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Author Topic: 4 Mb ? (Read 4181 times)

Offline OA128

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4 Mb ?
« on: 2009.February.28. 00:03:01 »
In an old french magazine (Tilt #8 or #17, I don't remember...), it was written that the Lansay 64 (Enterprise 64, in fact) can support a 4 Mb extension. Is is true? Was this extension available? I've never seen it...

Offline Zozosoft

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Re: 4 Mb ?
« Reply #1 on: 2009.February.28. 11:22:45 »
The 4 Mb is the total addressable memory range, including all RAM and ROM. Such as the other Z80 machines, for the memory paging, the Z80 64K memory area divided to four pages. And also the 4 Mb area divided to 256 memory segments. You can page any segments to any Z80 page, without any limitations! This is very unique features at the 8 bit machines! Most of the machines you can select memory configuration from some limited variations.
And also very unique: this high memory capacity is fully supported by the operating system! (At this time the PC can handle 640K RAM + 384K reserved for the ROM extension from the total 1024K address space. And the MS-DOS memory barriers will be always a problems until the end of the DOS era)

The first four segments (64K): 00-03 is allocated to the onboard ROM. The second four: 04-07 for the cartridge.
The last 64K (FC-FF) is the onboard memory, it is also the video memory. And the EP128 have a 64K internal RAM expansion, at the F8-FB address. You can see these segment numbers during the RAM test: TESTING FE, FD, FC, etc
In the EP128 the segments 08-F7 is remaining free for any RAM or ROM expansion. Theoritically you can builad a 3840K RAM expansion, and got a 3968K total RAM   :)
It is most recommended to use my unofficial EXOS 2.31 to avoid the too long RAM test. And with the EXOS 2.31 you can also put RAM in the cartridge (62256 SRAM ICs), this is extra 64, then the total is 4032K  :ds_icon_cheesygrin:

But it is a little foolish configuration :-) Normally needed to left some space for the ROM extensions, for example the EXDOS ROM :)

At now from a modern components very easy builad a RAM/RAM expansion: buy a 512K SRAM ICs (AS6C4008, K6X4008C or other compatible) and 512K FLASH ROM (MX29F040, AM29F040B or other compatible). Only other addtional logical circuit needed for the address decoding: 74LS138 (or compatible). At the 138 eigth outputs you got a Enable signals for the eigth 512K memory area: 00-1F, 20-3F, ... C0-DF, E0-FF
The first and the last cannot be used, becaus conflict with the mother board memories. The remaining is can be used. (For using the 08-1F and E0-F7 areas needed to add more logical circuits to your expansion board.)

I have a MICROTEAM type EXDOS card, this is originally have a 512K RAM and 3x32K ROM sockets. I modified the ROM sockets for 2x512K SRAM and 1x512K FLASH ROM, now I have a 1.5M RAM and 512K ROM in this card  :ds_icon_cheesygrin:

Offline pepax

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Re: 4 Mb ?
« Reply #2 on: 2009.March.01. 12:16:19 »
Very cool feature.
Any photos, schematics and howtos for the Dummies?  ;-)

Offline OA128

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Re: 4 Mb ?
« Reply #3 on: 2009.March.01. 20:23:06 »
Thank you for your answer!

So I guess the 4Mb are theorically and the company never sold that.

Anyway the Enterprise is really a nice machine...

The latest Thomson 6809 computers (TO8, TO9+ and MO6 - and its clone italian Olivetti PC128) have a gate array that allows a bank switching : it's possible to put any 16 Ko RAM bank (from 512 Ko) at the adress of the RAM or even the ROM - but some adress are reserved.