This is an example of "emphatic do." Some examples: Four basic forms (example verb "work") He works at the paint factory. [positive statement] He does work at the paint factory. [emphatic statement to insist on truth of the statement] He does not [doesn't] work at the paint factory. [negative statement] Does he work at the paint factory? [interrogative statement] Simple past examples: He worked at the paint factory. [positive statement] He did work at the paint factory. [emphatic statement to insist on truth of the statement] He did not [didn't] work at the paint factory. [negative statement] Did he work at the paint factory? [interrogative statement] More information: https://www.thoughtco.com/what-is-the-emphatic-do-1690590

Did is a auxiliar verb of simple past. I wrote. I didn’t write. Did I write?