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2.6.2 Converting Formatted Offset into Position Slot and Position Footer Values

The formatted offset is subdivided into a position slot and a position footer. The position slot defines the most significant bits of the formatted offset in the form of a base position as shown in the following table. The position footer defines the remaining least significant bits of the formatted offset. As the following table shows, the number of bits dedicated to the position footer grows as the formatted offset becomes larger, meaning that each position slot addresses a larger and larger range.

The number of position slots available depends on the window size. The number of bits of position footer for each position slot is fixed and is shown in the following table.

Position slot number

Base position

Footer bits

Range of base position and position footer (formatted offset)

0 (R0)

0

0

0

1 (R1)

1

0

1

2 (R2)

2

0

2

3 (offset 1)

3

0

3

4 (offset 2..3)

4

1

4-5

5 (offset 4..5)

6

1

6-7

6 (offset 6..9)

8

2

8-11

7 (..etc..)

12

2

12-15

8

16

3

16-23

9

24

3

24-31

10

32

4

32-47

11

48

4

48-63

12

64

5

64-95

13

96

5

96-127

14

128

6

128-191

15

192

6

192-255

16

256

7

256-383

17

384

7

384-511

18

512

8

512-767

19

768

8

768-1023

20

1024

9

1024-1535

21

1536

9

1536-2047

22

2048

10

2048-3071

23

3072

10

3072-4095

24

4096

11

4096-6143

25

6144

11

6144-8191

26

8192

12

8192-12287

27

12288

12

12288-16383

28

16384

13

16384-24575

29

24576

13

24576-32767

30

32768

14

32768-49151

31

49152

14

49152-65535

32

65536

15

65536-98303

33

98304

15

98304-131071

34

131072

16

131072-196607

35

196608

16

196608-262143

36

262144

17

262144-393215

37

393216

17

393216-524287

38

524288

17

524288-655359

39

655360

17

655360-786431

40

786432

17

786432-917503

41

917504

17

917504-1048575

42

1048576

17

1048576-1179647

..etc..

..etc..

17 (all)

..etc..

288

33292288

17

33292288-33423359

289

33423360

17

33423360-33554431

The following pseudocode demonstrates how to determine the position slot and the position footer.

position_slot ← calculate_the position_slot from the formatted_offset 
position_footer_bits ← determine the number of footer bits from the position slot value
if position_footer_bits > 0 
    position_footer ← formatted_offset & ((2^position_footer_bits)-1) 
else 
    position_footer ← null
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