Home ยป Brainfuck Interpreter written in x86 Assembly

Brainfuck Interpreter written in x86 Assembly


  • The comment ; *argv should be ; argv, since you are not yet dereferencing the pointer.

  • After a cmp instruction, you should prefer je over jz, since it is nicer to the human reader.

    Oh, the old times, where you had to tell the assembler to jmp short because it couldn’t figure it out on its own. ๐Ÿ™‚

  • In the run the BF program section, I would have changed esi and edi so that the s register points to the source code and the d register points to the data. But that is just for fun.

  • In bfprogram_memory_inc, you can just say inc byte [esi]. The inc instruction has an r/m8 encoding that allows incrementing a value in memory directly, without needing the indirection.

  • The safe to do optimization is nice.

  • Since you rely on the ASCII encoding anyway, you should define JUMP_PAST_CODE as '[' instead of 91, if possible. Not every reader knows the ASCII codes by heart.

  • Does NASM support local labels? That would make the label names in bfprogram_jump_past_loop a little shorter and thereby easier to read.

  • Instead of calling _fdopen, is stderr a linker-visible symbol so that you can access it directly?

  • Since your error messages don’t contain percent characters, you should call fputs instead of fprintf.

  • The error messages should include a trailing newline.

    (Or are they missing because Windows adds this newline anyway? If so, then it’s ok, since it is not necessary to write strictly conforming ANSI C code when you target a specific platform.)

  • The bfprogram_jump_table is very large. You can probably make it smaller by encoding the jump target relative to a known location, which should only take a single byte per entry. times 43 db 0, db bfprogram_memory_inc - run_program_loop_end, ....

  • format_int seems to be unused.

  • STDERR should be called STDERR_FILENO (although that name comes from POSIX, not from Windows, it is still widely known).

  • Since 0xFF is a valid instruction in a Brainfuck program, using it as the EOF marker is not a good idea. When loading the code from the file, you could replace every character above 93 or below 43 with 44. This change would also allow you to make the jump table smaller.

Overall, it’s a nice little program and the code does the obvious thing. It’s a pleasure to read.

Jump instructions are slow and should be minimized. One jump instruction in the interpreter loop can be removed as follows:

    dec      edi       ; on first pass, compensate for inc edi
run_program_loop:      ; jump here after executing an opcode
run_program_loop_end:  ; old label for reference
    inc     edi
    movzx   eax, byte [edi]

    jmp     [bfprogram_jump_table + 4*eax]      ; addresses are dword, ASCII is translated to byte offsets

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