Registers in Logisim-evolution

A register is a row of D flip-flops sharing one clock: it stores a whole word (4, 8, 16 bits) instead of a single bit. Registers sit at the heart of CPUs — the program counter, the accumulator, and every pipeline stage are registers.

Building a 4-bit register from D flip-flops

Step 1 — Place four D flip-flops. From the Memory library, place four D Flip-Flops side by side on the canvas.

Step 2 — Share one clock. Add a Clock component (Wiring library) and use a Splitter or plain wire branches to connect the same clock signal to all four flip-flops’ clock inputs. One clock edge must update all bits simultaneously — that is what makes it a register rather than four independent bits.

Step 3 — Bundle the data lines. Add four Input Pins (D3…D0) and four Probes (Q3…Q0), or use a Splitter to work with a 4-bit bus. Enable Simulate → Ticks Enabled (Ctrl+K) and poke in a value like 1010: on the next clock edge, all four bits latch together.

The built-in Register component

For real designs, use the Register component from the Memory library instead: it is exactly what you just built, in one box, with a configurable data width. Select it and set Data Bits to 4, 8, or 16 in the attributes panel. It also gives you a clear input and an enable pin for free.

Making an accumulator

The classic next step: feed the register’s output back through an Adder (Arithmetic library) into its own input, with a constant on the adder’s other input. Each clock tick adds the constant to the stored total — you have built an accumulator, the core of the CPU in our Building a Simple CPU tutorial.

Try it on a breadboard too
Registers make much more sense once you have clocked a real 74-series chip by hand. A starter kit has everything for the comparison experiment.
ELEGOO starter kits

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Go further

Turn registers sideways into shift registers, or see them in action inside a CPU and a MIPS processor.