Yaffs is supplied as an EPK (eCos Package) file, the latest version of which can be downloaded from http://www.ecoscentric.com/devzone/yaffs.shtml.
Open up the eCos Configuration Tool.
Open up the Administration dialog, from the Tools menu.
Press the Add button.
A file browser windows opens. Navigate to the Yaffs EPK file.
The License screen shows. You must accept the license in order to install the package. Press Yes if you do.
Advanced users may alternatively use the ecosadmin.tcl tool from the command line. You will be prompted to accept the license at the appropriate time during the procedure.
tclsh $ECOS_REPOSITORY/ecosadmin.tcl add yaffs-v1_2_3.epk