In 8051, consider the following instructions: MOV PSW, #18H MOV R3, #55H MOV A, R3. From which register bank is R3 selected in the instruction MOV A, R3?
Bank 3
The 8051 keeps four banks of eight general-purpose registers (R0–R7), called Bank 0, Bank 1, Bank 2 and Bank 3, occupying internal RAM addresses 00H–1FH (8 bytes per bank). Only one bank is active at a time, and the active bank is chosen by two bits inside the Program Status Word (PSW): RS1 (bit 4) and RS0 (bit 3).
The instruction MOV PSW, #18H loads the PSW with 18H. Converting to binary:
| Bit | 7 | 6 | 5 | 4 (RS1) | 3 (RS0) | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|---|
| Value | 0 | 0 | 0 | 1 | 1 | 0 | 0 | 0 |
So RS1 = 1 and RS0 = 1. The bank-select mapping is: 00 → Bank 0, 01 → Bank 1, 10 → Bank 2, 11 → Bank 3. Since RS1 RS0 = 11, the active bank is Bank 3, whose registers live at addresses 18H–1FH; here R3 corresponds to address 1BH.
Therefore MOV R3, #55H writes 55H into R3 of Bank 3, and MOV A, R3 reads that same R3 from Bank 3. That is why the correct answer is Bank 3.
Why the other options are wrong:
- Bank 0 would require RS1 RS0 = 00 (PSW value 00H), but the loaded value 18H sets both select bits to 1.
- Bank 1 corresponds to RS1 RS0 = 01, and Bank 2 to RS1 RS0 = 10; neither matches the 11 pattern produced by 18H.
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