When generating addresses, the processor performs address arithmetic; it adds three components: the contents of the 64-bit GPR, the displacement (a 12-bit value), and (optionally) the contents of the 64-bit index register. Then, the processor checks the addressing mode and truncates the answer accordingly. For AMODE 24, the processor truncates bits 0 through 39; for AMODE 31, the processor truncates bits 0 through 32; for AMODE 64, no truncation (or truncation of 0 bits) occurs. In S/390® architecture, the processor added together the contents of a 32-bit GPR, the displacement, and (optionally) the contents of a 32-bit index register. It then checked to see if the addressing mode was 31 or 24 bits, and truncated accordingly. AMODE 24 caused truncation of 8 bits. AMODE 31 caused a truncation of bit 0.

The addressing mode also determines where the storage operands can reside. The storage operands for programs running in AMODE 64 can be anywhere in the 16-exabyte address space, while a program running in AMODE 24 can use only storage operands that reside in the first 16 megabytes of the 16-exabyte address space.