Is it possible to know what 都 means in "Subj + 都 + Predicate + 了"?
It expresses "already" with the structure: Subj. + 都 + Predicate + 了


Only '了' indicates 'completion of a verb' in this structure

'都' in [Subj. + 都 + Predicate + 了]  mean 'even'.  Try write a sentence in this structure without 了, and you wouldn't say 都 expresses "already". 

Example:


"他(都)说 不知道" = "(even) he said he didn't know" 
"他(都)说 不知道 (了)" = "He (has already) said he didn't know" or "(Even) he said he didn't know"



  It is used for "all" or "both" with the structure: Subj. + 都 + [Verb Phrase]


The 'all' and 'both' meanings can only apply with multiple objects 

Example: 


[A and B (都) + Verb Phrase] = [A and B (both) + Verb Phrase]
[subjects (都) + Verb Phrase] = [subjects (All) + Verb Phrase]


For a singular subject, 都 cannot expresses the meaning of 'all' or 'both' . It can only expresses 'Even' or 'too/ also' (Cantonese)