I discovered the most irritating thing last week setting up routers and prepping kit for my LAB. The Cisco 3640 does not support the TFTPDNLD in ROMMON mode. This is particularly painful, when you don’t have a IOS image loaded, replacing or upgrading the Flash Memory.
The only way to do this, if you do not have a spare (8Mb or 16Mb ONLY) PCMCIA Flash card formatted with File system Class “B” and a valid 3640 IOS image on it, is via the Grandfather of file transfer protocols: XMODEM.
The good news is that you can use Ymodem protocol extension with -y switch, which is a bit more efficient than old Xmodem, in that your transfer time is less.
xmodem -y c3640-ik9s-mz.124-19b.bin
Be sure to change you Baud Rate before hand, as per the following article at Cisco Land :
http://www.cisco.com/en/US/products/hw/routers/ps259/products_tech_note09186a008015bfac.shtml
Routing-Bits 