From the documentation....

\c{MOVSB} copies the byte at \c{[ES:DI]} or \c{[ES:EDI]} to
\c{[DS:SI]} or \c{[DS:ESI]}. It then increments or decrements
(depending on the direction flag: increments if the flag is clear,
decrements if it is set)

Is this a mistake? Logic would tell me that MOVSB would take the source ESI and place it in the destination EDI. And I have code that seems to work that way.

What made me look at the documentation was I have a function that I recently made use MOVSB and it didn't seem to be working correctly. Although I have other functions that work perfectly with this using the opposite of what the doc says.

I've been using ESI as a source and EDI as the destination There must be a bug in the ( nasm ) documentation. I'll change it..