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Banks - Graham72 - 03-11-2026

Hi all,
On the Premium Plus (1024K), I'd like to confirm how port 50h selects the 32 lower-memory banks of 32K, with ROMs off (bit 2 = 1).

Bits 0–1 are clearly bank bits 0–1 (with bit 1 inverted when ROMs are off). For the other three bank bits I've found two different readings:

Bank bit

2 port 50h bit 5 port 50h bit 6
3 port 50h bit 6 port 50h bit 7
4 port 50h bit 7 port 50h bit 5

Thanks g

RE: Banks - MbeeTech - 03-11-2026

(03-11-2026, 11:18 AM)Graham72 Wrote: Hi all,
On the Premium Plus (1024K), I'd like to confirm how port 50h selects the 32 lower-memory banks of 32K, with ROMs off (bit 2 = 1).

Bits 0–1 are clearly bank bits 0–1 (with bit 1 inverted when ROMs are off). For the other three bank bits I've found two different readings:

Bank bit

2 port 50h bit 5 port 50h bit 6
3 port 50h bit 6 port 50h bit 7
4 port 50h bit 7 port 50h bit 5

Thanks g
You can use them either way (they are just blocks of ram), but the way its wired is:
2 port 50h bit 5 
3 port 50h bit 6 
4 port 50h bit 7

RE: Banks - Graham72 - 04-11-2026

Thanks Ewan, I have changed my setup. A subsequent question now arises. Is 51h used in non ROM models for anything?

RE: Banks - MbeeTech - 04-11-2026

(04-11-2026, 07:33 AM)Graham72 Wrote: Thanks Ewan, I have changed my setup. A subsequent question now arises. Is 51h used in non ROM models for anything?

Because the lower [A2..0] address lines are not decoded, the port selects are in groups of 8 addresses and in the case of port 50h, any port access in the range 57h..50h 
will be decoded as port 50h.  This is the way it is on the 128k DRAM coreboard.

RE: Banks - Graham72 - 09-11-2026

One more question on a 1mb Machine is 58h free?

RE: Banks - MbeeTech - 09-11-2026

(Yesterday, 03:09 PM)Graham72 Wrote: One more question on a 1mb Machine is 58h free?

Port 0x58h is not used on any Microbee.

RE: Banks - Graham72 - 09-11-2026

(Yesterday, 03:51 PM)MbeeTech Wrote:
(Yesterday, 03:09 PM)Graham72 Wrote: One more question on a 1mb Machine is 58h free?

Port 0x58h is not used on any Microbee.

Thanks Ewan.