How OP_CHECKSIG knows the source of sig in pay to pubkey hash transaction

How OP_CHECKSIG knows the source of sig in pay to pubkey hash transaction

https://en.bitcoin.it/wiki/Script#Standard_Transaction_to_Bitcoin_address_.28pay-to-pubkey-hash.29

The last step of pay to pubkey hash transaction is to check the signature of the previous transaction tx hash and pubkey .

But how the script interpreter knows the sig the signature of the previous transition hash, what if the sig is the signature of some other content?

https://ift.tt/2PSbg0K