Question

How many special function registers are there in 8051 microcontroller?

• A. 5
• B. 8
• C. 10
• D. 20

Hint:

SFRs are located at RAM addresses $80\text{H}$ to $FF\text{H}$.
Only those SFR addresses that are divisible by 8 (e.g., $80\text{H}$, $88\text{H}$, etc.) are bit-addressable.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the count of Special Function Registers (SFRs) present in the standard 8051 microcontroller.

Step 2: Key Formula or Approach:
Special Function Registers (SFRs) are dedicated registers used to control and monitor the hardware peripherals of the microcontroller, such as timers, I/O ports, serial communication, and interrupts.
They are mapped to the upper 128 bytes of internal RAM (addresses $80\text{H}$ to $FF\text{H}$).

Step 3: Detailed Explanation:

• The 8051 microcontroller has 21 officially documented Special Function Registers (SFRs) in the original standard design.

• These registers include:
- Ports: P0, P1, P2, P3
- Math/Control: ACC (Accumulator), B, PSW (Program Status Word), SP (Stack Pointer), DPTR (DPL, DPH)
- Timers: TMOD, TCON, TH0, TL0, TH1, TL1
- Serial Port: SCON, SBUF
- Power/Interrupts: PCON, IE, IP

• While there are 21 registers, they are often rounded or grouped close to 20 in several examinations.

• Out of the given options, the number 20 is the closest.

Step 4: Final Answer:
The number of special function registers is 20, which corresponds to Option (D).