Electronic – 8086 memory decoder logic

For most RAM, the mux turns into a column select for a large grid, and an entire line of data is read out or written at once. The grid structure gives you the efficiency you need.

http://docencia.ac.upc.edu/master/DTM/docs/03-Memory%20Structures.pdf is a good PDF on the subject. It also shows a "predecode" structure which makes the decoder less complex than it would be otherwise.

These 256K x 1 chips are only 1 bit wide. With four of them you would only get 1 nibble at a time, and the Z80 doesn't do 4-bit data.

It is possible to use them, but not what I'd call practical, and would take far more than 8 chips, unless you want to use an FPGA to make a RAM controller. You need a controller anyways, in order to generate your dynamic RAM timing. But let's concentrate on simple read/write operations.

You need, at a minimum, a 9-channel 2x1 multiplexer for the addresses. You also need some time delays to generate RAS and CAS strobes. With a single 41256 you could produce a 32K x 8 RAM, by using page mode. Using the upper 15 address bits as the 32K address and the 3 lsbs of address to provide the 8 bits of data, you would perform a page mode read or write , then step through the 3 data addresses to sequentially read or write the 8 data bits into or from the RAM. Page mode cycle time takes 100 to 150 nsec/cycle, depending on your chip speed, so a single read or write cycle would take on the order of 1 to 1.5 usec.

Chip count would be something like: 2 ea. 74HC157 2-1 mpx (although this would restrict you to a 16k equivalent), 1 ea 74HC374 for the RAS addresses, 2 ea 74HC161 for the CAS addresses, 1 ea. 74HC161 for the data bit counter, 1 ea 74HC151 8-1 mpx for writing data, 1 ea 74HC374 as a read shift register to store the output data before sending it to the Z80, and a 74HC244/245 to drive the data onto the Z80 data bus. Plus a few more for glue logic, at a guess. That's a minimum of about 11 chips. Plus, assuming you're using an 8-MHz Z80, it will run at an equivalent of about a 2-MHz chip. All your memory reads and writes will take a loooong time.

You can, of course, speed things up by using 4 of them with a two-stage read or write cycle with almost no loss in speed.

What you really need to do is use 8. Then you have a good match between your CPU and your memory, and you should be able to get decent speed and minimal chip count.