When the MAXDATA value is known, the RU size (Request Unit size) is calculated as follows: MAXDATA value minus 34 bytes (header of PIU) equals the RU size, e.g. 2048 - 34 = 2014 bytes. (See the explanation of message 9204E in manual BSA Messages and Codes on how to determine the MAXDATA size.) If no matching equal value is returned in the table, e.g. 2014, then the next lower value must be used for the maximum permitted RU size, e.g. 1920, displayed as a hexadecimal X'F7F7'.
The formula for calculating the RU size value is: a x 2b or X'abab', displayed as a hexadecimal.
The formula specifies the maximum length in bytes of the RUs (request units) that the primary and the secondary logical unit
can send to each other. The RU sizes are specified as four hexadecimal figures: X'abab'.
The first digit is the mantissa (a) and must be within the range X'8'-X'F'. The second digit is the exponent (b) and must
be within the range X'0'-X'F'. The leftmost two digits apply to the secondary logical unit and the rightmost two digits apply
to the primary logical unit. These sent and received units must always be of the same size.
X'F7F7' specifies that the maximum size of the unit sent or received is 15 x 27 or 1920 bytes.
The table below lists the exponents, mantissas, and the values for RU sizes.
|
|
Mantissa (a) |
|||||||
|
Exponent (b) |
8 |
9 |
A (10) |
B (11) |
C (12) |
D (13) |
E (14) |
F (15) |
|
0 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
|
1 |
16 |
18 |
20 |
22 |
24 |
26 |
28 |
30 |
|
2 |
32 |
36 |
40 |
44 |
48 |
52 |
56 |
60 |
|
3 |
64 |
72 |
80 |
88 |
96 |
104 |
112 |
120 |
|
4 |
128 |
144 |
160 |
176 |
192 |
208 |
224 |
240 |
|
5 |
256 |
288 |
320 |
352 |
384 |
416 |
448 |
480 |
|
6 |
512 |
576 |
640 |
704 |
768 |
832 |
896 |
960 |
|
7 |
1024 |
1152 |
1280 |
1408 |
1536 |
1664 |
1792 |
1920 |
|
8 |
2048 |
2304 |
2560 |
2816 |
3072 |
3328 |
3584 |
3840 |
|
9 |
4096 |
4608 |
5120 |
5632 |
6144 |
6656 |
7168 |
7680 |
|
A (10) |
8192 |
9216 |
10240 |
11264 |
12288 |
13312 |
14336 |
15360 |
|
B (11) |
16384 |
18432 |
20480 |
22528 |
24576 |
26624 |
28672 |
30720 |
|
C (12) |
32768 |
36864 |
40960 |
45056 |
49152 |
53248 |
57344 |
61440 |
|
D (13) |
65536 |
73728 |
81920 |
90112 |
98304 |
106496 |
114688 |
122880 |
|
E (14) |
131072 |
147456 |
163840 |
180224 |
196608 |
212992 |
229376 |
245760 |
|
F (15) |
262144 |
294912 |
327680 |
360448 |
393216 |
425984 |
458752 |
491520 |