Marco Antonio Simon dal Poz <[EMAIL PROTECTED]> wrote:
> > Is it possible to make the MegaRAM yourself using all new parts?
> > Or do you need old cartridges?
> > Anybody has any schematics?
>
> It's perfectly possible. You can use the same memory IC's used in Memory
> Mapper. The difference is only the block switching circuit.
>
> I don't know anybody who has the schematics, but it's not very difficult
> to create one. The special feature is the port 8Eh, that selects between
> BLOCK SELECT mode and WRITE ENABLE mode.
>
> In a Megaram, when you execute a "IN A,(8Eh)", you put Megaram in WRITE
> ENABLE MODE. The accumulator receives garbage. When you execute a
> "OUT (8Eh),A", you put Megaram in BLOCK SELECT MODE (exactly as a Megarom
> cartridge), whatever the contents in the accumulator.
>
> In Brazil, we have Megaram models of 256kb, 512kb and 768kb.
Okay, just so that anyone can build this, or emulate it (oops...),
can you please fill in the details?
-IN xx,(8Eh) sets "write enable mode"
-OUT (8Eh),xx sets "block switch mode"
I suppose this can be made using a single 1-bit register, that
responds to access of I/O port 8Eh, and taking either read- or
write-signal as data input. D0-D7 lines are don't cares when
accessing this I/O port.
-Is this a single I/O port (8Eh), or mirrored on other I/O-addresses?
2? 4? 8 I/O ports?
-If in "write enable mode", I suspect you can write to the current
selected blocks in certain memory ranges. Is this correct? Or just
one special block or so?
-What size blocks are switched? 8 KB? 16 KB? Smaller? Bigger?
-EXACTLY what memory ranges do these blocks occupy?
-EXACTLY what memory ranges can be used to switch the blocks (if in
"block switch mode")?
(tells the hardware designer which address-bits are don't cares)
-What block numbers are allowed (# of bits)?
-How are unused bits in block numbers handled?
Suppose you have 32 blocks (5 bit block numbers). What happens when
you try to access block numbers of 32 and above? Are higher blocks
'empty'? Are unused bits in block numbers treated as don't cares?
-Can you read the selected block numbers back, or are these
write-only? (for instance: most ROM mappers: write only (mapper
registers, that is, not the ROM data ofcourse), normal memory mapper:
read & write block numbers)
-What's the power-on / reset state? Everything "don't know"?
Or blockswitch-mode, but currently selected blocks undefined? Or:
block xx in memory range yyy-zzzz, block ... in memory range ....
etc.?
Finally this: what software exists, that actually USES this MegaRAM?
Greets,
Alwin Henseler ([EMAIL PROTECTED])
http://huizen.dds.nl/~alwinh/msx MSX Tech Doc page
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