Spectrum +2A floating bus port

Post Reply
User avatar
chernandezba
Posts: 205
Joined: Tue Mar 11, 2014 4:30 pm

Spectrum +2A floating bus port

Post by chernandezba »

Hi

As I discovered in the past, Spectrum +2A (and probably +3) do have floating bus port. But it is not shown on all unused ports, only on some ports in these situations:

When the paging is disabled (bit 5 of port 32765 set to 1), we will
always read 255; when it is not, we will have the byte that the ULA reads
(with bit 0 set) if the port number has the following mask:
0000XXXXXXXXXX01b, I mean, starting from port 1, stepping 4, until port 4093
(1,5,9,13,..., 4093)

You can see a simple test running on my old Spectrum +2A:

https://www.youtube.com/watch?v=Oj9ygM9U5lQ

I think it's important to remember this, so most of the information on the net says that +2A does not have floating bus port, but it's incorrect.
I think Spectrum +3 have the same behavior.
Trying some emulators also return incorrect value FFH for all unused ports (except ZEsarUX)

Cheers
Cesar
----

ZEsarUX
ZX Second-Emulator And Released for UniX
https://github.com/chernandezba/zesarux
Post Reply