While memory architecture has gotten more complex, the fundamental organization of a DRAM has not changed much. Basically, DRAM is organized as an array of rows and columns. In modern chips, a single column location on a row actually contains more than one memory cell (bit). The number of cells equals the width of the DRAM’s data bus. For example, the following illustration shows a chip with an 8-bit data bus, so a location at any row/column coordinate holds 8 bits, all of which are accessed simultaneously.
A DRAM may have multiple banks of these memory arrays, as shown in the simplified diagram of an older 32MB chip. The 32MB DRAM is divided into four banks, each having 8,192 rows and 1,024 columns. Multiply everything (8Kx1Kx8x4), and you’ll see that this device is a 256 megabit memory—far smaller than the multi-gigabits in newer chips.
Controlling Rows and Columns Through RAS and CAS
To understand modern memory architectures, it’s helpful to look back at devices where chip package pins had more-direct control of the memory circuits. A combination of two control signals, the Row Address Strobe (RAS) and Column Address Strobe (CAS), selects a specific memory location (one byte, in our example) for reading or writing. Note that although the diagram shows external pins for these signals, these pins no longer exist; memory chips now produce CAS and RAS signals internally in response to external commands.
Sometimes you’ll hear the total bits in a single row referred to as a page, which is the number of cells that the chip can simultaneously activate for reading or writing. The page size is usually the number of columns multiplied by the width of the data bus. In our diagram, where a row has 1,024 8-bit column locations, a page contains 8,192 bits (1,024 columns per row times 8 bits per column location). When given a RAS signal, a group of sense amplifiers will sense the stored value on the tiny capacitors at each bit location in a page. A CAS signal would then allow the system to rapidly read the data bits at each column location of the opened page. Continued…