[Hunble]

Darn, my very first computer was the IBM 360!

But MVC DDD(LL,B),DDD(B) has me rather stumped.

When prorammers invented a decent assembly language, we changed the world!

[NCjim]

The instruction is a storage-to-storage instruction, capable of moving up to 256 bytes from the storage location at the address specified by the second operand to the storage location at the address specified by the first operand.

Storage is fetched one byte at a time from left to right. Knowing this, back when storage was at more of a premium than processor cycles, you could set 256 bytes of storage to blanks with two instructions:

MVI 0(R2),C' '
MVC 1(255,R2),0(R2)

Kind of grows on you, doesn't it? :-)