The word Shahnameh usually refers to the Persian epic poem written by Ferdowsi in 977-1010 CE, although this versification was mostly a rewriting of existing mythological and historical stories of the history of Iran/Persia and more broadly the Greater Iran region, and some of these other stories are also called "Shahnameh". According to Wikipedia (although this sentence is uncited), just some parts of Ferdowsi's Shahnameh were his own original writing rather than based on pre-existing tales:
The Shahnameh is a monument of poetry and historiography, being mainly the poetical recast of what Ferdowsi, his contemporaries, and his predecessors regarded as the account of Iran's ancient history. Many such accounts already existed in prose, an example being the Abu-Mansuri Shahnameh. A small portion of Ferdowsi's work, in passages scattered throughout the Shahnameh, is entirely of his own conception.
Which parts did Ferdowsi conceive of himself rather than using older sources? Are there any detailed studies analysing this? I'd be interested to know in which chapters/parts they appear and which events they describe, so that I can find them in the presumably abridged English translation I'm reading. Also, how is it known that he did create some parts himself, rather than drawing on sources which have since been lost?