Playing with S/370 assembler specifically the simple MVC instruction. Realized I’d forgotten basics by examining storage with the SNAP macro, after trying various moves. Came to realize that the length is only determined by the first operand. Verified this later by looking at yellow card. Figured out one way to move values to a 80 byte CARD storage. If I have 50 bytes of ‘X’ in STUFF. Then I can initialize the 1st 50 bytes by “MVC CARD(50),STUFF” and the rest by “MVC CARD+50(30),STUFF” I guessed at that, probably an educated guess from stuff in the recesses of my mind. (it didn’t look right but CARD(30)+50 looked worse. Below is an explanation of the assembly on the left of that code in the listing. Also if memory serves I believe there was some trick to initialize storage.
From Assembly Listing
00006C D21D C1FD C190 00203 00196 117 MVC CARD+50(30),STUFF
Since Op Code D2 is MVC..we know this is a SS Instruction with Operands D1(L,B1),D2(b2)
So SS instruction is 6 bytes [S/370 instructions can be 2,4 or 6 bytes]
D21D C1FD C190
D2 – Op Code MVC
1D – Length=29 zero based
C – Base Reg #1=12
1FD – Displacement #1=509
C – Base Reg #2=12
190 – Displacement #2=400