8051 Internal Data Memory Capacity What is the size of the internal RAM (lower internal data memory) available in a standard 8051 microcontroller?

Introduction / Context:
Memory organization is fundamental to programming the 8051. The classic 8051 family divides internal memory into lower internal RAM and a separate SFR space, with some derivatives adding upper RAM. Correctly identifying capacities avoids addressing errors and stack overflow problems.

Given Data / Assumptions:

• Reference to the original 8051 architecture.
• Lower internal RAM includes register banks, bit-addressable area, and general RAM.
• Upper internal RAM (if present) is device dependent and not universal.

Concept / Approach:
The original 8051 implements 128 bytes of lower internal RAM from addresses 00H to 7FH. This space comprises four register banks (00H–1FH), a bit-addressable region (20H–2FH), and general purpose RAM (30H–7FH). SFRs occupy 80H–FFH and are not counted as RAM bytes.

Step-by-Step Solution:
1) Identify address map: 00H–7FH is lower RAM.2) Count size: 7FH–00H + 1 = 128 bytes.3) Conclude that classic 8051 internal RAM is 128 bytes.

Verification / Alternative check:
Many modern variants add extra internal RAM up to 256 bytes or more, but the canonical answer for the base 8051 remains 128 bytes.

Why Other Options Are Wrong:

• 32 bytes, 64 bytes: Too small, do not match the documented map.
• 256 bytes: Applies to some derivatives with upper RAM, not to the baseline device.

Common Pitfalls:
Counting SFR space as RAM or assuming all 8051 derivatives share the same RAM size.

Final Answer:
128 bytes