How does MASM .DATA? directive work internally [duplicate]

This question already has an answer here: How to see the memory occupied by initialised array vs uninitialised array (1 answer) 开发者_如何学Python Closed 5 years ago.

In Kip Irvines book I came across the following :

The .DATA? directive declares uninitialized data. When defining a large block of uninitialized data, the .DATA? directive reduces the size of a compiled program. For example, the followingcode is declared efficiently:

.data?

bigArray DWORD 5000 DUP(?) ; 20,000 bytes, not initialized

The following code, on the other hand, produces a compiled program 20,000 bytes larger:

.data

bigArray DWORD 5000 DUP(?) ; 20,000 bytes

What exactly is the .data? directive doing under the hood in the above example to make the program 20k smaller.

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The uninitialized data need not be in the compiled binary, just a byte count that the OS loader allocates at run-time when executing your program.