Why does this SRAM chip have more physical bits than declared by the manufacturer?

Question

I noticed while scanning the datasheet for a 23K256 SRAM chip that it has 32768 bytes (+262Kbit.)

The manufacturer clearly identifies this chip as 256Kbit.

Reading through the datasheet it clearly says "32768 x 8" which confirms my scan result - but it doesn't say what those extra 6Kbit are for.

Anyone can shed some light on this:

1. Why 262 while its rated 256 and the actual documented maximum address is 0x7FFF (32767)?
2. Can I use this extra space? Is it safe?
3. Can bits on the SRAM (or maybe bytes) get damaged over time?

Answer 1

There are two kinds of memory: memory where the structure and addressing and continuity clearly demands a number of cells that is a power of two (or at best a small multiple of such), and "bulk" memory where it doesn't. Flash memory is in the middle since the raw cell counts clearly are powers of two but wear management requires setting some of that aside and at least some consumer SSDs routinely store non-binary values in a cell.

The primordial bulk memory are hard disks. Long before prefixes "Ki" and "Mi" were introduced for referring to 1024 and 1048576, respectively (\$2^{10}\$ and \$2^{20}\$, respectively), memory sizes in binary computers were measured in terms of powers of two. This is still the case for RAM: nobody states having a computer having 17GB of RAM even when the exact number are 17179869184 bytes. Flash memory capacities are similarly advertised with powers-of-two based units of raw capacity since "32GB" (actually GiB) as the power-of-two number sounds better than a net bulk size after level wear management of "30GB" or similar.

Hard disk manufacturers were the first to realize that stuff looked better in powers-of-ten based unit multipliers, leading to a long intermediate period where some manufacturers boosted their sizes by diverging from what was in common use (annoying if you try allocating enough sectors to swap out 16GB of RAM).

Now of all current and historical perversions of units, probably the most insulting one is the 1.44MB floppy disk which has an actual size of 1440KiB.

To return to your original question: RAM is consistently specified in terms of unit multipliers based on powers of two as of now, even if the ostensibly more correct "Ki", "Mi", and "Gi" prefixes are not at all consistently employed in marketing and documentation instead of the historic "k", "M" and "G" prefixes.

Answer 2

256 Kbit is referring to the binary power prefix, sometimes referred to as Kib (kilo-binary bits) which is common for memory specification.

So that means it is referring to 256 * 2^10 bits = 262144 bits = 32768 x 8.

Answer 3

It's the difference between powers of 2 and powers of 10.

RAM chips are always addressed by an integer number of address lines, which makes it natural for them to have capacities that are a power of 2. Early on, someone noticed that 2¹⁰ (1024) was conveniently close to 1000, so they started using K to represent 1024 instead of the traditional 1000.

As capacities go up, the difference between binary powers and decimal powers gets larger. 1024 vs. 1000 is a 2.4% difference. 1099511627776 (2⁴⁰) vs. 1000000000000 (10¹²) is a 10% difference. Since RAMs are still constrained to be powers of 2 by the addressing, they're still specified using binary powers. Hard disk makers switched to decimal powers early on, but confusingly the operating systems (such as Windows) report capacities in binary powers leading to much confusion.

Answer 4

Tom already addressed the first part of your question - regarding the wear and tear you refer in point 3, this is not a failure mode that I am aware of for SRAM (or DRAM for that matter).

You are probably thinking of flash memory, where each write cycle wears the floating gate structure ever so slightly - but in SRAM the memory cell is basically a flop and is not subject to such damage.

Of course it is still silicon, so I guess that over long periods of time the chip can get damaged, for electromigration as an example, but not for reason specific for it being an SRAM chip.