IBM System/360

If DEVTAB is specified, the next three full words contain the
following information:
Word 3
Word 4
Word 5
Bytes 1-2 The numcer of physical cylinders on the
device.
Bytes 3-4 The number of tracks per cylinder.
Bytes 1-2 Maximurr. track length.
Byte 3
Byte 4
Byte 1
Block Overhead the number of bytes
required for gaps and check bits for each
keyed clock other than the last block on a
track.
Block Overhead the number of bytes
required for gaps and check bits for a
keyed block that is the last block on a
track.
Block Overhead - the number of bytes to be
subtracted if a clock is not keyed.
Byte 2 bits 0-6 Reserved
bit 7 If 1, a tolerance factor must be
applied to all blocks except the
last block on the track.
Bytes 3-4 Tolerance Factor - this factor is used to
calculate the effective length of a block.
The calculation shculd be perforwed as
follows:
step 1 - add the block's key length to the
block's data length.
Step 2 - test tit 7 of byte 2 of word 5.
If bit 7 is 0, perform step 3. If bit 7
is 1, multiply the sum computed in step 1
by the tolerance factor. Shift the result
of the multiplication nine bits to the
right.
step 3 add the appropriate block
overhead to the value obtained above.
System Macro-Instructions 127