Reply #8: I shall try [View All]

[jmowreader]

A bus in a computer is a group of wires. They call it that because the computer bus carries information like a city bus carries passengers. (Well, they hadda call it something.)

There are two kinds of buses inside your computer--address buses and data buses. The address bus is used to tell the computer to look at a particular spot in the computer's high-speed memory--RAM and ROM. It's important to the user mainly because it sets an upper limit on the amount of RAM you can have. (When the Macintosh 128k was released, we knew that the processor could address a for-that-time stupendous 16MB RAM because it had a 24-bit address bus. No one had any idea what you would ever need 16MB RAM for, although we were plenty pissed that the design of the Mac limited it to 4MB RAM. We didn't know what you'd ever want with that much RAM either. Before you start chortling, remember that neither DOS nor the Macintosh operating system would run more than one program at a time. And before you start laughing about how limited the Mac's measly memory capability was, remember that the 8088 processor in the IBM PC and PC-XT had a 20-bit address bus and would only address 1MB RAM.)

Which brings us to the data bus, the subject of this discussion.

Modern processors have two: the front-side bus and the back-side bus. The back-side bus is where your cache memory lives. Cache is a very small amount of very fast memory--just a couple of megabytes. I can explain what it does later if you'd like. It's on a separate bus because if the CPU was making cache requests along the same bus as it was making main-memory requests, it would slow the cache down so much that having it would be pointless. It's as small as it is because (1) it's expensive as hell, (2) it generates a lot of heat, and (3) nowadays, they put a lot of it in the processor itself and they don't want the processor to be the size of a Buick.

The front-side bus is where your main memory lives. (This is also the bus the computer uses to talk to the video card, hard drive, modem and all the other things your computer is running, but that's not important now.) You know when you look at a computer ad, it says "This computer has 512MB RAM!" That's main memory. It's sitting on the front-side bus. And the faster that bus runs (it hasn't run at processor speed in decades) the faster your memory calls run and the faster your computer goes. Fast is good. The bus is rated in megahertz, same as your computer used to be before they started selling GHz PCs, and 800MHz is a very fast one.

Short answer: this computer is nice and fast.