30&31&32 together) All of this on the 27C1000 EPROM itself, no mods to the computer and no damage to the EPROM either. This worked and acted like an 8300. Switched to 8201 mode with the OUT command. etc. No extensive testing, but seemed to work fine. Then I put my 8201s and 8300s away awile ago. But found some of my notes. I only had the 1 27C1000 scrounged from an old computer when I figured out what I needed so I don't have any more. I remenber other similar 128K x 8bit UV EPROMS 27C1001 ? and such didn't have the pinout that matched up perfectly like the ST 27C1000-20F1 I was luck to have. Don't know if that is the right speed IC but it seemed to work. Hope this is helpful, again some info is from memory Kyle
