TurboIMAGE/XL Database Management System Reference Manual (30391-90012)

490 Chapter10
Internal Structures and Techniques
Run-Time TurboIMAGE/XL Control Blocks
Run-Time TurboIMAGE/XL Control Blocks
As mentioned in chapter 4, TurboIMAGE/XL uses control blocks resident in privileged
mapped files to provide and control user access to a database through the
TurboIMAGE/XL procedures. The contents of these control blocks are maintained by
TurboIMAGE/XL. Although it is not necessary to know the details in order to use the
TurboIMAGE/XL procedures, the following descriptions are provided for those who prefer
to understand the control blocks and their functions.
Local Database Access
The following structures are involved in local database access:
• Database System Control Block (DBS) -- Contains pointers to all of the DBGs on the
system.
• Database Globals Control Block (DBG) -- Contains global information required during
run-time and space for DBLOCK's lock descriptors. The DBG also contains a pointer to
the DBB and pointers to the DBUs.
• Database Buffer Area Control Block (DBB) -- Contains a set of addresses and
temporary locks used to coordinate access to the data set, a set of buffer headers, and a
pointer to the DBG.
• Database User Local Control Block (DBU) -- Contains information pertaining to each
access path (each DBOPEN) to the database. The DBU also contains pointers to the DBG,
DBB, and DBS.
• Database User Local Index Control Block (DBUX) -- Contains the addresses of all the
DBUs and DBRs (Remote Database Control Blocks) belonging to a specific process. The
DBUX also contains dynamic roll-back information.
• Database Lock Table (TURBOLKT) -- Contains information pertaining to locks on the
database and is used to avoid deadlocks.
• Multi-Database Transactions (TURBOGTX) -- Contains information pertaining to
dynamic transactions constituting multiple databases.
• QOPEN Table (QOPEN) -- Contains information about user logging process for active
DBOPENS of modes 1-4.
• QLOCK Table (QLOCK) -- Contains information about all writers to databases and a
pointer to QOPEN.
The DBS is a permanent file, TURBODBS.PUB.SYS, that is created by DBOPEN if it does not
exist beforehand. Thereafter, it is opened when the first user opens any database on the
system. It is reinitialized after a system abort. Each system has only one DBS; it contains
pointers to the current DBGs for any currently open database(s).
Both the DBG and the DBB are created when the first user opens a database (DBOPEN).
They remain allocated until the last user closes the database. Each opened database has
only one DBB and one DBG, regardless of the number of users. Both of these control blocks
are part of a permanent mapped file called dbnameGB located in the same group and