Hi Guys, I've got a Cromemco System-3 which I've been wrestling with on and off for quite a few months now ... I've got it to the point where it will boot and run CDOS, however Cromix hangs during boot. I've "boiled it down" to just the ZPU, 16FDC, and the RAM cards which are a pair of 256KZs, with the 948 and 949 addressing ROMs in them (sw.1 is ON for the 948 card and OFF for the 949 card as described in the documentation). I want to run a comprehensive memory test... I written such a test and can "load" it by having a PC "type" the HEX code into RDOS's SM command - then run it with 'G' - that all works fine, and I have verified that the default 32k that comes up with RDOS appears to be OK. Now I want to test all of the possible banks in the 256KZs and so far I can not find any documentation as to how the bank select works. From the 16KZ docs I see that the normal bank select register is 40h, and this does do "something" to the 256KZs ... I've determined that by writing individual bits 1, 2, 3 or 4 I can swap in four distinct 32k blocks into the upper half, while nibbles with other than one bit set all seem to map to the same block which is different from the other 4 ... This makes 5x32k = only 160k. I've also determined that writing individual bits 4, 5, 6 or 7 causes the system to die, which I'm guessing is because it is swapping the lower 32k (where my program is running)... It it works the same as the lower-nibble/upper-block, then that would give a total of 10x32k = 320k that I can access from the 512k physical memory on the cards ... ? Can anyone provide me with details of exactly how the bank select works with dual 256KZ cards installed, and how to map in the entire 512k (obviously a section at a time)? Unfortunately I don't have the 947 address ROM which would have shipped with single cards, so I can't test the cards by thenselves... Anybody got a spare? Also, I note that RDOS will not come up if the ZPU is set for 2MHZ ... It appears to only run at 4MHZ - is this normal? Regards, Dave -- dave04a (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.parse.com/~ddunfield/museum/index.html