On a 2314 or 2319 disk, the blocks are actually grouped fifteen aOO-byte blocks per even/odd pair of tracks. An even/odd pair of tracks
is called a track group. On a 3330 disk, the blocks are grouped
fourteen SaO-byte blocks per track. On a 3340 disk, the blocks are
grouped into eight aOO-byte blocks per track. When the system is not in use, a user's resides on the File Directory; during a session it is maintained on disk, but also
resides in real storage. QMSK is of variable length, depending on how many cylinders exist on the disk.
Each bit is associated with a particular block on the disk. The
first bit in QMSK corresponds to the first block, the second bit to the
second block, and so forth, as shown in Figure 17. When a bit in QMSK is set to 1, it indicates that the corresponding block is in use and not available for allocation. A O-bit indicates
that the corresponding block is available.. The data blocks are referred
to by relative block numbers throughout disk space management, and the
disk I/O routine, DMSDIO, finally converts this number to a CCHHR disk
address. A table called QQMSK indicates which 200 byte segments (QQTRK) are
available for allocation and which are currently in use. QQ!SK contains 100 entries, which are used to indicate the status of up to 100 QQTRK
records. An entry in QQMSK contains either a disk address, pointing to
a QQTRK record that is available for allocation, or zero. QQ8SK is used
only for 2314 files; for 3330, 3340, and 3350, the first chain link
occupies the first 200-byte area of an aOO-byte block.
The QMSK and QQMSK tables for read-only disks are not brought into
storage, since no space allocation is done for a disk while it is
read-only. They remain, as is, on the disk until the disk is accessed
as a read/write disk. CMS Method of operation and Progra. Organization 2-91

Figure 16. OMSK for 2314 or 2319 0 0 0 0 0 0 1 2 3 4 0 0 0 1 1 1
9 10 11 12 0 0 0 0 1
Byte 0 Byte 364
Byte 380 Byte 382
Byte 384
Byte 599
Byte 600 .... .... ------ 2 Bytes --------- .. Disk Address of 1st FST Block Disk Address of 2nd FST Block (if any) · · · Disk Address of Nth FST Block (if any) Sentinel Disk Address of 1st OMSK extension (if any) · · · Disk Address of Nth OMSK extension (if any) · · · T
Not used - Zero filled
T · · · / ADTNUM,ADTUSED,ADTLEFT,ADTLAST
(4 bytes each) Not used (zero)
ADTCYL /. First 215 Bytes of OMSK V- 1 UNIT·TYPE Entire 200·Byte OOMSK Table / TL __________ (fo_r_2_3_14_0_n_IY_) __________ Structure of the Master File Directory
1 bit 1 bit r---- OMSK for 3330 0 0 0 0 I: 0 0 0 0 5 6 7 8 0 0 0 g 1 1 1
1 bit mt1bit where: g g g 1 2 3 g g g g 4 g 13 14 15 1
C = Cylinder H
= Head
9 10 11 12 0 0 0 0 i R = Record 0 0 0 0 1 J J Bit Value Meaning
o Block available J
1 Block in use I Number of OMSK Extensions Number of Cylinders on Disk
Required (if any) 2314 or 2319 3330 3340 3350 0 1 -11 1 6
1 12 -54 7 30 2 55 -96 31 54
3 97 -139 55 78
4 140 - 182 79 - 102 5 183 - 203 103 - 126
6 127 -150
7 151 -174
8 175 -198
9 199 -223 10 224 -246 g 5 g 13 0 1
7
Figure 17. Disk Storage Allocation Using the QMSK Data Block g g 6 7 g 14 1 0 0 a J 2-92 IBM VM/370 System Logic and Program Determination--Volume 2 g 8 2 0 16 J