how to use floppy as the ROOT_DEV

I want to set floppy as the ROOT_DEV, and test the rd_load() function. So I made the following two changes:
1. change Makefile: ROOT_DEV=FLOPPY 
2. change bootsect.s:
 .equ ROOT_DEV, 0x000
But it failed to mount root file.
Can you help me?