First of all, your existing programs work, unchanged, in z/Architecture® mode. This information describes
how z/Architecture processes S/390® instructions. The best way to describe this
processing is through examples of common S/390 instructions.
First, consider a simple Add instruction: A R3,NUM31. This instruction
takes the value of a fullword binary integer at location NUM31 and
adds it to the contents of the low-order half of GPR3, placing the
sum in the low-order half of GPR3. The high-order half of GPR3 is
unchanged. 
Second, consider the LOAD instruction: L R3,MYDATA. This instruction
takes the 4 bytes of data at location MYDATA and puts them into the
low order bits of GPR3. 
The high-order half is not changed by the ADD instruction or the LOAD instruction. The register forms of these instructions (AR and LR) work similarly, as do Add Logical instructions (AL and ALR).