Lesson 6 — Intermediate

Control Unit (Hardwired & Microprogrammed)

Instruction fetch ho gayi, decode ho gaya — ab kaun decide karega ki kaunsa register padhein, kaunsa bus enable ho, ALU kya kare? Control Unit. Is lesson mein hum dekhenge CU ke signals kaise bante hain, hardwired aur microprogrammed dono styles ka farak, aur microprogramming ka poora khel.

⏱ 14 min• Intermediate• Computer Arithmetic

Control Unit kya hai

WHAT

Control Unit (CU) CPU ka conductor hai — instruction padh kar saare control signals (read, write, register load, ALU op, bus enable) generate karta hai taaki instruction sahi sequence mein execute ho. Khud data process nahi karta.

WHEN

Har instruction ke har step pe — fetch se lekar write-back tak. Jitne bhi control signals jaate hain, wo CU se hi nikalte hain.

WHERE

CPU ke andar, ALU ke saath. CU ke do tarike hain — hardwired (circuit) aur microprogrammed (stored program).

Ek line mein: ALU karta hai, CU batata hai kab kya karna hai. CU ke paas instruction ka decode hota hai aur wo timing ke saath control signals baantta hai — bilkul railway station master jaisa.

Control signals aur timing

Common control signals :
  MemRead / MemWrite      → memory padho ya likho
  RegRead / RegWrite      → register file access
  ALUSrc, ALUOp           → ALU ka input source aur operation
  PCWrite, IorD, MtoReg   → PC update, bus select, result select
  BusRd/Wr, IRQAck        → bus aur interrupt handshake

Instruction ADD R1, R2, R3 ka control sequence :
  T1 : PC → MAR, MemRead           (fetch ka address)
  T2 : MDR → IR, PC = PC + 1       (instruction aayi)
  T3 : decode → R2, R3 padho       (register read)
  T4 : ALU = R2 + R3               (execute)
  T5 : result → R1                 (write back)

Har step = ek clock cycle (single-cycle) ya pipeline stage

Yaad rakho: Control signals teen jagah se nikalte hain — instruction register (kya), timing generator (kab), aur flags (condition). Teeno ka combination milke CU ka output banata hai.

Hardwired control unit

Control signals combinational logic circuits se bante hain (gates, decoders, flip-flops).
Fast — kyunki koi memory access nahi, seedha circuit output.
Rigid: design badalna = circuit badalna — naya instruction set aaya toh chip dobara.
FSM-based: state machine (states = timing steps) control banati hai.
Modern RISC processors isse chalte hain (simple instruction set, tez zaroorat).

Hardwired flow :
  IR (opcode) → Decoder → Logic gates → Control signals
  Timing counter → kaun sa step chal raha hai

  Faida : SPEED (no lookup), Cost : flexibility
  RISC style : simple fixed instructions → circuit manageable

Microprogrammed control unit

Idea : har machine instruction ko chhote MICRO-INSTRUCTIONS
       mein tod do, wo control memory mein store karo
       (jaise Von Neumann ne program memory mein rakha tha)

Control Memory (CSROM) : microprograms stored yahan
Microprogram           : instruction → sequence of micro-operations
Microinstruction       : ek step ke saare control signals (one line)
Microsequencer         : agla microinstruction ka address decide

HORIZONTAL microprogramming :
  - wide microinstruction — ek bit ek control signal (1-hot)
  - bahut signals ek saath (parallel) — tez, par memory badi

VERTICAL microprogramming :
  - narrow, encoded fields (jaise opcode bits)
  - kam parallelism, chhoti microinstruction — sasta

Ex: x86 jaisa CISC = microprogrammed (complex instructions todne padte)
    ARM/MIPS = hardwired (simple instructions)

Microprogrammed = flexible software-jaisa — naya instruction set = sirf control memory update. Isliye CISC processors mein use hua, jahan 200+ complex instructions ko handle karna tha.

Interview comparison: Hardwired = fast, costly to change, RISC. Microprogrammed = slower (memory lookup), easy to modify, CISC. Ye 4-line answer interview mein bol do — selection ka guaranteed question.

Exercise

Question: Microprogrammed control unit mein microinstructions kahan store hote hain? (control memory / ram / general registers)

Question: Hardwired control unit kis se banti hai — combinational logic se ya microcode se?

Question: Micro-instruction ke chhote steps ko kya kehte hain? (micro-operations / macros / interrupts)

Common mistakes

CU ko processing unit samajhna: CU khud calculate nahi karta — wo sirf control signals banata hai. Calculation ALU karta hai. Ye distinction har interview mein aata hai.
Hardwired ko slow bolna: Hardwired FAST hai (direct circuit), microprogrammed thoda slow (control memory access). Order ulta mat karna.
Microprogrammed ko compiler samajhna: Microprogram instruction ko todta hai micro-operations mein — ye hardware/control ka concept hai, software compiler nahi.

Desi example: Railway station master

Station master train nahi chalata — sirf signal deta hai (control signals). Train = instruction, signal = CU output:

Instruction = train (ADD aa gaya)
CU (station master) : padha, decode kiya
Signals : platform do (register read), raasta khole (bus enable),
          aage bhejo (ALU op) — har step ka signal
Hardwired = manual signal board (fast, fixed)
Microprogram = stored rulebook (flexible, slow)

Exam mapping (GATE / UGC / Interviews)

GATE: hardwired vs microprogrammed comparison — short/direct question.
UGC-NET: control memory, microinstruction, horizontal vs vertical definitions.
Interview: "CU kya karta hai?" + "RISC mein kaunsa CU?" (hardwired).
Trap: "Microprogrammed CU is faster than hardwired" — galat, flexibility ka price speed hai.

Lesson complete?

Control unit clear — ab tum jaante ho instructions convert kaise hote hain signals mein. Agla lesson — Memory Hierarchy — mein dekhenge itni saari memories kyun aur kaise organise hoti hain.