Special Function Register address is___.

Question

Examoogle · Accepted Answer

80h to FFh

Explanation: In the 8051 microcontroller architecture, the internal RAM address space is divided into two distinct regions: the lower 128 bytes (00h to 7Fh) for general-purpose data memory and the upper 128 bytes (80h to FFh) reserved for Special Function Registers (SFRs). Accessing addresses in the range of 80h to FFh triggers the microcontroller to read or write to specific control, configuration, or status registers rather than general-purpose RAM.

Working Principle: The 8051 memory map separates standard RAM from SFRs to simplify peripheral management. Addresses from 80h to FFh are mapped to the SFR space, which contains registers such as P0, P1, TMOD, TCON, SCON, and the Accumulator (ACC). When an instruction targets an address in this range, the decoder logic routes the data bus to the specific peripheral control hardware instead of the data memory cells.

Key Formulas: $Memory_{Total} = 2^{8} = 256$ bytes total addressable internal space; $SFR_{Range} = [80h, FFh]$

Key Points: The 8051 uses a 256-byte internal RAM architecture.; Addresses 00h to 7Fh are strictly for General Purpose RAM.; Addresses 80h to FFh are reserved for SFRs, though not all 128 slots are physically implemented.; Direct addressing is mandatory for accessing SFRs.

Advantages: Efficient control of peripherals via memory-mapped I/O.; High-speed access to status and control flags.

Disadvantages: Non-implemented addresses in the SFR range are reserved and should not be accessed.; Limited space forces careful management of peripheral configuration registers.

Applications: I/O Port configuration; Timer/Counter initialization; Serial communication setup; Interrupt control

Additional Info: Only a subset of the 128 byte range (80h-FFh) is actually used by the 8051; unassigned addresses are reserved.. Option A is incorrect because 00h to FFh includes both standard RAM and SFRs, not just SFRs.. Option B is incorrect as it describes part of the lower RAM range.. Option D is incorrect as it is a sub-section of the General Purpose RAM.

Answer: C is correct — The Special Function Register (SFR) space in an 8051 microcontroller is defined from address 80h to FFh.

Exam Tip: Always remember that SFRs are accessed using 'direct' addressing mode, whereas internal RAM locations can be accessed using either 'direct' or 'indirect' (using R0/R1) addressing.

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