GHC's runtime system is a slightly scary beast: 50,000 lines of C and C-- code, much of which seems at first glance to be completely obscure. What on earth does the RTS do? Here are the highlights:
It includes all the bits required to execute Haskell code that aren't compiled into the code itself.
For example, the RTS contains the code that knows how to raise an exception when you call error,
code to allocate Array# objects, and code to implement takeMVar#.
It includes a sophisticated storage manager, including a multi-generational garbage collector with copying
and compacting strategies.
It includes a user-space scheduler for Haskell threads, together with support for scheduling Haskell threads
across multiple CPUs, and allowing Haskell threads to call foreign functions in separate OS threads.
There's a byte-code interpreter for GHCi, and a dynamic linker for loading up object code into a GHCi session.
Heap-profiling (of various kinds), time-profiling and code coverage of Haskell code are included.
Support for Software Transactional Memory.
Next, we try to make sense of how it all fits together.