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Bare machine C with floating-point processing

If you want to use floating-point processing in an application without the C library you must:

  • Re-implement __rt_raise() if you are using the heap.

  • Not define main(), to avoid linking in the library initialization code.

  • Write an assembly language veneer that establishes the register state required to run C. This veneer must branch to the entry function in your application. The register state required to run C primarily comprises the stack pointer. It also consists of sb, the stack base, if Read/Write Position-Independent (RWPI) code applies.

  • Provide your own RW/ZI initialization code.

  • Ensure that your initialization veneer is executed by, for example, placing it in your reset handler.

  • Use the appropriate FPU option when you build your application.

  • Call _fp_init() to initialize the floating-point status register before performing any floating-point operations.

Do not build your application with the --fpu=none option.

If you are using software floating-point support and a floating-point mode that requires a floating-point status word (--fpmode=ieee_fixed or --fpmode=ieee_full), you can also define the function __rt_fp_status_addr() to return the address of a writable data word to be used instead of the floating-point status register. If you rely on the default library definition of __rt_fp_status_addr(), this word resides in the program data section, unless you define __user_perthread_libspace() (or in the case of legacy code that does not yet use __user_perthread_libspace(), __user_libspace() ).