RAM - Memory Technology Overview

An Anecdote

Getting the whole picture of how memory performance impacts system performance is still a very difficult task. If all this talk of timings and latencies has not helped, let us provide another comparison. Think of the CPU as a cook at a restaurant, busily working to keep up with customer demand. There is a process that occurs. Waiters or cashiers take the orders and send them to the cook, the cook prepares the food, and the final result is delivered to the customer. Sounds simple enough, right? Let's look at some of the details.

When an order for a dish comes in, certain common items (e.g. fries, rice, soup, salads, etc.) may already be prepared, so delivering them to the customer occurs rapidly. We can think of this as the processor finding something in the L1 cache. This is great when it occurs, but it only occurs for a very limited number of items. Most of the time, the cook will need to begin preparing the order, so he will get the items from the cupboard, freezer and refrigerator and begin cooking them. This time, the ingredients are in the L2/L3 cache. So far so good, but where does RAM come into play?

As items are pulled from the fridge, freezer, etc., the restaurant will need to restock them. The supplies have to be ordered from headquarters or whomever the restaurant uses. This is akin to system RAM (or maybe even the hard drive, but we'll leave that out of the analogy for now). If the restaurant can anticipate needs properly, it can order the supplies in advance. Sometimes, though, supplies run low - or maybe you didn't order the correct amount of supplies - and you need to send someone off to a local store for additional ingredients. This is a cache miss, and the store is the system RAM. In a time-critical situation such as this one, the cook wants the ingredients ASAP. A closer store would be better, or perhaps a store with faster checkout lanes, but provided that the trip does not take a really long time, any store is about as good as another. Basically, system RAM with its timings and latencies can have an impact, but a really fast memory controller (i.e. a store next door) with slower RAM (slow checkout lanes) can be more important than having the fastest RAM in the world.

This is all well and good for smaller restaurants and chains, but a large corporation (e.g. McDonald's) cannot simply walk next door to pick up some frozen burgers. In this case, the whole supply chain needs to be highly efficient. Instead of ordering supplies once a week, inventories might be checked every night, and orders placed as necessary. Headquarters has forecasts based on past requirements and may send orders to their suppliers months in advance. This supply chain correlates loosely with the idea of outstanding memory requests, prefetch logic, deeper buffers, etc. Bandwidth also comes into play here, as a large chain might have several large trailers of supplies en route at any point in time, while a smaller chain might be able to get by with only one or two moderately-sized delivery vans.

With faster processors, faster buses, faster RAM, etc., the analogy is moving towards all processors being large corporations with huge demands. Early 8088 and 8086 processors could just wander to the local store as necessary - like what most adults do for their own cooking needs. As the amount of data being processed increases, though, everything becomes exponentially more difficult. There is a big jump from running one small restaurant that serves a few dozen people daily to serving hundreds of people daily, to running several locations, to running a corporation that has locations scattered across the world. That is essentially what we have seen in the world of computer processors. We have gone from running a local "mom-and-pop" burger joint to running McDonald's, Burger King, and several other hamburger chains.

This analogy is probably flawed at numerous levels, but hopefully it helps. If you think about it, the complexity of any one subsystem of the modern PC is probably hundreds of times greater than that of the entire original IBM PC. The change did not occur instantly, but even the largest of technology corporations are going to have a lot of trouble staying at the top of every area of computers.