I am reading "Porting Guide".

It says:
"...
int startBlock

The block number (not the address in bytes) on the chip of the first block of this logical yaffs_Device.  Beware, yaffs needs the first block to be free so you should not set this to 0, if it is then yaffs will add on one to it itself and also add on one to the end block which could be an issue if you wanted your device to start at block 0 and finish at the last block which might mean yaffs trying to write to a non-existent block.
..."