Memory

The memory in a computer is split between RAM and ROM.

• Random access memory (RAM) is the stuff you can modify by storing your programs and data in.
• Read only memory (ROM) is the stuff that cannot be written, which the computer needs to start up. Typically video game cartridges are also ROMs.

Memory capacity (RAM) is the part of computer systems which tend to be outgrown the fastest, because standard memory sizes double every few years, and programs add features to fill the capacity.

Memory Addressing

Every word in the memory is defined by its location. To access a given word in memory, you must specify its address, which describes its location.

The address is given as a binary number. The maximum amount of memory which can possibly be accessed by a given CPU is limited by its word length, meaning the length of each number the computer works with.

Old 16-bit computers could access kilobytes of memory. Current 32-bit computers can access gigabytes of memory. This will soon start to be a bottleneck, so 64-bit processors are starting to hit the market.

In random-access memory (RAM) every location can be assessed equally fast. Note that this is different than data stored on a tape.

The Memory Hierarchy

There are several different types of `memory' in modern computer systems, which differ in speed and cost. These define a hierarchy from fastest to slowest:

• Registers are the 8 to 64 words of memory which are on the CPU to hold quantities immediately being worked with, like the numbers we are adding. These are most expensive because of the fast circuitry to operate on them.
• Cache is a very fast, small (say 16 kilobyte) memory to hold the section of the program currently running, for fastest possible access of the next instruction.
• Main memory are silicon chips which hold the program currently running, and the data being worked on. Typical memory sizes today are 4 to 32 megabytes. Main memory costs about $25 per megabyte, on little boards of 9 chips called SIMMs. The contents of main memory are lost when the power goes off.
• Secondary storage today is typically a hard disk, which holds from 1 to 8 gigabytes, at a cost of about $500 a gigabyte. Floppy disks are more often used today for moving/storing files rather than on-line use.

Moore's law states that computer power doubles every 1.5 to 2 years, and has been amazingly accurate to date.

Buses

Buses are the paths along which data flows in a computer.

These paths consist of wires, but differ according to: (1) the type of connector, (2) the speed with which they move data around, and (3) the internal protocols which they use.

Every processor has an internal bus where data is moved from the memory to the CPU.

Every motherboard has a bus where expansion cards are connected to it.

Different types of buses use different types of connectors, i.e. plugs and sockets. Assembling a computer system from a shipping carton is hopefully just a process of plugging in matching connectors.

Knowing the difference between male and female connectors is important when ordering cables. Think about human anatomy!

Power Supplies

Power surges on electric lines can damage computer equipment.

Line filters are inexpensive devices which supposedly prevent voltage spikes from reaching your computer.

An uninteruptable power supply (UPS) is a line filter with a battery, so even if the power goes off briefly your machine will not crash.

Electronics will not work at high temperatures. Fans are used to keep computers cool. Don't block the fans or you are asking from trouble.

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