ibm:vm370-lib:cms:dmssbd.assemble_abs
Table of Contents
DMSSBD Abstract
Statistics
Type ASSEMBLE
Source Lines : 642
Fixes Applied : 1
Most Recent Version : Tuesday, December 12, 1978
Last Fix ID : [R09009DS]
Synopsis
Module
DMSSBD (DMSSBD - BASIC DIRECT ACCESS METHOD)
Function
THE CMS BDAM MACRO ROUTINE IS USED TO ACCESS DATA SET
RECORDS DIRECTLY BY ITEM NUMBER. IT CONVERTS RECORD
IDENTIFICATIONS GIVEN BY OS BDAM MACROS INTO ITEM
NUMBERS AND USES THESE ITEM NUMBERS TO ACCESS
RECORDS. THE CMS BDAM MACRO ROUTINE SUPPORTS ALL THE
RELEASE 20 OS BDAM MACRO FUNCTIONS EXCEPT THOSE
LISTED AS RESTRICTIONS.
Attributes
REENTRANT, NUCLEUS RESIDENT
Operation
FOR RELATIVE BLOCK, RELATIVE TRACK AND ACTUAL
ADDRESSES, THE LOW ORDER TWO BYTES OF A RECORD
IDENTIFICATION ARE USED FOR AN ITEM NUMBER. FOR
RELATIVE TRACK ADDRESS AND ACTUAL KEY, THE LOW ORDER
BYTE OF THE RELATIVE TRACK ADDRESS IS USED TO ACCESS
A TABLE OF KEYS WHICH IF NOT ALREADY IN CORE IS
BROUGHT IN AND SEARCHED FOR THE CORRECT KEY.
CMS DOES NOT SUPPORT ACTUAL KEY I/O SO THE CMS BDAM
ROUTINE SIMULATES IT. IN CMS, ALL KEYS ARE KEPT AT
THE END OF THEIR DATA FILE. WHEN THE DATA FILES IS
OPENED, EITHER THE KEYS ARE ACCESSED FROM THE END OF V0016
THE DATA FILE AS IN THE CASE OF A FIXED FILE OR A V0016
VARIABLE FILE THAT IS ONLY BEING READ OR TWO NEW V0016
FILES ARE CREATED WITH THE SAME V0016
FILETYPE, BUT WITH FILENAMES OF $KEYTEMP AND
$KEYSAVE. BOTH THESE FILES CONTAIN ALL THE KEYS IN
THE ORIGINAL DATA FILE. $KEYTEMP IS USED FOR
UPDATING KEYS AND $KEYSAVE IS USED TO SAVE ALL THE
KEYS IN CASE OF A RE-IPL OR SYSTEM CRASH. FOR EVERY
ITEM IN THE ORIGINAL FILE THERE IS A CORRESPONDING
KEY SPACE IN THE $KEYTEMP FILE. EACH ITEM IN THE
$KEYTEMP FILE IS A KEY TABLE THAT CONTAINS 256 KEYS.
WHEN THE DATA FILE IS CLOSED, THE $KEYTEMP FILE IS
WRITTEN AT THE END OF THE DATA FILE, AND THE $KEYTEMP
AND $KEYSAVE FILES ARE ERASED.
THE CMS BDAM ROUTINE GETS CONTROL FROM THE CMS BSAM
ROUTINE WHICH IN TURN GETS CONTROL FROM AN OS READ OR
WRITE MACRO. UPON ENTRY TO DMSSBD, A CHECK IS MADE
TO SEE IF DYNAMIC BUFFERING IS NEEDED. IF SO, A KEY
BUFFER AND OR DATA BUFFER IS ACQUIRED OR RETURNED
DEPENDING ON WHETHER A READ OR WRITE IS REQUESTED.
NEXT THE RELATIVE OR ACTUAL ADDRESS IS CHECKED TO
MAKE SURE IT DOES NOT EXCEED TWO BYTES. THIS ADDRESS
IS CONVERTED INTO AN ITEM NUMBER AND, IF KEYS ARE NOT
INVOLVED, THE FEEDBACK OPTION IS TAKEN CARE OF AND
CONTROL IS
PASSED BACK TO DMSSBS.
IF KEYS ARE ACCESSED AND THE KEY TABLE CONTAINING THE
KEY WANTED IS NOT IN CORE, IT IS BROUGHT IN AND
SEARCHED OR UPDATED. IF A SEARCH IS SPECIFIED, THE
ITEM NUMBER OF THE KEY TABLE CONTAINING THE KEY IS
COMBINED WITH THE POSITION NUMBER OF THE KEY IN THE
TABLE TO FORM THE ITEM NUMBER OF THE DATA. IF THE
EXTENDED SEARCH OPTION IS NOT SPECIFIED, ONLY ONE KEY
TABLE OF 256 KEYS IS SEARCHED. IF THE EXTENDED
SEARCH OPTION IS SPECIFIED, THE LIMIT PARAMETER IN
THE DCB IS CONVERTED TO A NUMBER OF KEY TABLES AND
THAT NUMBER OF KEY TABLES IS SEARCHED FOR A MATCHING
KEY. AFTER THE KEY TABLE HAS BEEN READ, UPDATED OR
SEARCHED, THE ITEM NUMBER, IF FEEDBACK
IS REQUESTED, IS STORED IN THE CORRECT FEEDBACK
ADDRESS AND CONTROL IS
RETURNED TO DMSSBS.
CORE FOR THE KEY TABLE AND ITS CONTROL PARAMETERS IS
ACQUIRED THE FIRST TIME A KEY IS ACCESSED. THE
ADDRESS OF THIS CORE IS STORED IN THE FCB AND THE
CORE IS NOT FREED UP UNTIL THE DATA SET IS CLOSED.
THE FORMAT OF THE DISK KEY TABLE AND THE IN-CORE KEY
TABLE AND CONTROL WORDS IS DESCRIBED BELOW.
KEY TABLE
| KEYLENGTH |
-----------------
| LST KEY | N IS 256
-----------------
| 2ND KEY |
-----------------
| |
-----------------
| NTH KEY |
-----------------
IN-CORE KEY TABLE AND CONTROL WORDS
KEYTABL DSECT
KEYLNGTH DS 1F KEY LENGTH
ENDDATA DS 1F LAST DATA ITEM IN FILE
KEYOP DS 2F COMMAND NAME
KEYNAME DS 2F FILENAME OF KEY FILE
KEYTYPE DS 2F FILETYPE OF KEY FILE
KEYMODE DS 1H FILEMODE OF KEY FILE
KEYTBLNO DS 1H ITEM NUMBER OF KEY TABLE
KEYTBLAD DC A(KEYTABLE) ADDRESS OF KEY TABLE
TBLLNGTH DS 1F BYTE SIZE OF KEY TABLE
KEYFORM DC C'F' FORMAT OF KEY TABLE
KEYCHNG DC X'00' BYTE TO SIGNIFY CHANGE IN KEY TBL
KEYCOUT DC X'0001' NUMBER OF TABLES PER ITEM
DS IF USED BY RDBUF FOR RESIDUAL COUNTS
KEYTABLE DS 0F TABLE OF KEYS
RESTRICTIONS
THE FOUR METHODS OF ACCESSING BDAM RECORDS ARE:
1. RELATIVE BLOCK RRR
--
2. RELATIVE TRACK TTR
--
3. RELATIVE TRACK AND KEY TT KEY
- -
4. ACTUAL ADDRESS MBBCCHHR
--
THE RESTRICTIONS ON THESE METHODS ARE:
. BDAM SPANNED RECORDS ARE NOT SUPPORTED IN CMS.
. THE R AND RU OPTIONS OF READ AND WRITE (ADDED
IN OS RELEASE 19) ARE NOT SUPPORTED.
. SINCE CMS FILES ONLY HAVE A TWO-BYTE RECORD
IDENTIFIER, ONLY THE BDAM IDENTIFIERS
UNDERLINED ABOVE CAN BE USED TO REFERENCE
RECORDS.
. CMS BDAM FILES ARE ALWAYS CREATED WITH 255
RECORDS ON THE 1ST LOGICAL TRACK AND 256 RECORDS
ON ALL OTHER LOGICAL TRACKS REGUARDLESS OF THE
BLOCKSIZE. IF BDAM METHODS 2, 3 OR 4 ARE USED
AND THE RECFM IS U OR V, THE BDAM USER MUST EITHER
WRITE 255 RECORDS ON THE 1ST TRACK AND 256
RECORDS ON EVERY TRACK THEREAFTER OR HE MUST NOT
UPDATE THE TRACK INDICATOR UNTIL A NO SPACE
FOUND MESSAGE IS RETURNED ON A WRITE. FOR
METHOD 3 (WRITE ADD), THIS IS WHEN NO MORE
DUMMY RECORDS CAN BE FOUND ON A WRITE
REQUEST. FOR METHODS 2 AND 4, THIS WILL NOT
OCCUR, AND THE TRACK INDICATOR WILL ONLY BE
UPDATED WHEN THE RECORD INDICATOR REACHES 256
AND OVERFLOWS INTO THE TRACK INDICATOR.
. TWO FILES WITH KEYS AND THE SAME FILETYPE
CANNOT BE OPEN AT THE SAME TIME. IF A
PROGRAM THAT IS UPDATING KEYS DOES NOT CLOSE
THE FILE IT IS UPDATING FOR SOME REASON,
E.G., A SYSTEM CRASH OR A RE-IPL, THE
ORIGINAL KEYS FOR A VARIABLE OR UNDEFINED DATA V0016
FILE WILL BE IN A TEMPORARY FILE V0016
WITH THE SAME FILETYPE AND A FILENAME OF
$KEYSAVE. THE KEYS FOR A FIXED DATA FILE WILL BE V0016
INTACT AT THE END OF THE DATA FILE. TO FINISH THEV0016
UPDATE, RUN THE UPDATE PROGRAM AGAIN. V0016
. ONCE A FILE IS CREATED USING KEYS, THE FILE
MUST NOT BE ADDED TO WITHOUT USING KEYS AND
SPECIFYING THE ORIGINAL KEY LENGTH.
. THE MINIMUM DCBLRECL FOR A CMS BDAM FILE WITH KEYV0016
IS 8 BYTES. V0016
Calls to Other Routines
DMSBRD, DMSFRE, DMSBWR, DMSFNS, DMSSVT
ibm/vm370-lib/cms/dmssbd.assemble_abs.txt · Last modified: 2023/08/06 13:35 by Site Administrator