@Muse - This mini 4-bit CPU core implements an arithmetic datapath with an accumulator register, status flag detection, and real-time output visualization.
### Architecture Overview
1. **Operand & Mode Input**:
- Operand **B** ($B_3 B_2 B_1 B_0$) is set via logic toggles #B3 through #B0.
- Operand **A** ($A_3 A_2 A_1 A_0$) is fed back dynamically from the 4-bit accumulator register outputs (#a3, #a2, #a1, #a0).
- Mode switch #OP controls the arithmetic operation:
- #OP = $0$: **Addition** ($A + B$, with initial carry $C_{\text{in}} = 0$).
- #OP = $1$: **Subtraction** ($A - B = A + \overline{B} + 1$, using 2's complement by XOR-inverting $B$ and setting $C_{\text{in}} = 1$).
2. **4-Bit Arithmetic Logic Unit (ALU)**:
- Built from a 4-stage full adder/subtractor array using XOR, AND, and OR gates.
- Computes sum/difference bits #s0, #s1, #s2, and #s3.
- Produces the carry/borrow-out line #cout and detects the zero state (#zero) via NOR gate logic across all sum bits ($Z = \overline{S_0 + S_1 + S_2 + S_3}$).
3. **Accumulator Register (#REG0–#REG3)**:
- Four edge-triggered D flip-flops capture and hold the ALU result bus on the rising edge of #CLK.
- The stored values immediately feed back to the ALU's **A** operand port, enabling iterative accumulation, counting, and subtraction loops.
4. **Output & Status Indicators**:
- **7-Segment Display (#DISP)**: Driven by the BCD/hexadecimal decoder #DEC, rendering the current accumulated value in hexadecimal ($0$ to $\text{F}$).
- **Carry/Borrow LED (#FLAG_C)**: Illuminates red when the ALU generates an arithmetic carry or borrow out.
- **Zero Flag LED (#FLAG_Z)**: Illuminates green whenever the current ALU computation results in $0000_2$.
