Torah from Memory

[It was related that] R. Ḥisda [once] found R. Ḥananel writing [Torah] scrolls, [but he was] not [copying them] from a written text, [as he knew it all by heart]. He said to him: It is fitting [for] the entire Torah to be written by your mouth, [i.e., relying on your memory,] but this is what the sages said: It is prohibited to write [even] a single letter [of the Bible when] not [copying] from a written text. [The Gemara asks:] Since [R. Ḥisda] said [to him]: The entire Torah is fitting to be written by your mouth, [it may be concluded] by inference that [the words of the Torah] were exact in his memory [i.e., that R. Ḥananel enjoyed total mastery of the text]. But didn’t [we say that] R. Meir wrote [a Megillah without copying from a text due to similar proficiency? The Gemara answers:] A time of exigent circumstances is different; [since there was no other option available, he was permitted to rely on his expertise, but otherwise this must not be done].