Shell Programming and Scripting

cat memusageAIX.sh

#!/usr/bin/ksh
#
# Memory usage under AIX
#

USED=`svmon -G | head -2 | tail -1 | awk '{ print $3 }'`
USED=`expr $USED / 256`
TOTAL=`lsattr -El sys0 -a realmem | awk '{ print $2 }'`
TOTAL=`expr $TOTAL / 1024`
FREE=`expr $TOTAL - $USED`

echo "\nMemory Information"
echo "=================="
echo "total memory = $TOTAL MB"
echo "free memory  = $FREE MB"
echo "used memory  = $USED MB"

exit 0

Question:

Why USED is divided by 256 in the above shell script :

USED=`expr $USED / 256`

How did we arrive at 256 figure? 

What is the significance of it?