"High density" RAM

There is a lot of confusion about high denisty (or more accuratly DIMMs based on 64Mb x 4bit chips)... here are the facts...

The two common configurations for a 512MB DIMM are:
- 16 chips of 32M x 8bit each
- 16 chips of 64M x 4bit each

The term "high density":
This is a marketing/sales term. In any one family you can get 32MBx8 and 64MBx4 chips - the silicon density is *exactly* the same.

Why are there two kinds?
You can only have so many chips wired together on a single data pin of a data bus. In large servers/mainframes with huge memory requirements this starts to become an issue. Using 64Mx4 chips you have half the number of chips on each data pin. As a result you can have twice as many chips, thus twice as much RAM on the same bus. For PCs it doesn't matter.

Motherboard compatibility:
32Mx8 based chips use 10 column control signals (A0 to A9).
64Mx4 based chips use 11 column control signals (A0 to A9 plus A11).
A large number of motherboards don't support using A11 as a column signal and so can only detect half the capacity.

Cachecard compatibility
The cachecard supports both 32Mx8 and 64Mx4 based DIMMs.
All 32Mx8 based DIMMs will work.
All 64Mx4 based DIMMs the follow the JEDEC standard for DQM wiring will work (99% of 64Mx4 based DIMMs).