This directory contains some limited C runtime features. It will take care of booting the binary, setting up some minimal environment and then call the main function. It also contains some basic implementations of standard C functions which are required by the compiler when doing self-hosted compilation.

Using very bare C runtime

The file crt0.s is the entry point of the C runtime. It will set up the stack, and call the main function. It will not perform any other initialization beyond clearing the BSS section.

Using C++ runtime

The file crt0cxx.s is the entry point of the C++ runtime. It will set up the stack, and then go to the cxxglue.c file for further initialization, which will then call the main function. The cxxglue.c file also contains the minimal implementation of the C++ runtime that is required by the C++ compiler.

Memory C runtime functions

The files memory-c.c and memory-s.s contain the 4 basic memory functions that are required by the self-hosted C and C++ compilers. These are memcpy, memset, memcmp, and memmove. These are implemented in a very basic way, being very small, and are not optimized for speed. They also contain optimized versions of memcpy and memset for the MIPS architecture, available using the LDFLAGS -Wl,-wrap,memcpy and -Wl,-wrap,memset toggles. They are somewhat bigger, but much faster. Note that these 4 functions are available directly when writing C or C++ code using the __builtin_memcpy, __builtin_memset, __builtin_memcmp and __builtin_memmove functions, with no need to include any header file.