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
 endif