Testing RAM for a 4Mb A3000 Upgrade

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daveejhitchins
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Testing RAM for a 4Mb A3000 Upgrade

Postby daveejhitchins » Wed Sep 20, 2017 10:12 pm

OK, I'm not too much into DRAM. So would appreciate some assistance, please!

I'm thinking of a Block of Memory 2 x 1Mx4bit attached to the 1MHz Bus (if it can handle that amount of data?). The test board would need the DRAM RAS and CAS generation plus any Paging requirements. There may be something out there already?

Any other scheme would do just as well!

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ABR, ATI, AP6, MGC, AP5 . . .
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crj
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Re: Testing RAM for a 4Mb A3000 Upgrade

Postby crj » Thu Sep 21, 2017 5:01 am

A lot depends on precisely what you mean by "handle".

At the most basic level, the 1MHz bus exists as 512 bytes in the BBC Micro's memory map. In that sense, it can only handle half a kilobyte.

However, at least in theory, you're not the only person using the 1MHz bus.

That 512 bytes is divided in two. The bottom half, FRED, is &FC00 to &FCFF, 256 bytes for directly-mapped things. The top half, JIM, &FD00 to &FDFF is "paged" to create a 64Kbyte address space.

Suppose you want to read from JIM address &1B29. You write &1B to the JIM paging register in FRED at &FCFF then, having selected page &1B you read address &29 from it, by reading &FD29.

However, at least in theory, you're also not the only person using the paged memory. The Music 5000, for example, takes up 4Kbytes.

But that doesn't mean you can't do some more paging of your own. If you wanted to access a megabyte you could, for example, decide to use, say, JIM pages &E0-&EF and FRED location &F3. Now you can divvy up your 1,024Kbytes into 256 "clusters" of 4Kbytes each, further subdivided into 16 pages of 256 bytes each. To access address &xxyzz you would:
  • Write &xx to your private paging register, &FCF3
  • Write &Ey to the JIM paging register, &FCFF
  • Access address &FDzz
As you can tell, while strictly speaking possible, this is faff. Certainly, the memory isn't usable in any normal way by any normal program. It's more of a ramdisk, and indeed as a ramdisk is probably the best way to use an extra megabyte of RAM in a BBC Micro.

(Oh, and you'd also need a DRAM refresh circuit...)

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1024MAK
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Re: Testing RAM for a 4Mb A3000 Upgrade

Postby 1024MAK » Thu Sep 21, 2017 1:26 pm

crj wrote:(Oh, and you'd also need a DRAM refresh circuit...)
Some DRAM chips can automatically refresh themselves. You will need to read the relevant data sheet...

Mark
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daveejhitchins
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Re: Testing RAM for a 4Mb A3000 Upgrade

Postby daveejhitchins » Thu Sep 21, 2017 9:50 pm

1024MAK wrote:Some DRAM chips can automatically refresh themselves. You will need to read the relevant data sheet...
Yes they do . . . I used some of the first commercially available pseudo static RAM when I built a memory card for my Elf 1802 computer - it must have been back in the late 70s :shock: I've still got the board with the chips on. When I next come across it I take some photos. In the meantime I've received some photos of my AQR here - I'm thinking I'll reverse engineer the board and build a test cartridge using the Elk.

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ABR, ATI, AP6, MGC, AP5 . . .
For a price list, contact me at: Retro Hardware AT dave ej hitchins DOT plus DOT com


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