© Thomas Kunz 2000
SCE 574
75
   What is a Segment?
ä20 bit address can access 1 MB of memory
ä16 bit registers can only access 64KB of memory
äNeed some strategy for accessing more memory!
äApproach
äUse value in segment + offsets from register
äNotation SEG:OFF
äBoth value in segment and offset are 16 bits
äTo create a 20 bit address
ä(1) Shift segment value left by 4 bits (shift in 0s)
ä(2) Add offset to get 20 bit value
1000 0000 0001 0000
1000 1110 0000 1000
Segment register
Offset register
Bottom four
bits from the
segment register
are zero
0000
Act as  4 0’s
1000 1000 1111 0000 1000