8-Bit ALU System Circuit Simulator

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Was du lernst

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.

So funktioniert es

An 8-bit ALU scales the 4-bit demo to byte-sized operands — taking two 8-bit inputs (range 0–255 unsigned, or −128 to 127 signed) and producing an 8-bit result plus status flags. This is the size of the ALU in classic 8-bit CPUs like the Intel 8080, MOS 6502, and Motorola 6800.

Operations typically supported: - Arithmetic: ADD, SUB, ADC (add with carry), SBC (subtract with borrow), INC, DEC. - Logical: AND, OR, XOR, NOT. - Shift / rotate: SHL, SHR, ROL, ROR (some ALUs include these; others have a separate shifter). - Compare: CMP (subtract A − B and update flags without storing the result).

The operation-select code (typically 4 bits for ~16 operations) drives an output MUX that picks among the functional units' results. Status flags (Z, N, C, V) are computed from the final result and side signals.

For 8-bit addition/subtraction, the carry chain has 8 stages — slow if pure ripple-carry. Real 8-bit CPUs used various optimizations: carry-lookahead in faster chips, simple ripple-carry in cheap chips. The trade-off is speed vs. transistor count.

The 8-bit ALU is a substantial educational milestone: it's the central computational engine of an 8-bit CPU. Once you've built one, the other CPU components (registers, control unit, memory interface) are conceptually simpler.

Schritt für Schritt ausprobieren

Stelle die Eingänge in der Einbettung oben ein, lies was passieren sollte und überprüfe es.

1

A = 00010101 B = 00001010 Op = ADD

Erwartet: Result = 00011111 (= 31)

Was du siehst: 21 + 10 = 31. Watch the result bits and verify no carry-out (sum fits in 8 bits).
2

A = 11111111 B = 00000001 Op = ADD

Erwartet: Result = 00000000, C=1, Z=1

Was du siehst: 255 + 1 = 256, which wraps to 0 in 8 bits. Carry-out fires; zero flag fires; result rolls over.
3

A = 11110000 B = 00001111 Op = AND

Erwartet: Result = 00000000, Z=1

Was du siehst: Bitwise AND — no overlapping 1 bits, so result is all zeros. Zero flag fires.
4

A = 11001100 B = 00110011 Op = OR

Erwartet: Result = 11111111

Was du siehst: Bitwise OR — every bit is 1 in at least one input, so result is all 1s.
5

A = 01010101 B = 01010101 Op = SUB

Erwartet: Result = 00000000, Z=1, C=1

Was du siehst: 85 − 85 = 0. Zero flag fires; carry-out indicates no borrow (the standard convention for two's-complement subtract).

Verwendete Komponenten

Praxisanwendungen

8-bit microcontrollers. PIC, AVR, 8051 — all have 8-bit ALUs at their core. Billions of these are deployed in embedded systems.

Classic gaming consoles. NES (6502), SNES (65C816), Master System (Z80) — all built around 8-bit ALUs.

Educational CPUs. Many university CPU-design courses build an 8-bit CPU around this kind of ALU as the first complete processor.

Embedded data processing. 8-bit ALUs handle byte-stream processing in protocols like USB low-speed, UART, and many sensor interfaces.

Bit-slice predecessors. Before commodity CPUs, designers built custom processors from 4-bit bit-slice ALUs (AMD 2901) chained for any width. The 8-bit ALU is two slices.

Häufig gestellte Fragen

What's the difference between this and the 4-bit ALU?

Bit width. Same architecture, more bits per operand. The carry chain is twice as long, the zero-detect is wider, and the operation count typically grows (8-bit ALUs often add shift/rotate beyond the 4-bit minimum).

How fast is an 8-bit ALU?

In modern CMOS at 5 nm, well into the multi-GHz range — limited primarily by the carry chain through 8 full-adder stages. Classic 1980s 8-bit CPUs ran at 1–4 MHz, with the ALU completing in a single cycle.

Why doesn't the 8-bit ALU include 32-bit multiply?

Multiplication needs more transistors and longer paths than basic ALU ops. Many 8-bit micros omitted it (NES 6502 has no multiply); some included a small multiplier (8-bit by 8-bit → 16-bit result). Modern CPUs always have dedicated multipliers separate from the basic ALU.

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.

Could I build a 16-bit ALU from two 8-bit ALUs?

Yes — chain them with the lower ALU's carry-out as the upper ALU's carry-in. This is the bit-slice approach used historically before single-chip 16-bit CPUs were common. AMD's 2901 was a 4-bit slice that designers chained for 16 or 32 bits.