The compiler inserts a jump to the C routine _sdcc__external__startup()at the start of the CODE area. This routine can be found in the file SDCCDIR/sdcc51lib/_startup.c , by default this routine returns 0, if this routine returns a non-zero value , the static & global variable initialization will be skipped and the function main will be invoked, other wise static & global variables will be initialized before the function main is invoked.