How computers store everything

01 A computer only knows two things

Inside your Mac there are billions of microscopic switches. Each one is either off or on. That is the entire alphabet. We write off as 0 and on as 1, and we call one switch a bit.

One bit alone is nearly useless. It can say yes or no and nothing more. So computers work with bits in groups of eight, and a group of eight is called a byte.

02 Building a number from bits

Each position in a byte has a value, and the values double as you move left. The rightmost bit is worth 1, then 2, 4, 8, and so on up to 128 at the far left.

To read a byte, you add up the values of every position that is switched on. That is all binary is. Click the switches below and watch the number change.

Try to make the number 65. You will need the 64 switch and the 1 switch. Notice what character appears when you get there.

03 Why nobody writes binary

Binary is correct but exhausting. The word "Hacker" in binary is forty-eight characters of zeros and ones. Nobody can read that.

So we use hexadecimal, or hex for short. Hex counts to sixteen instead of ten. After 9 it keeps going with letters:

Here is the reason hex won. One hex digit is exactly four bits, so two hex digits are exactly one byte. Always. That clean fit is why hashes, memory dumps, network captures and colour codes are all written in hex.

Reading a two-digit hex number

The left digit counts sixteens. The right digit counts ones. So 6b is six sixteens plus eleven, which is 107.

04 From numbers to letters

A number like 72 is still just a number. Turning it into the letter H takes one more step, and that step is a lookup table called ASCII.

ASCII is not clever. Decades ago people agreed that 72 would mean H, 97 would mean a, and so on. Every computer follows the same list. That is the whole trick.

Three anchors are worth memorising, because you can work out most of the table from them:

Lowercase always sits exactly 32 above its uppercase twin. So if A is 65, then a is 97. If H is 72, then h is 104. One bit is the only difference between a capital and a small letter.

Try it on your own words

Type anything below. Watch each character break into its character, hex, decimal and binary.

Try it. The name of a file is a label. The first bytes are the truth.

05 Why this matters for security

This is not trivia. Reading raw bytes is a daily task in real security work.

• Malware hides its instructions. A virus that contains the text delete everything is easy for antivirus to spot, so attackers store it as hex or base64 instead, and rebuild it while running.
• Evidence lives in raw bytes. A deleted file often still sits on the disk. Forensic investigators read the drive as raw bytes to recover it.
• File types are proven by their first bytes. Every PNG image begins with the same signature, no matter what the filename says. Renaming virus.exe to photo.png does not change what is inside.
• Network traffic is bytes. When you inspect a captured packet later in Phase 00.4, you will be looking at exactly this.

06 Do it on your real machine

Playgrounds are useful, but you should see this work in a real terminal too. Open the Terminal app on your Mac (press Command and Space, type Terminal, press Enter) and run these.

Turn the number 65 into binary, hex and a character:

python3 -c "print(bin(65), hex(65), chr(65))"

Turn text into raw hex bytes:

echo -n "Hi" | xxd

Turn hex back into text:

echo -n "48 61 63 6b 65 72" | xxd -r -p

See the three anchor characters and the gap of 32 between cases:

python3 -c "[print(f'{c}  dec={ord(c):<4} hex={ord(c):02x}') for c in 'AaHh0']"

Your checklist

• Built the number 65 with the bit switches and saw the letter A appear
• Can explain why two hex digits equal exactly one byte
• Know the three anchors: A is 65, a is 97, the digit 0 is 48
• Ran all four commands above in the real Terminal
• Can say out loud why hex is not encryption

07 Final check

✓ Key takeaways

The things from this lesson worth carrying into the next one. If you remember nothing else, remember these.