CPUSim64 is an emulation of a simple 64-bit microprocessor. It is useful for exploring the basics of programming with assembly language without getting bogged down in the various idiosyncrocies of real world microprocessor assembly languages such as Intel/AMD64, ARM or PowerPC. Because it is an emulator, you don't risk crashing your computer with errant assembly language.
CPUSim64 is implemented in Java which means that you must have a Java 17 or later JDK installed on your system to run the assembler and emulator. There are scripts to make common functions like assembling your source, disassembling object files, tracing through your program execution and running your program easy to do from the command line. You will need a good programming editor to write your CPUSim64 programs.
There are four documents plus the CPUSim64.zip archive that you will need to read to get up to speed with programming CPUSim64.