MIX ENTA

Hello - Can anyone help me understand the ENTA instruction from the MIX computer. I dont understand the part about when M=0.

FYI, the description of it is:

"ENTA (Enter A) C = 48; F = 2.
The quantity M is loaded into rA. The action is equivalent to "LDA" from a memory word containing the signed value of M. If M=0, the sign of the instruction is loaded."

what instruction is loaded?

>>7
When M = 0, the sign is the ONLY thing copied.  That way you can negate a number just by using the instruction to replace the sign bit.