Computer Memory: Units, Primary and Secondary, Data Recovery

Memory units: bit to petabyte

A bit (binary digit) is a 0 or a 1. It is the smallest unit.

• 4 bits = 1 nibble
• 8 bits = 1 byte (one letter like "A")
• 1024 bytes = 1 kilobyte (KB)
• 1024 KB = 1 megabyte (MB)
• 1024 MB = 1 gigabyte (GB)
• 1024 GB = 1 terabyte (TB)
• 1024 TB = 1 petabyte (PB)
• Then exabyte (EB), zettabyte (ZB), yottabyte (YB), each ×1024.

Why 1024 and not 1000? Because 1024 = 2¹⁰, and computers count in powers of 2.

How to convert

Going to a smaller unit: multiply by 1024. Going to a bigger unit: divide by 1024.

Primary memory

Primary memory is the memory the CPU uses directly. It is fast but small.

• RAM (Random Access Memory): holds programs and data you are using now. You can read and write. It is volatile: it loses everything when power goes off.
• ROM (Read Only Memory): holds start-up instructions (BIOS/firmware). It is non-volatile: it keeps data without power. Normally we only read it.
• Cache: a very fast, small memory between the CPU and RAM. It keeps data the CPU needs again and again.
• Registers: the fastest, tiniest stores, inside the CPU.

Secondary memory (storage)

Secondary memory keeps data for a long time. It is non-volatile, large and cheap, but slower. The CPU cannot use it directly; data is first brought into RAM.

• Hard disk drive (HDD): spinning magnetic plates.
• Solid state drive (SSD): flash chips, no moving parts, much faster.
• Pen drive, memory card: small flash storage.
• CD, DVD, Blu-ray: optical discs read by a laser.

Primary vs secondary

Primary: fast, small, costly per GB, mostly volatile, used directly by CPU. Secondary: slower, large, cheap per GB, non-volatile, used for saving.

Data deletion, recovery and security

When you delete a file, it usually goes to the Recycle Bin first. Even after emptying it, the computer only removes the file's entry from its list and marks the space as free. The actual bits stay until new data is written there.

Data recovery means getting back deleted or lost data using special tools. It works only if the space has not been overwritten.

This is a risk: if you sell an old phone or pen drive, someone could recover your photos.

Keeping data safe

• Secure deletion: use a shredder tool that writes random data over the file, or a factory reset with encryption.
• Encryption: data is scrambled; only the right key or password can read it.
• Backup: keep a copy on another drive or cloud, so a crash does not lose everything.
• Strong passwords, antivirus, and never plugging in unknown pen drives.

Try it: size detective

Open your phone's storage settings. Note the total (like 64 GB) and how much photos use. Convert the photo size to MB (×1024). Then in step 6 of the 3D, set the same number and unit and check your answer.

Key formulas and definitions

• 1 byte = 8 bits; 1 nibble = 4 bits
• 1 KB = 1024 B; 1 MB = 1024 KB; 1 GB = 1024 MB; 1 TB = 1024 GB; 1 PB = 1024 TB
• Bigger → smaller unit: × 1024; smaller → bigger: ÷ 1024
• RAM: volatile; ROM: non-volatile

Worked examples

Common mistakes

• Using 1000 instead of 1024 in school conversions.
• Saying ROM is volatile. ROM keeps data without power; RAM is volatile.
• Thinking "delete" or "empty bin" destroys data for ever. It can often be recovered.
• Mixing bits and bytes: 8 Mbps internet is 1 MB per second, not 8 MB.