How does newlib support ARM

[Chen Yang]

I'm building newlib for ARM under cygwin.
In the lib/sys/arm/syscall.c,I can see two 
conditional compiler switch:
#ifdef ARM_RDI_MONITOR
or else.
 With newlib,the application uses
ARM_RDI_MONITOR,the whole
thing is done by 
  do_AngelSWI .
 all the functions such as open,close,exit are 
inplemented by do_AngelSWI,
But in Linux,this should be done by syscalls:
 such as "swi syscall_no".
How is it done?
Thanks.