1 Meg Mother on Rev 6A Boards
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Owners of Rev 6a A500's will notice that they only have 4 RAM chips as
opposed to the 16 in the Rev 5 and earlier machines They will also note thatin between the RAM chips are empty sets of holes which look like something ought to go into them 
       
Firstly fear not - the reason you have a quarter the number of RAM chips as older machines is that the 6a uses 44256 4-bit x 256K chips which are four times as large as the 41256 1-bit x 256K chips used in the 5 and earlier revisions All the modern Amiga memory hardware (except possibly the A501) uses these larger chips because they are slightly cheaper on a dollar-per- megabyte basis For example the older Series I A590 hard drives use the 41256 and they will accept up to 2Mb The new Series II A590's use the 44256 (and consequently have enough space to fit 8Mb of RAM). 

But back to those extra holes They are indeed designed to accept something;viz another set of four 44256's to take your machine up to 1Mb without havingto buy an A501 or other expander (this is particularly useful if you have amemory expansion which uses the A501 slot and you want to have 1Mb chip-RAM).If you want to install your own RAM there you should do as follows : Purchasefour 20-pin IC sockets (preferably machined-pin). Make sure you have asolder-sniffer Now look at those holes and you will see that they are cloggedby solder (this is due to the way the boards are soldered; the process covers all exposed copper areas with solder). You must clean thissolder out by heating the board from one side and sniffing the solder out onthe other (or blowing the solder through; if you choose this method make sureto remove all the little splashes of solder from the board and your lap). Whenyou have cleaned out all the holes solder in the sockets Now plug in four44256 chips and you have 1Mb of memory for slightly less than the cost of anA501 (in Australia the difference is roughly A$20). 

With respect to the speed of RAM chips required I use 150ns chips with no
problems at all; however I suspect that Commodore would recommend 100nsdevices The price difference is minimal so you may as well buy 100ns chips sothat if you ever buy a series II A590 you can shift the chips over and have100%-guaranteed-not-to-overheat fast-RAM