Disk Address of 11 2nd Chatn Link Disk Address of Disk Address of
3rd Chain Link 1\. Oth Data Block I Disk .Aridress of
1 I 1\. 1st Data Block \ Chain Linkage 1 'I Directory
l • • Disk Address of 40th Chatn Link I
J • 1
Disk Address of
41 st Chatn Link r • T
Disk Address of I I 1st Data Block I I Disk Address of
! I I I 2nd Dala Block -1 ! I I ...l, rL T J Disk Address of h 398 th Data Block I Disk Address of 1\+ 3991h Data Block I D,Sk Address of 1 59th Data Block A" (n-21 • 400' 61 I I wheren = Cha!n Link, Number
Disk Address of 60th Data Block
Figure 14. Format of the First Chain Link and Nth Chain Links
Data block structure for file consisting of fixed-length records Data block structure for file consisting of variable-length records I sao
1st record 1 --------------
2nd record
1st record
sao Sq«l L2J - - - 2nd record T
- ---------'------------ [9
-------[9-------- L3 3rd record L 4
- -------
3rd record SOO sao ----------------1 ____ _
4th record SOO sao
4th record I L Tt 5th record I 800 I Figure 15. Arrangement of Fixed-Length Records
Records in Files
and Variable-Length eMS Method of Operation and Program Organization 2-89

THE MASTER FILE DIRECTORY The master file directory (MFD) is the major file management table for a
virtual disk. As mentioned earlier, it resides on cylinder 0, track 0, record 4 of each virtual disk. Six types of information contained in
the master file directory: • The disk addresses of the FST entries describing user files on that
disk. • A 4-byte "sentinel," which can be either FFFD or FFFF. PFPD specifies that extensions of the QMSK (described below) follow. PFPF specifies that no QMSK extensions follow. • Extensions to the QMSK, if any. • General information describing the status of the disk: ADTNUM -- The total number of BOO-byte blocks on the user's disk. ADTUSED ADTLEFT ADTLAST disk.
The number of blocks currently in use on the disk.
Nu.ber of blocks remaining for use (ADTNUM - ADTUSED). Relative byte address of the last record in use on the ADTCYL -- Number of cylinders on the user's disk. Unit Type --A 1-byte field describing the type of the disk: oa
for a 2314, 09 for a 3330. A bit mask called the QMSK, which keeps track of the status of the
records on disk. The QMSK is described in more detail below.
Another bit map, called the QQMSK, which is used only for 2314
disks and performs a function similar to that of QMSK. Figure 16 shows the structure of the master file directory. Figure
12 shows the relationship of the Master Pile Directory, which resides on
disk, to data blocks brought into storage for file management purposes,
for example, FSTs and chain links. KEEPING TRACK OF READ/WRITE DISK STORAGE: QMSK AND QQMSK Because large areas of disk space need not be contiguous in CMS, but are
composed of aOO-byte blocks chain-linked together, disk space management
needs to determine only the availability of blocks, not extents. The
status of the blocks on any read/write disk (which blocks are available
and which are currently in use) is stored in a table called QMSK. The
term QMSK is derived from the fact that a 2311 disk drive has four aOO-byte blocks per track. One block is a "quarter-track", or QTRK, and
a 200-byte area is a "guarter-quarter-track", or QQTRK. The bit .ask
for 2314, 2319, 3340, or 3330 records is called the QMSK, although each aOO-byte block represents less than a quarter of a track on these
devices. 2-90 IBM VM/370 System Logic and Program Determination--Volume 2