MIPSie is a basic MIPS simulator written in Prolog.
It allows you to execute common MIPS assembler programs in your computer (without macros) with a syscall interface like MARS. However, it is not a comprehensible and formal definition of the MIPS architecture nor a real assembler (it will not produce machine code) and it doesn't want to. Some tricky programs won't work also due to un underlying simplified memory layout.