DRAM Memory Access: In DRAM memory chips (not SRAM ones), memory locations are read from by specifying two values: A RAS (Row Address Strobe) value, and a CAS (Column Address Strobe) value. Although computer memory is, from the user's point of view, basically just numbered in order from 0 to (however much RAM you have), on the actual DRAM chip, memory is accessed more like in a grid: First you specify the row, then the column of the memory cell (byte) in the grid that you want to read from. A DRAM has a RAS pin and a CAS pin on it; First the row access number is put on the address pins (of the address bus), then the RAS pin is triggered to make the chip read the row number. RAS is an "active-low" pin, meaning to activate it, the signal to it is actually turned off to indicate that a row number is to be read. (RAS is left low until CAS is activated as well.) Next, the column access number is put on the address line, and the CAS pin is triggered. (Like RAS, CAS is an active-low signal.) If the Write Enable pin is not active, the contents of the selected address are then placed on the data bus. If the Write Enable pin is active, the data bus is read from and its contents placed in the selected memory address. (Like RAS and CAS, Write Enable is an active-low signal too.) DRAM must be refreshed very frequently (normally a few hundred times every second), or the data in it will be lost. If you are going to build a circuit that uses DRAM, you must add some supporting chips to refresh the DRAM as well. (Forget any ideas of operating DRAM by hand, sending signals manually into the pins; It is humanly impossible to do it fast enough.) For this reason, if you just want a small RAM-using circuit that isn't too complicated, the use of SRAM is suggested; It's more expensive, but it's much simpler and you can operate it manually. To refresh DRAM, you do not need to keep writing the same data to it constantly; DRAM is refreshed by simply *reading* the data in it. The memory controller chip typically reads each row of RAM to refresh it, and it goes through the entire RAM in about 2 milliseconds. Thus, the RAM is refreshed about 500 times a second.