8-Bit ALU with Output Enable
A packaged 8-bit ALU provides eight arithmetic, Boolean, and shift operations plus Zero, Carry/Borrow, Negative, and Overflow outputs. OE releases all twelve outputs to high impedance when LOW.
What You'll Learn
- Perform arithmetic and logical operations on 8-bit operands.
- Read 8-bit results in binary, decimal, and hex.
- Generate and interpret status flags (Z, N, C, V).
- Recognize the 8-bit ALU as the heart of classic 8-bit microprocessors.
- Trace the operation-select code through to the output MUX.
How It Works
This circuit exposes the complete interface of one packaged 8-bit Arithmetic Logic Unit. Sixteen switches provide A0–A7 and B0–B7, three switches provide Op0–Op2, and a twentieth switch drives OE (output enable). Eight lamps display R0–R7; four more display Zero (Z), Carry/Borrow (C), Negative (N), and signed Overflow (V).
Op2Op1Op0 selects exactly eight operations: 000 ADD, 001 SUB, 010 AND, 011 OR, 100 XOR, 101 NOT A, 110 shift A left by one bit, and 111 shift A right by one bit. The result is constrained to eight bits. ADD wraps modulo 256 and asserts C on carry-out. SUB computes A−B modulo 256 and uses the DigiSim convention C=1 when an unsigned borrow is required (A Z is HIGH when the 8-bit result is zero. N copies the result's most-significant bit. V reports signed overflow for ADD and SUB and a sign change for left shift; it is distinct from unsigned C. When OE is HIGH, all eight result bits and four flags are driven. When OE is LOW, the component releases all twelve outputs to the high-impedance state. The displayed OE switch is therefore part of the circuit's functional input set, not a decorative control.
Try It Step-by-Step
Set the listed controls in the embedded circuit, then compare the observed modeled state with the expected result.
- 1A=00010101B=00001010OE=1Op=ADDExpected:
Result = 00011111 (= 31)What you'll see:21 + 10 = 31. Watch the result bits and verify no carry-out (sum fits in 8 bits). - 2A=11111111B=00000001OE=1Op=ADDExpected:
Result = 00000000, C=1, Z=1What you'll see:255 + 1 = 256, which wraps to 0 in 8 bits. Carry-out fires; zero flag fires; result rolls over. - 3A=11110000B=00001111OE=1Op=ANDExpected:
Result = 00000000, Z=1What you'll see:Bitwise AND — no overlapping 1 bits, so result is all zeros. Zero flag fires. - 4A=11001100B=00110011OE=1Op=ORExpected:
Result = 11111111What you'll see:Bitwise OR — every bit is 1 in at least one input, so result is all 1s. - 5A=01010101B=01010101OE=1Op=SUBExpected:
Result = 00000000, Z=1, C=0What you'll see:85 − 85 = 0. Zero is HIGH; the simulator's borrow-style C flag remains LOW because no borrow is required. - 6A=00010101B=00001010OE=0Op=ADDExpected:
R0–R7 and Z/C/N/V = high impedanceWhat you'll see:Set OE LOW. The ALU releases all result and flag outputs to high impedance; this is a bus-disconnect state, not a numeric zero result.
Components Used
Applications and Design Context
8-bit processors and microcontrollers. Byte-wide ALUs perform integer arithmetic, masks, comparisons, and shifts in architectures such as the 6502, Z80, AVR, and 8051 families. Their exact instruction sets and flag conventions differ from this teaching component.
Educational CPU datapaths. An 8-bit ALU can sit between operand registers and a result bus while a control unit supplies the operation code and output-enable signal.
Embedded control. Byte-sized arithmetic and bitwise operations are common in register manipulation, protocol fields, sensor values, and compact state machines.
Flag-driven control flow. Z, C, N, and V support equality, unsigned-range, sign, and signed-overflow decisions when the instruction-set architecture defines matching branch semantics.
Bus-oriented designs. OE demonstrates how a functional unit can release a shared bus; a complete system must ensure that at most one enabled driver controls that bus at a time.
Frequently Asked Questions
What's the difference between this and the 4-bit ALU?
Both components implement the same eight operation codes. This one widens A, B, and R from four to eight bits, displays all four Z/C/N/V flags, and adds an output-enable input. The 4-bit template displays only its Zero and C outputs.
Does this simulation predict hardware clock speed?
No physical timing can be inferred from this behavioral simulator component. Hardware delay depends on logic family, process, voltage, temperature, adder architecture, routing, and load. Use the template to verify logic and flags, not to predict silicon frequency.
Why does this ALU not include multiplication?
Multiplication requires a wider result and substantially different logic from the eight operations modeled here. A processor may implement it with a separate combinational multiplier, an iterative shift-add unit, or software; it is not part of this component's Op table.
What status flags does an 8-bit CPU typically have?
Zero (Z), Negative/Sign (N), Carry (C), Overflow (V). Some add Half-Carry (H) for BCD arithmetic, Parity (P) for serial-protocol checks. The 6502 has N, V, B, D, I, Z, C — 7 status bits in one register.
Can two of these components be chained directly into a 16-bit ALU?
Not by wiring these two packaged components directly: this ALU exposes C but has no carry-in input. A 16-bit arithmetic design needs a component or gate-level slice interface that propagates carry between halves; logical operations can be applied independently to each byte.