# Step-by-step solution (with final answer) Below is a concise, correct, and practical 8051 solution that (1) adds the bytes at internal RAM `55H` and `56H`, (2) stores the low byte of the sum at `57H`, and (3) snapshots the `PSW` to `60H` for verification — done safely (no side effects). --- ## 1) Goal * Read `55H` and `56H` (on-chip internal RAM). * Compute 8-bit sum → store low byte at `57H`. * Capture `PSW` immediately after the `ADD` so you can inspect `CY`, `AC`, `OV`, `P`, etc. --- ## 2) High-level algorithm 1. Load `55H` into accumulator `A`. 2. `ADD A, 56H` — adds `56H` to `A` and updates PSW flags. 3. Store `A` → `57H`. 4. Save PSW to `60H` without disturbing A or PSW (use `PUSH PSW` / `POP 60H`). 5. Halt (infinite loop). > Note: `MOV 60H, PSW` is **not** a legal 8051 instruction (8051 does not support `MOV direct, direct`). Also avoid `MOV A, PSW` if you want to preserve the accumulator's value and parity from the addition. --- ## 3) Final assembly program (commented) ```assembly ; Add RAM[55H] + RAM[56H] -> RAM[57H] ; Snapshot PSW to RAM[60H] for verification ORG 0000H MOV A, 55H ; A <- RAM[55H] ADD A, 56H ; A <- A + RAM[56H] ; PSW flags updated (CY/AC/OV/P) MOV 57H, A ; store low byte of sum at RAM[57H] ; Save PSW safely (no change to A or flags) PUSH PSW ; push PSW onto stack POP 60H ; pop from stack into RAM[60H] (snapshot of PSW) HERE: SJMP HERE ; infinite loop (stop) END ``` --- ## 4) Instruction-by-instruction explanation * `MOV A, 55H` — direct addressing reads internal RAM `55H` into accumulator. * `ADD A, 56H` — adds direct RAM `56H` to `A`. This updates PSW bits: `CY`, `AC`, `OV`, and parity `P`. * `MOV 57H, A` — stores the result (lower 8 bits) into `57H`. Any carry (overflow beyond 8 bits) is reflected in `CY`. * `PUSH PSW` / `POP 60H` — pushes current PSW onto the stack, then pops it into internal RAM `60H`. This records PSW exactly as it is after the `ADD`, without disturbing `A` or flags. * `SJMP HERE` — infinite loop to stop execution. --- ## 5) PSW bits (after the `ADD`) — how to read `RAM[60H]` PSW (bit7 .. bit0): * **b7 CY** — Carry: `1` if unsigned sum > `0xFF`, else `0`. * **b6 AC** — Auxiliary carry: `1` if carry from bit3 → bit4 (BCD relevance). * **b5 F0** — User flag (not affected by `ADD`). * **b4 RS1**, **b3 RS0** — Register bank select bits. * **b2 OV** — Overflow: `1` if signed overflow (carry into MSB ≠ carry out). * **b1 —** — Reserved (usually unused). * **b0 P** — Parity of accumulator `A` **after** the add: `1` = *odd* parity, `0` = *even* parity. --- ## 6) Quick verification examples **Example A** * `55H = 0xF0`, `56H = 0x30` → sum = `0x120` * `57H = 0x20` (low byte) * `CY = 1`, `AC = 0`, `OV = 0`, `P = 1` (0x20 has one ‘1’ bit → odd parity) * `RAM[60H]` will contain PSW with those bits set accordingly. **Example B** * `55H = 0x7F`, `56H = 0x01` → sum = `0x80` * `57H = 0x80` * `CY = 0`, `OV = 1` (signed 127 + 1 → −128 overflow), `AC = 1`, `P = 1` --- ## 7) Optional: store carry as high byte (16-bit view) If you want a 16-bit result (high byte = `CY`) stored at `58H`: ```assembly ; after MOV 57H,A and after PSW snapshot if desired PUSH ACC MOV A, #00H ADDC A, #00H ; A becomes 0x00 or 0x01 depending on CY MOV 58H, A ; store carry (high byte) POP ACC ``` `PUSH/POP ACC` preserves the original accumulator value. --- ## Final answer (concise) Use the program shown in section **3**. It implements the required addition, stores the low byte at `57H`, and snapshots `PSW` to `60H` correctly and safely (no illegal instructions, no side effects).