First of all, your existing programs work, unchanged, in z/Architecture® mode. This section 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.