what os the is use of pack 'v' for?

The documentation explains it :) 

v  An unsigned short (16-bit) in "VAX" (little-endian) order.
V  An unsigned long (32-bit) in "VAX" (little-endian) order.

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In reply to Re^4: Converting negative number to binary with specific width by Anonymous Monk
in thread Converting negative number to binary with specific width by pjkang7

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