Posted By
JamesD
on 2013-04-24
12:53:48
|  Re: Flash memory through 3+1 ROM sockets?
ROM chips don't have a connection for a write line so there would be no need to have them on the sockets. Technically, it's a read/write line but since ROMs are only read... there is no need. FLASH or RAM makes no difference.
I would hope that in order to keep address decoding simple, writes would also activate the chip selects but it doesn't require a lot of gates to disable the chip selects on write. I don't have the right equipment to capture the buss signals to test this though.
Given the capacity of the ROMs vs the capacity of FLASH chips, you could use a switch to select between the default Plus/4 ROMs and other ROM sets. That switch could be mechanical or electronic in nature. This could be used to make things like custom editors, assemblers, graphics utilities, better word processors, spreadsheets, or games available at the flick of a switch or with a simple POKE.
[edit]
Page 10 of the Service Manual shows the ROM selection logic.
http://www.pagetable.com/docs/ted/Service%20Manual%20Model%20Plus%204%20Computer.pdf
[edit]
The OCR of the document is poor but I didn't do it so... not my fault.
From page 5(?):
"Selectior, of either ROM or RAM is accomplished bw
writir,.:3 a bit. in Q Ted resister,. When RAM is selected, the
whole 64K men.ory m2P is coalPrised of RAM with the exception of
1 re~ist~rs for 7501 portl 1 page for Ted control resistersJ
~rld 1 pa!e for I/O. This alethad yields 60,671 bytes of RAM
avail;;ble for Basi,c progralT, storage. When ROM is selected, the
prosr~~1 residin~ irl ROM ap?ears in pla~e of RAM. Tht e:(ception
to this is a write a~eratiDrl to ROM will alwa~s 'bleed throlJsh
tc 'Jrld,erl~ir.g RAM."
http://www.pagetable.com/docs/ted/TED%20System%20Hardware%20Manual.pdf
The last sentence is important. All writes to ROM bleed through to RAM.
You won't be able to use FLASH like a disk drive because you'll corrupt RAM but you could program it while in the system.
This behavior could be changed but I'm sure existing software depends on it and I recommend against it.
[edit]
I looked around and the smallest FLASH memory chip I could find is 128K and uses 16K sectors.
That means you'd need to rewrite sectors 16K at a time.
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