Re: HP 50g and SD Memory
Message #8 Posted by James M. Prange (Michigan) on 1 Sept 2006, 4:19 a.m.,
in response to message #7 by Marcus von Cube, Germany
Note that "RAM" ("Random Access Memory") and "ROM" ("Read-Only
Memory") are rather misnamed.
As far as I know, both "RAM" and "ROM" apply only to solid-state
memory, and any particular memory cell in them can be accessed "at
random", without (serially) accessing other memory cells.
Originally, ROM was written as part of its manufacture, and can't
be erased or rewritten (this still applies to plain ROM), so it's
essential that ROM be non-volatile, that is, it doesn't require
any power to retain its contents. In actual use of the names "RAM"
and "ROM", volatility for RAM and non-volatility for ROM are the
real distinguishing characteristics, so we also have PROM
(Programmable ROM) that can be written only once after
manufacturing, EPROM (Erasable Programmable ROM) that can be
erased and rewritten, and EEPROM (Electrically Erasable
Programmable ROM) that can be erased without resort to such
cumbersome methods as exposure to ultraviolet light.
Magnetic and optical discs are non-volatile and pretty much
"random access" (as compared to the serial access of a magnetic
tape), but we never refer to them as either ROM or RAM; they're
considered to be mass-storage media rather than "memory", although
a hard disk is often used for "virtual memory".
EEPROM (such as flash memory) can easily be erased and rewritten,
so it's not really "read-only", but because it's non-volatile,
it's considered to by a type of ROM. Of course, the flash cards
are used as removable mass-storage media as well, used much as we
would use removable magnetic or optical discs.
The blocks of flash memory erased at one time are relatively large
(32KB in a SanDisk document that I read), and writing anything,
even a single bit, to flash entails reading the whole block,
changing the contents, erasing the block, and writing the block,
so writing to flash is rather slow and uses a lot of power, at
least as compared to the SRAM (Static RAM) used in the
calculators. Writing to flash requires more power than anything
else that the calculators do.
Erasing flash memory also "wears it out". I think that typically,
flash memory is designed for about 10,000 to 100,000 erasures per
block (depending on the grade). The controllers for the cards also
do "wear leveling" by remapping the "logical sectors" on the card
to different physical "sectors", for example, to avoid always
writing the FATs to the same block(s).
The non-volatility of flash memory makes it very suitable for uses
in which this is desired and it won't be written to all that many
times, such as upgradable "firmware", occasional storage or
transfer, using libraries in port 2 of the 49G, 49g+, or 50g, and
use in digital cameras, cell phones, and music and video players.
But the file systems used on the flash cards make them unsuitable
for attached libraries, although they can indeed be used for
storing and transferring library files. The low write speed, high
power requirement for writing, and limited number of write cycles
make flash memory unsuitable for "merging" with a calculator's
"System RAM", or the frequent writing that a PC's operating system
typically does to its hard drive.
The calculators can indeed use a card with a "Master Boot Record"
and partitions, but will use only one partition, so if the card is
intended for use only with the calculator, there's no advantage to
formatting it this way. Formatting a card more like a floppy, with
just a "Boot Record", works just fine.
"Secure Digital" (SD) cards are based on the older MultiMediaCard
(MMC), and the "Secure" refers to added "security features" for
copy protection; perhaps more secure for copyright holders, but no
more secure for the user. To be sure, an SD Card does have a
lock/unlock switch, but note that early revisions of the 49g+ will
write to the card even with the switch in the lock position, so I
surmise that it's up to the device to determine the state of the
switch and act accordingly.
Having firmware (including the "ROM-based" operating systems in
the calculators) upgradable is no doubt an advantage (no need to
replace the device or a chip to fix any bugs), but it has the
disadvantage that developers may initially release a device with
"beta-quality" firmware as "good enough", expecting that they can
always fix the bugs later, as users discover and complain about
them.
Regards,
James
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