The program includes CPU-emulation, assembler, disassembler, debugger, eeprom upload function, this wonderful package can be downloaded.
Here. (63 Kbytes, DOS version, for PC/XT and up).
Here. (83 Kbytes, Windows version, for 386 and up).
Bugfixes, updates, improvements, including emulation of (or redirection to) external hardware available upon request (if I could get some money for it would be great, otherwise some motivating feedback would be nice either).
Just for curiosity, here are standalone versions of the specs (extracted from the help text/manual which is built-in into the emulator/debugger).
Here. (67 KBytes, Text version).
Here. (78 KBytes, Html version).
The XED Editor (separately required for DOS version only)
The no$x51 built-in source code assembler is parts of able to communicate with my XED editor (download here). That is, access the editor by hitting ALT+E from inside of no$x51, edit your source code, hit F9 in the editor, this will assemble your program and, if no errors occured, autostart it in the emulator, otherwise, in case of assembly errors it'll switch back to the editor, and automatically move it to the first error-line.
Installation notes: Copy XED into a directory which is part of your PATH (eg. your C:\Windows directory, if you are using windows), and select XED as preferred editor in no$x51 setup!