8-bit assembler simulator alternative: build the CPU that runs it
The Simple 8-bit Assembler Simulator by Marco Schweighauser is a small, free (MIT) browser simulator at schweigi.github.io/assembler-simulator. It uses a simplified assembler syntax based on NASM and simulates an x86-like 8-bit CPU with four general-purpose registers (A, B, C, D), 256 bytes of memory and console output. You type a program, assemble it, and run or step it while the registers, flags and every byte of memory are shown. Its author also wrote a blog series on how to build such a simulator in JavaScript. The assembler simulator runs your program on a finished 8-bit CPU; digiwleea has you build an 8-bit CPU yourself, gate by gate.
What the assembler simulator is good at
A first taste of assembly with almost nothing to learn: MOV, ADD, jumps and a stack on a tiny x86-like machine, with every register and all 256 bytes of memory on one screen, in the browser. It is quick for trying out an instruction or showing a class how a loop runs. (digiwleea is an independent project, not affiliated with the simulator or its author.)
How digiwleea is different
digiwleea does not run x86-like assembly; it shows the machine underneath. In the browser you build NOT and NAND from CMOS transistors, then adders, an ALU, registers, RAM, a program counter and a control unit, through a 40+ level course where every part is checked against its truth table, with an AI tutor (Wleea). The CPU you finish with is an 8-bit accumulator machine whose capstone runs a stored LOAD, ADD, STORE, HALT program; Act IV builds the parts behind SUB, JMP, JZ and OUT.
| Simple 8-bit Assembler Simulator | digiwleea | |
|---|---|---|
| Focus | Writing assembly for a small x86-like CPU | Digital logic from transistors to a simple CPU |
| The CPU | Given: 8-bit, registers A to D, 256 bytes | Built by you: an 8-bit accumulator machine |
| Runs on | Any browser | Any browser |
| Price | Free, MIT license | Free lab and Act I; Premium for the full course |
The same addition on both machines
Add 5 and 10 and store the sum. On the left, the program we ran in the assembler simulator on 2026-10-31: it halts with A = 0F (15), and it assembles to the bytes 06 00 05, 0D 00 0A, 04 E8 00 and 00. On the right, the program of digiwleea's capstone CPU, which the simulator runs to an accumulator of 15 and a stored 15.
| Assembler simulator | digiwleea capstone CPU |
|---|---|
MOV A, 5 | LOAD 14 (memory 14 holds 5) |
ADD A, 10 | ADD 15 (memory 15 holds 10) |
MOV [232], A | STORE 13 |
HLT | HALT |
The MARIE, LC-3 and 8085 comparisons put the same program side by side on other teaching machines.
No sign-up is needed to start: open the lab and place your first transistor, or read the free theory course. The account (and the AI tutor) are optional.
More comparisons: CPU and assembly simulators
See for yourself: build a logic gate from transistors in the simulator, then follow the path from sand to an 8-bit CPU.
Open the digiwleea lab →Frequently asked
What is the Simple 8-bit Assembler Simulator?
A free, open-source browser simulator by Marco Schweighauser for a small x86-like 8-bit CPU with four registers and 256 bytes of memory, using simplified NASM-style assembly.
Is there an online assembly simulator for beginners?
Yes. The Simple 8-bit Assembler Simulator runs in the browser and shows every register and byte of memory while you step. For a specific chip, there are browser simulators for the 8085, MARIE and LC-3 too.
Can I see how a CPU runs an instruction inside?
In digiwleea you build the CPU's parts from transistors and watch each instruction move through them; the CPU simulator page walks through fetch, decode and execute step by step.