In the 8085 microprocessor, what operation is performed by the instruction SUI 20H?
Subtracts 20H from the accumulator
The 8085 microprocessor provides a family of arithmetic instructions that operate on the accumulator (register A). Within this family, the mnemonic suffix distinguishes the exact operation: instructions ending in I use an immediate operand (a constant supplied in the instruction itself), while variants with a B involve the borrow/carry flag.
- SUI data stands for Subtract Immediate from Accumulator. It performs A ← A − data, subtracting the 8-bit immediate value directly from the accumulator, and it does not include any borrow.
- Therefore SUI 20H computes A ← A − 20H. If A initially holds, say, 50H, then after this instruction A = 30H. The result is stored back in A, and the flags (Sign, Zero, Carry, Auxiliary Carry, Parity) are updated according to the outcome; a borrow sets the Carry flag.
Hence the correct description is that it subtracts 20H from the accumulator. The other options confuse SUI with related mnemonics. Subtracting with borrow describes SBI (A ← A − data − CY), which additionally deducts the carry flag. Adding 20H would be the job of ADI (A ← A + data). Comparing 20H with the accumulator is done by CPI, which performs the subtraction internally only to set flags but leaves A unchanged. SUI, by contrast, actually alters the accumulator and involves no borrow.
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