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Memory (''storage'') in System/360 is addressed in terms of [[8-bit]] bytes. Various instructions operate on larger units called ''halfword'' (2 bytes), ''fullword'' (4 bytes), ''doubleword'' (8 bytes), ''quad word'' (16 bytes) and 2048 byte storage block, specifying the leftmost (lowest address) of the unit. Within a halfword, fullword, doubleword or quadword, low numbered bytes are more significant than high numbered bytes; this is sometimes referred to as [[Endianness#Big-endian#with 8-bit atomic element size and 1-byte (octet) address increment|big-endian]]. Many uses for these units require aligning them on the corresponding boundaries. Within this article the unqualified term ''word'' refers to a ''fullword''.
The original architecture of System/360 provided for up to 2<sup>24</sup> = 16,777,216 bytes of memory. The later [[IBM System/360 Model 67|Model 67]] extended the architecture to allow up to 2<sup>32</sup> = 4,294,967,296<ref group=NB>Twice the size of the later System/370</ref> bytes of virtual memory.
==Addressing==
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