Primary Memory: RAM, ROM and Cache

Hello students, welcome to Kwickprep. You type a long answer in a document, and the power goes off. Why is your unsaved work gone, but the computer still knows how to start? Today we will learn about primary memory: RAM, ROM, cache and registers, and why each one exists.

First, why does a computer need memory at all? The CPU, or central processing unit, is the brain that processes data, but it can hold only a tiny amount. Memory is the place that holds the data and the instructions the CPU is working on right now. When you type, your input first goes into memory, and the CPU picks it up from there. The answers the CPU produces also wait in memory before they are shown or printed.

Think of your study desk and your school bag. The desk is like primary memory, small but right in front of you, so work is fast. The school bag is like secondary storage, which holds much more but takes time to open. You keep only the books you need right now on the desk, just as the computer keeps only running programs in primary memory.

Primary memory is the memory the CPU can reach directly, and it has four members. RAM is volatile and holds the programs that are running. ROM is non volatile and holds the start up instructions. Cache is volatile, a very fast store between the CPU and RAM. Registers are tiny volatile stores inside the CPU itself. Volatile means the data is lost when power goes off, and we will see this next.

RAM stands for random access memory. It holds the programs and data you are using right now, such as an open browser and a game. It is read and write memory, so the computer can change its contents any time. RAM is volatile, which means everything in it is lost when the power goes off. That is why unsaved work disappears. More RAM lets more apps run together smoothly, which is why phones advertise eight or twelve GB of RAM.

There are two kinds of RAM, static RAM and dynamic RAM. SRAM stores each bit in a circuit called a flip flop, while DRAM stores each bit as a tiny charge in a capacitor. A capacitor leaks its charge, so DRAM must be refreshed thousands of times a second, but SRAM does not need refreshing. Because of this, SRAM is faster than DRAM. SRAM costs much more for the same size, so DRAM is cheaper. So SRAM is used for cache, and DRAM is used as the main memory of computers and phones.

Let us pause and predict. You are writing an essay in a word processor. Suddenly the power goes off before you save. Which memory lost your essay? The answer is RAM, because RAM is volatile, so save your work often.

ROM stands for read only memory. It is non volatile, which means it keeps its data even when the power is off. Normally the computer only reads from it, and the user does not change it. It holds firmware, the built in start up program, called BIOS or UEFI in a PC. When you switch on, this program checks the hardware and then loads the operating system from storage. This start up process is called booting.

ROM has grown over the years into these types. PROM is programmable ROM, written once by the user with a special device and then it can never be changed. EPROM is erasable programmable ROM, erased by shining strong ultraviolet light through a small window on the chip, then rewritten. EEPROM is electrically erasable programmable ROM, erased and rewritten with electric signals while it stays inside the computer. Flash memory is a kind of EEPROM that erases in blocks, and it stores BIOS updates today.

Board exams often ask you to compare RAM and ROM. RAM is volatile, but ROM is non volatile. RAM can be read and written freely, while ROM is mostly read. RAM holds the running programs, and ROM holds the start up firmware. A PC has many GB of RAM, but only a few MB of ROM.

Now, cache memory. Cache is a small amount of very fast SRAM that sits between the CPU and RAM. It keeps copies of the data and instructions the CPU uses again and again. The CPU looks in the cache first, and only goes to the slower RAM if the data is not there. Modern processors have levels of cache called L1, L2 and L3, where L1 is the smallest and fastest. It is like keeping your pen in your hand instead of in the bag.

Now we reach the top of the ladder. Registers are the smallest and fastest memory of all, and they are inside the CPU itself. They hold the single instruction or value the CPU is working on at this very moment. For example, the program counter holds the address of the next instruction to fetch. The accumulator holds the result of the current calculation.

All these memories form a ladder called the memory hierarchy. At the top are registers, the fastest and smallest. Next is cache, very fast with a few MB. Then RAM, fast with several GB. At the bottom is secondary storage like an SSD or hard disk, slower but huge, and it is not primary memory. As we go down, speed and cost per GB fall, while size grows.

Let us revise what we learned today. A computer needs memory to hold the data and instructions in use. RAM is volatile, SRAM is fast and used for cache, and DRAM needs refreshing and is main memory. ROM is non volatile, and its types are PROM, EPROM and EEPROM. Cache is fast SRAM between the CPU and RAM. Registers are the fastest memory, inside the CPU. Try drawing the memory hierarchy from memory before your next class.