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FASM — x86 assembler and debugger

Write and run FASM assembly programs. Debug step by step, set breakpoints, and inspect registers and memory.

Source code

CPU

Changed on this step Changed during session Flag is set

Registers

ax ····
bx ····
cx ····
dx ····
si ····
di ····
bp ····
sp ····
cs ····
ip ····
ds ····
es ····
ss ····

Flags

CF ·
PF ·
AF ·
ZF ·
SF ·
IF ·
DF ·
OF ·

Disassembly

···

How to use this tool

Write and run an educational FASM assembly program. Step through execution, set breakpoints, and inspect registers and memory.

Steps

  1. Write a FASM program using use16 and org 100h. Run and Debug automatically rebuild modified source.
  2. Use Run for normal execution, Debug to stop at the entry point, and Restart to repeat the last launch mode.
  3. Set breakpoints from the source gutter, then inspect registers, flags, stack, memory, and persistent disassembly history while debugging.

Useful for

  • x86 and DOS laboratory exercises
  • Checking FASM .COM programs
  • Learning registers, flags, stack, and memory

Limits and accuracy

  • The environment targets DOS .COM programs; protected mode and modern operating systems are outside its scope.
  • Restart the program to begin again; copy any unsaved changes before closing the page.

Sources