7.2 Lab 4: lab  (Page 3/7)

Assembly fft routine

As the list of operations indicates, bit-reversal and FFT computation are to be done in both C and assembly. For theassembly version, make sure that the line defining C_FFT is commented out in lab4.c . We are providing you with a shell assembly file, available at v:\ece420\55x\lab4\c_fft_given.asm and shown in Appendix B , containing many useful declarations and some code. The code for performingbit-reversal and other declarations needed for the FFT routine are also provided in this section. However, we would like you to enter this code manually, as you will be expected to understand itsoperation.

The assembly file c_fft_given.asm contains two main parts, the data section starting with .sect ".data" and the program section starting with .sect ".text" . Every function and variable accessed in C must be preceded by a single underscore _ in assembly and a .global _name must be placed in the assembly file for linking. In this example, bit_rev_fft is an assembly function called from the C program with a label _bit_rev_fft in the text portion of the assembly file and a .global _bit_rev_fft declaration. In each assembly function, the macro ENTER_ASM is called upon entering and LEAVE_ASM is called upon exiting. These macros are defined in v:\ece420\55x\macro.asm . The ENTER_ASM macro saves the status registers and AR1 , AR6 , and AR7 when entering a function as required by the register use conventions. The ENTER_ASM macro also sets the status registers to the assembly conventions we have been using (i.e, FRCT =1 for fractional arithmetic and CPL =0 for DP referenced addressing). The LEAVE_ASM macro just restores the saved registers.

Parameter passing

The parameter passing convention between assembly and C places the parameters into registers depending on the size of the parameters. Data pointers (16 or 23 bit) will be placed in (X)AR0 through (X)AR4 in that order. 16-bit data will be placed in T0, T1, and AR0 through AR4 in that order. 32-bit data will be placed in accumulators AC0 through AC2. If there are no available registers of the correct type, the parameters will be passed onto the stack. For more details, see page 6-16 of the Optimizing C/C++ Compiler User's Guide ( spru281e ).

Registers modified

When entering and leaving an assembly function, the ENTER_ASM and LEAVE_ASM macros ensure that certain registers are saved and restored. Since the C program may use any and all registers, the state ofa register cannot be expected to remain the same between calls to assembly function(s). Therefore, any information that needs to be preserved across calls to an assembly function must besaved to memory!

Now, we explain how to use the FFT routine provided by TI for the C55x. TI provides a library of commonly used functions. These functions include FFT, FIR, IIR, and some math operations. More information can be found in the DSP Library Programmer's Reference . The CFFT function will be used for the DSPLIB implementation. Refer to the reference guide to figure out the correct syntax in calling the function.