View Full Version : Upgrading my 386SX to 64kb cache?

Anonymous Coward
May 19th, 2008, 09:53 PM
I have an AMI 386SX-XT Series 20 motherboard that was manufactured around 1990. This one is special because it is equipped with 32kb cache memory which makes it much faster than similarly clocked 386SX boards.

Here's a picture of the board:


I currently have 16mb of RAM installed on this board, and I would like to be able to cache all of it. In order to do so I will need to upgrade the cache to 64kb. However, this is not trivial since on older motherboards that cache and tag rams aren't standardized like on the later 486 boards. I have had very little luck finding datasheets for the chips on this board. Infact, the cache arrangement just confuses the hell out of me. There are exactly 7 SRAMs that make up the cache.

Here's a picture:


There are four chips to the left. They are Motorola MCM6290BP25 and are each 24-pin. According to the spec sheets they are 16k x 4 arrangement rated for 25ns. By my math:

(16,384 bits x 4) / 8 = 8kbytes. Therefore four of these chips would make up the 32kb cache. Because this is an SX motherboard only four of the chips are needed since memory data path is just 16-bits wide (each chip is 4 bits wide).

So now here's where the mystery comes in. What are those other chips for? They are Motorola MCM6288BP25. Experience with 486 and 386DX motherboards tells me it should be TAG RAMs. But why three of them? The only other board I have ever seen that needed three TAG RAMs was a special server board that used writeback and supported 1024kb cache. Now the bigger mystery. According to the specsheets for these, they are also 16k x 4 at 25ns. It really seems strange to have such a large capacity for TAG RAMs. Is it possible there was a mistake and these are actually 16k x 1 arrangement?

In any case, it seems I will need to replace all 7 chips with 32k x 4 (or possibly 64k x 4) to upgrade to 64kb cache. Just wondering if anyone has the datasheets for these or knows of equivilant parts. I haven't been able to turn anything up so far. Perhaps someone has a dead older model 386 board they can scavenge for parts.

Anonymous Coward
May 21st, 2008, 05:47 PM
Perhaps the 64kb upgrade isn't necessary. I ran CTCM on this machine, and it indicated all RAM was being cached.