What x86 register denotes source location in movsb instruction?
In 32-bit mode,
How to extract that information from the manual
Now that you've logged into osdev.org, it's time to pay your moral debt by actually learning it. :-)
Intel Manual Volume 2 Instruction Set Reference - 325383-056US September 2015 section "MOVS/MOVSB/MOVSW/MOVSD/MOVSQ—Move Data from String to String" says:
Minimal test program
Finally, you must make a minimal program that uses the instruction to see if you've understood it correctly:
Version of this with assertions on GitHub.