... how can this work !!!
I wanted to do some test regarding C3 mode with my 464 and my memory 512k RAM extension. This extension works well on my 6128 and 6128+ however, it is not working on the 464.
Working, not working
Test show that everything written to the extension (in mode C4 to C7) is OK, what is wrong is that the base ram in 0x4000-0x7FFF is written as well.
Using a logic analyser, I can see that the DRAM is written as if no extension were connected, despite the RAMDIS signal being asserted. I also checked that RAMDIS was properly propagated from extension to the main board (pull down resistor and or gate).
From a schematic point of view, RAMDIS only prevent data read from DRAM to drive the Z80 databus. There is no direct connection between RAMDIS and the DRAM or the Gate Array. And only the Gate Array has connection to the DRAM RAS/CAS/WE, so only the Gate Array could inhibit the write.
My CPC having a 40007 GA (with heatsink), I also tested it with a 40010. Result are the same.
I cannot test with a DKTronic extension as I do not own any.
So my question is : how the write to DRAM is inhibited when an extension RAM is selected ?