1.1.3 · Coding › How Computers Work
Computers sirf do cheezein jaante hain: 0 (false / off / low voltage) aur 1 (true / on / high voltage). Boolean algebra inhi do values ki math hai. Jaise normal algebra mein + aur × hote hain, Boolean algebra mein AND , OR , NOT wagera hote hain. Tumhara CPU jo bhi calculation karta hai — addition, comparison, memory — sab in choti logic operations ko wire karke banta hai. Ye 6 gates seekh lo aur tumhare paas saare digital hardware ki alphabet aa jaayegi.
Definition Chhe operations
Maano A , B ∈ { 0 , 1 } .
NOT (¬ A , A ˉ ): bit ko flip karta hai. Unary (ek input).
AND (A ⋅ B , A ∧ B ): 1 sirf tab jab dono 1 hon.
OR (A + B , A ∨ B ): 1 jab kam se kam ek 1 ho.
XOR (A ⊕ B ): 1 jab inputs alag hon.
NAND (A ⋅ B ): AND ka NOT.
NOR (A + B ): OR ka NOT.
Kyunki ye arithmetic ki tarah behave karte hain. A ⋅ B multiplication jaisa kaam karta hai: 0 ⋅ 0 = 0 , 1 ⋅ 1 = 1 . A + B addition jaisa kaam karta hai sivaaye 1 + 1 = 1 ke (ye saturate ho jaata hai, 1 se aage overflow nahi kar sakta). Yahi analogy poori wajah hai ise algebra kyun kehte hain.
Truth table har possible input combination ke liye output list karta hai. n inputs ke saath 2 n rows hoti hain.
A
B
AND
OR
XOR
NAND
NOR
0
0
0
0
0
1
1
0
1
0
1
1
1
0
1
0
0
1
1
1
0
1
1
1
1
0
0
0
NOT (ek input): ¬0 = 1 , ¬1 = 0 .
AND = "strict" gate (sabka agree karna zaroori hai).
OR = "generous" gate (ek haan kaafi hai).
XOR = "disagreement detector" (output 1 = dono alag hain).
NAND / NOR = literally AND/OR columns ulte kar do (1 ↔ 0 ).
XOR ka formula yaad nahi karte — tum use banate ho. "Inputs kab alag hote hain?" Ya (A = 1 , B = 0 ) ya (A = 0 , B = 1 ). Har English word ko algebra mein translate karo:
"A = 1 aur B = 0 " ⇒ A ⋅ B ˉ
"A = 0 aur B = 1 " ⇒ A ˉ ⋅ B
"ya...ya" ⇒ +
A ⊕ B = A B ˉ + A ˉ B
Ye step kyun? Truth table ki har row jahan output = 1 hota hai, ek AND-term (minterm ) banta hai; hum un sab terms ko OR karte hain. Ise Sum of Products (SOP) kehte hain aur isse tum kisi bhi truth table ka formula derive kar sakte ho.
Intuition NAND ek universal gate hai
De Morgan se, NAND NOT, AND, OR — teeno bana sakta hai. To ek chip factory sirf NAND gates mass-produce karke poora CPU bana sakti hai. Manufacturing ke liye ye bahut badi baat hai.
NOT: A ⋅ A = A ˉ
AND: A ⋅ B = pehle NAND phir NOT.
OR: A ˉ ⋅ B ˉ = A + B (De Morgan).
Worked example 2. XOR sirf NAND se banao
Goal: ek famous result. A ⊕ B NAND (↑ ) use karke.
Maano g = A ↑ B = A B .
Tab A ⊕ B = ( A ↑ g ) ↑ ( g ↑ B ) .
Ye step kyun? Expand karke: A ↑ g = A A B = A ( A ˉ + B ˉ ) = A B ˉ = A ˉ + B . Similarly g ↑ B = B ˉ + A . Inhe NAND karo: ( A ˉ + B ) ( A + B ˉ ) = A ˉ A + A ˉ B ˉ + A B + B B ˉ = A ˉ B ˉ + A B = ( A + B ) ( A ˉ + B ˉ ) = A B ˉ + A ˉ B = A ⊕ B . ✓
A + A ˉ B simplify karo
"Pehle andaza lagao": shayad lage ye complicated rahega. Absorption-style trick se verify karo:
A + A ˉ B = ( A + A ˉ ) ( A + B ) = 1 ⋅ ( A + B ) = A + B .
Ye step kyun? Distribution X + Y Z = ( X + Y ) ( X + Z ) use kiya, phir A + A ˉ = 1 . Clean result A + B . Truth table se check karo — match karta hai. Ye 20% hai jo 80% deta hai: zyaadatar simplification = De Morgan + distribution + A + A ˉ = 1 .
Common mistake Classic errors ko steel-man karna
(a) "OR ka matlab exactly ek hai, isliye 1 + 1 = 0 ." Kyun sahi lagta hai: English mein "chai ya coffee" ka matlab usually ek hota hai. Fix: Boolean algebra mein OR inclusive hai — 1 + 1 = 1 . "Exactly one" waala matlab XOR hai, ek alag gate.
(b) "A + B = A ˉ + B ˉ ." Kyun sahi lagta hai: "bas sab par bar laga do." Fix: De Morgan operator bhi flip karta hai : A + B = A ˉ ⋅ B ˉ . NOT AND↔OR swap karta hai.
(c) "NAND = NOT AND, to pehle NOT karo phir AND." Kyun sahi lagta hai: naam left-to-right padha jaata hai. Fix: ye A ⋅ B hai — pehle AND, phir result ka NOT , A ˉ ⋅ B ˉ nahi.
Recall Active recall — table dhako, phir se banao
Bina dekhe: XOR aur NAND ke liye saari 4 input rows ka output column likho. Phir De Morgan ke do laws batao aur ek single NAND se NOT prove karo.
Recall Feynman: ek 12-saal ke bacche ko samjhao
Light switches socho. AND do switches ek line mein hain — bulb sirf tab jalta hai jab dono on hon. OR do switches side by side hain — koi ek bulb jalaa deta hai. NOT ek ulta switch hai: dabaao aur light band ho jaati hai. XOR woh hallway waali trick light hai — koi bhi switch hilao aur wo change ho jaati hai, to on sirf tab hoti hai jab switches disagree karein. NAND/NOR bas AND/OR ke "opposite-day" versions hain. Itne saare toy switches wire karo aur tumne literally computer ka brain bana liya.
A ND = A ll needed (saare inputs 1).
O R = O ne is enough.
X OR = eX clusive / inputs differ.
N -anything = N egated (answer flip karo).
De Morgan: "Break the bar, change the sign" — lamba bar tod do aur AND↔OR.
When is A AND B equal to 1? Sirf tab jab A aur B dono 1 hon.
When is A OR B equal to 1? Jab kam se kam ek input 1 ho (inclusive OR; 1+1=1).
What does XOR output 1 for? Jab do inputs alag hon (A≠B).
Formula for XOR in terms of AND/OR/NOT? A ⊕ B = A B ˉ + A ˉ B .
State De Morgan's laws. A ⋅ B = A ˉ + B ˉ aur A + B = A ˉ ⋅ B ˉ .
How do you make NOT from a single NAND? Dono inputs ek saath jodo: A ⋅ A = A ˉ .
Why is NAND called a "universal" gate? Ye akela NOT, AND, OR — aur isliye koi bhi circuit bana sakta hai.
Difference between OR and XOR at inputs 1,1? OR 1 deta hai, XOR 0 deta hai.
What is NAND of A and B? A ⋅ B — pehle AND, phir NOT.
Simplify A + A ˉ B . A + B .
How many rows in a truth table with n inputs? 2 n .