[Coco] [Color Computer] Re: 128 or 512?

[John W. Linville]

On Thu, Mar 01, 2007 at 01:57:12PM -0500, Roger Merchberger wrote:
> Rumor has it that John W. Linville may have mentioned these words:
> >On Thu, Mar 01, 2007 at 11:17:40AM -0500, Roger Merchberger wrote:
> >> Rumor has it that John W. Linville may have mentioned these words:
> >> >On Thu, Mar 01, 2007 at 06:42:31AM -0800, Jim Cox wrote:
> >> >> Why James?  Would the issue be the limited writes of the
> >> >> flash cards?
> >> >
> >> >Because CF cards are not byte-addressable?  They look like a hard
> >> >drive, not memory.
> >>
> >> If I'm not mistaken (which I probably am) I *think* there's a mode that 
> >_on
> >> reads_ CF cards can be byte-addressed, but I think they can only be 
> >written
> >> by erasure & reprogramming in 32K or 64K chunks... but again, it's not 
> >like
> >> the CF spec is bedtime reading to me... ;-)
> >
> >I really don't want to quibble over terminology here.  The point is,
> >CF doesn't look like memory (i.e. LDA and STA don't work to read and
> >write any single random byte on the card in any random order).  So,
> >it wouldn't be appropriate to use it for a RAM card.
> 
> I'm not trying to quibble, either, but what I was trying to say, is I 
> _think_ there's a read-only memory mapped mode -- so LDA's *would work.* 
> STAs wouldn't, as you'd have to "format" a block & then reprogram the whole 
> sucker, but I'm essentially saying that if written correctly the first 
> time, you could set it up as a memory-mapped ROM and actually execute 
> programs from it, just like any other form of ROM. RAM "functionality," I 
> believe, is outta the question, tho.

The spec is readily available from www.compactflash.org.  While the
CF _bus_ has 11 available address bits (A0-A10) for cards to use,
CF _storage_ cards only use about 5 of them.  They do so in order to
implement the standard (more-or-less) ATA register set in order to
look as much like an IDE hard drive as possible.  Feel free to cite
the spec if I am in error.

Given the existince of 11 address lines, I suppose someone _could_
build a card that plugs into a CF adapter and provided  access to 2KB
of memory or a smaller window on a larger memory set.  Since they
would be building their own hardware, they might use flash, SRAM,
DRAM, magnetic core, photons, or servo-controlled Post-It notes.
I wouldn't even be totally surprised if someone has built such a
card for some purpose, and it might even be available for sale.
But it most certainly is not the CF card you will get down at the
local Wal-Mart or even on-line at TigerDirect or Fry's...

Hth!

John

P.S.  FWIW, I believe you are thinking of NOR-flash, not CF.
-- 
John W. Linville
linville at tuxdriver.com