An USB device with a particular VID/PID can be bound to a specific driver on the host system. This VID/PID combination can also be bound to a driver whose only function is to download firmware to the device. The EZ Loader USB Firmware Loader is used to create one such driver. When you plug in our USB device, the device comes up with a specific VID/PID. This particular VID/PID combination is then bound to the EZ Loader Driver. The EZ Loader driver downloads a firmware loader (loader.c) into the 8051. This loader then loads your firmware into the appropriate code memory location. After your firmware is loaded, the device would renumerate with the new VID/PID combination that is present in the firmware. This new VID/PID combination can then be bound to a new custom driver.
Please refer the application note EZ LOADER Custom USB Firmware Loader Driver for a detailed explanation on how to create the EZ Loader driver.