8085 simulator alternative: build the processor, not just program it
The Intel 8085 is an 8-bit microprocessor from Intel. Sim8085 is an open-source (BSD 3-Clause) 8085 simulator that runs in the browser at sim8085.com: a syntax-highlighted editor, an assembler with readable error messages, and a debugger that steps through a program while the registers (A, BC, DE, HL, SP, PC), the flags (S, Z, AC, P, C) and memory update. Its README says the simulator core is written in C and compiled with Emscripten, that it works offline once loaded, and that it is tested against classic 8080/8085 instruction exercisers and Intel's SDK-85 monitor ROM. The free plan saves up to 5 files; Sim8085 Plus, a one-time $8 for six months, adds unlimited saves, share links, AI help for assembler errors and no ads. Sim8085 lets you program a real 8085; digiwleea has you build a computer's hardware yourself, gate by gate.
What Sim8085 is good at
Writing and debugging 8085 assembly for a course or a lab: the editor, assembler and step-by-step debugger are in one browser tab, the documentation has worked programs (adding two 8-bit numbers, finding the largest number in an array, BCD to binary and more), and interrupts and instruction timing are modeled too. If your syllabus is the 8085, it is the tool to practise on. (digiwleea is an independent project, not affiliated with Sim8085 or Intel.)
How digiwleea is different
digiwleea is not an 8085 simulator and will not run 8085 programs. It covers the layer underneath: in the browser you build NOT and NAND from CMOS transistors, then adders, an ALU, latches, 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 machine you end with is far simpler than an 8085: an 8-bit accumulator CPU whose capstone runs a stored LOAD, ADD, STORE, HALT program, with the parts for SUB, JMP, JZ and OUT built in Act IV. What you gain is the hardware view of the words in an 8085 manual: what the accumulator, a flag or the program counter is made of, and why ADD changes the flags.
| Sim8085 | digiwleea | |
|---|---|---|
| Focus | Programming the Intel 8085 in assembly | Digital logic from transistors to a simple CPU |
| The CPU | Given: the 8085, registers and flags shown | Built by you: a small 8-bit accumulator machine |
| Runs on | The browser, offline after the first visit | The browser, nothing to install |
| Debugging | Step through instructions, registers, flags, memory | Probe any wire, step the clock, check against truth tables |
| Price | Free (5 saved files); Plus $8 for six months | 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, 8085 assembly: we assembled and ran it in Sim8085 on 2026-10-31, and it stops with A = 0x0F (15). On the right, the program of digiwleea's capstone CPU, which the simulator runs to an accumulator of 15 and a stored 15. The 8085 needs MOV B, A to keep the first number while LDA loads the second; the digiwleea CPU adds a memory value straight into its accumulator.
| Intel 8085 (Sim8085) | digiwleea capstone CPU |
|---|---|
JMP START | |
X: DB 05H | memory 14 holds 5 |
Y: DB 0AH | memory 15 holds 10 |
Z: DB 00H | memory 13 receives the sum |
START: LDA X | LOAD 14 |
MOV B, A | |
LDA Y | |
ADD B | ADD 15 |
STA Z | STORE 13 |
HLT | HALT |
To see what an accumulator and a flag are made of, build the accumulator and the 8-bit ALU, or read the CPU lesson. The MARIE and LC-3 comparisons do the same for the two common 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
Is there an online 8085 simulator?
Yes. Sim8085 runs in the browser at sim8085.com, with an editor, assembler and step-by-step debugger, and keeps working offline once loaded. It is open source under the BSD 3-Clause license.
Is Sim8085 free?
Yes, with a free plan that saves up to 5 files. Sim8085 Plus is a one-time payment of $8 for six months and adds unlimited saves, share links, AI help for assembler errors and no ads.
How do you add two numbers in 8085 assembly?
Load the first into the accumulator (LDA or MVI A), copy it to another register (MOV B, A), load the second, then ADD B. The sum is in A, and STA stores it to memory. 5 + 10 leaves A = 0x0F.
Can I build an 8085 in digiwleea?
Not as a finished chip. digiwleea's course builds a smaller 8-bit accumulator CPU from transistors up, and the lab lets you build the kinds of parts an 8085 contains, such as registers, an ALU and a program counter.