SDCC is a freeware, retargettable, optimizing ANSI-C compiler. The current version targets Intel 8051-based MCUs, the Zilog Z80, and the DS390. Targets for the Microchip PIC and Amtel AVR series are under development, and the compiler can be retargeted for other 8 bit MCUs or PICs. The entire source code for the compiler is distributed under GPL. SDCC uses ASXXXX and ASLINK, a freeware, retargettable assembler & linker. SDCC has extensive MCU (8051)-specific language extensions, which lets it utilize the underlying hardware effectively. It includes a source-level debugger/simulator and can generate debug info for the NoICE debugger.
