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

DYNAMIC STORAGE MANAGEMENT: ACTIVE DISKS AND FILES CMS disks and files contained on disk are physically mapped using the
data blocks described above: for disks, the QMSK, QQMSK, and the MFD; for files, the FST, chain links, and 800-byte file records. In storage,
all of this data is accessed by means of two DSECTs whose addresses are
defined in the DSECT NUCON, ADTSECT and AFTSECT. The ADTSECT DSECT maps information in the active disk table (ADT). This information includes data contained in the MFD, FST blocks; the QKSK, and QQMSK. The DSECT comprises of ten "slots," each representing one virtual ! slot contains siqnificant information about the
disk such as a pointer to the MFD for the disk, a pointer to the first FST block and pointers to the QMSK and QQMSK, if the disk is a R/i disk.
Also contained in ADTSECT is information such as the number of cylinders
on the disk, the number cf records on the disk.
Each open file is represented in storage by an active file table (AFT).
The AFT (defined by the AFTSECT DSECT) contains data found on disk in FSTs, chain links, and data records. Also contained in the AFT is such
information as the address of the first chain link for the file, the
current chain link for the file, the address of the current data block,
the fileid information for the file. Figure 1 shows the relationship
between the AFT and other CMS data blocks. CMS ROUTINES USED TO ACCESS THE FILE SYSTEM DMSACC the control routine used to access a virtual disk. In
conjunction with DMSACM and DMSACF, DMSACC builds, in virtual storage,
the tables CMS requires for processing files contained on the disk. The
list below shows the logical flow of the main function of DKSACC. ACCESS A VIRTUAL DISK: DKSACC Scans the command line to determine which disk is specified. DMSLAD: Looks up the address of the ADT for the disk specified on the coHaiid line. DMSACC: Determines whether an extension to a disk has been specified on th;-Command line and ensures that it is correctly specified. In the case where an extension has been specified, calls DKSLAD to ensure that the extension disk exists,. DMSLAD: Ensures that the specified disk is not already accessed as a R/W dIsk:-- CMS Method of Operation and Progra. Organization 2-93