How do I figure out RAM chips. WORD x BIT to Kilobyte?

[Ral-Clan]

So, I'm trying to put some RAM into my SupraDrive 500XP.

The manual says it can take 256k×4 or 1M×4 DRAM chips.

I have a bunch of old PC boards that have old RAM chips on them.
A Datasheet archive search tells me some of these chips are 262,144 Word x 4bit and some are 64,536 x 4-bit.

I'm thinking the former are the 256x4 bit chips I need.

But I've always wondered how the heck do you use these strange WORD and BIT numbers to calculate how much actual RAM (in Megabytes) you will get?

Does anyone know how to translate WORD x BIT numbers to Kilobytes and Megabytes?

Lets say I want to purchase enough chips to make 1MB of RAM (hypothetically).  How can I figure out how many 262,144 word x 4bit chips do I need?

I'd really love to be able to read a datasheet on a memory chip and figure out how much RAM I really will get.

Thanks.

[Ral-Clan]

It all depends on how many bits your SupraDrive needs.  If it's 16-bits, you'll need 4 of those x4 bits to get a 16-bit word size.  The number of words will also be multiplied.  So if you get 4 256x4 DRAMs that will be 512K of RAM.  If you get 4 of those 1Mx4 you'll get 8M of RAM.

-edit-
Going with the example you gave of wanting 1 Meg of RAM, you'd need 8 256Kx4 chips.

The SupraDrive needs 16bit RAM, I am pretty sure - as it fills the 8MB of Zorro II RAM space on the A500.

Okay, so I understand that if your computer requires 16-bit RAM and you are using 4-bit memory chips, then you must use 4 of them.  But I don't understand the "256" part of the "256 x 4bit" chip designation.  What does that number represent? Kilobytes on the chip?

256 x 4 (four chips) = 1024....which I would assume means 1024K of RAM....so how do four of these chips make 512K of RAM, as you say?

Same with the 1M x 4bit chips.  You say 4 of those will equal 8MB of RAM.  But 1024 x 4 (four chips) only equals 4,096 ---- four megabytes?

I'm not doubting you, I just don't understand, so I'm sure I have something wrong.  I'm sure my assumption that the 256 or 1M number equals an actual RAM value is incorrect.  But what does it represent?

[Ral-Clan]

Or in other words: to populate a bus of 16 bits with chips 4 bits wide (x4) you require 4 chips in parallel. 4x 256K x4 = 256K x16. In terms of bytes: 256K x16 = 512K x8.

Thanks guys, I'm going to have to read these explanations a couple of times for them to sink in, but I appreciate the straightforward explanations.  Those chips won't be a mystery to me anymore.

As an aside: does anyone know where I could buy (or trade for) 8MB of 1M x 4 RAM chips for my SupraDrive 500XP?  I'm hoping those chips shouldn't me more than 50 cents or so each nowadays.

[Ral-Clan]

Used to have a bunch of 512Kx4 but gave them away here a couple of years ago. I'll take a look if there are any 1Mx4 around...

Thanks!  I'd be interested to hear if you find those chips.

I did pull some 256x4bit chips off an old PC motherboard, which gave me 2MB for my Supra, but if I wouldn't mind maxing out the Supra if I can do it without too much expense.

I'm going to be using this A500 for DPAINT animation stuff - I'm not sure if 2MB chip (MegaChip) + 2MB fast RAM will be enough to do decent animations, or if I'll really wish I had the full 8MB in the SupraDrive expansion sidecar.

When I had an A2000 I had 32MB 32-bit RAM which I rarely maxed out.  So I rarely thought about how much RAM I was using.  Now that I'm back in A500 world I have to think about those things again.

I think a lot of people probably did good work on an A500 with 4MB RAM total back in the 1990s.