How a 1/2/4 mb simmcan be used with this design in stead of 8x 256k simms Maximum size of this kind of mapper is 2MB instead of 4 MB. Because this mapper maps 8kb per blocksize instead of 16 kb like a normal memory mapper. Take megaram.gif For using a 1 MB simm y'll need: Create an extra multiplexed adressline with A18' ( U9 pin 9) and A19' ( U9 pin 7) A18' ( U9 pin 9) goes to pin 6 U7. A19' ( U9 pin 7) goes to pin 5 U7. pin 7 U7 goes to pin 18 SIMM socket. Disconnect A18' from pin 1 U11. Disconnect A19' from pin 2 U11. Connect pin 1 and pin 2 to pin 8 U11. Voilá 1MB instead of 256kb ---------------------------------------------------------------------------- For using a 2 x 1 MB simm y'll need: Create an extra multiplexed adressline with A18' ( U9 pin 9) and A19' ( U9 pin 7) A18' ( U9 pin 9) goes to pin 6 U7 A19' ( U9 pin 7) goes to pin 5 U7 pin 7 U7 goes to pin 18 SIMM socket Disconnect A18' from pin 1 U11 Disconnect A19' from pin 2 U11 Disconnect A20' from pin 3 U11 Connect A20' to pin 1 U11 Connect pin 2 U11 and pin 3 U11 to pin 8 U11 Use a second SIMM connector Connect all the pins from SIMM 1 to SIMM 2 except pin 2 (CAS), pin19, pin 24, pin 26, pin 28 and pin 29. Connect pin 14 U11 to pin 2 second SIMM connector. Voilá 2MB instead of 256kb ---------------------------------------------------------------------------- ------------- For using a 4 MB simm y'll need: Create an extra multiplexed adressline with A18' ( U9 pin 9) and A19' ( U9 pin 7) A18' ( U9 pin 9) goes to pin 6 U7 A19' ( U9 pin 7) goes to pin 5 U7 pin 7 U7 goes to pin 18 SIMM socket Create an extra multiplexed adressline with A20' ( U9 pin 6) and Ground A20' (U9 pin 6) goes to pin 10 U7 pin 11 U7 goes to pin 8 U7 pin 9 U7 goes to pin 19 SIMM socket Disconnect A18' from pin 1 U11 Disconnect A19' from pin 2 U11 Disconnect A20' from pin 3 U11 Connect pin 1 U11, pin 2 U11 and pin3 U11 to pin 8 U11 Voilá 2MB instead of 256kb Ok, The last option uses only the half of the memory but it is a smaller design. Hans Oranje