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6.4.13 Extent Quad 1 - The QUAD is a 4-byte field that contains the granule allocation information for one extent of the file as well as the total quantity of granules contained in the file logically prior to this extent. Relative bytes zero and one contain the cumulative number of granules allocated to the file up to but not including the extent referenced by this field. This quantity is calculated by the system by adding up all the number of contiguous granules allocated in previous extents. Relative byte two contains the starting cylinder of this extent. Relative byte three contains the starting relative granule for the extent and the number of contiguous granules. Relative bytes two and three are obtained directly from an extent field of the directory entry record. Figure 6- 4 illustrates the Extent Quad. 6.4.14 Extent Quad 2 - This field contains information similar to the first Extent Quad but for a second extent of the file. 6.4.15 Extent Quad 3 - This field contains information similar to the first Extent Quad but for a third extent of the file. 6.4.16 Extent Quad 4 - This field contains information similar to the first Extent Quad but for a forth extent of the file. ___________________________________________ ... | | | |r g| | ... ... | # of contiguous |starting |e r|total| ... ... | granules up to |cylinder |l a|grans| ... ... | this extent | | n| | ... _|_________|_________|_________|___|_____|__ byte 0 byte 1 byte 2 byte 3 Figure 6-4 An FCB Extent Quad The File Control Block contains information on only five extents at any one time - one of which is always the first extent of the file (that which is placed into STARTING EXTENT. When a file is first opened, data for the STARTING EXTENT is extracted from the first extent of the file's primary directory entry record (the FPDE). If the file has more than one extent, data for the EXTENT QUADS is calculated for each additional extent that contains allocation information in the FPDE. This leaves, at a minimum, one EXTENT QUAD vacant. Each time a record is accessed, the system determines if the record is located in the starting EXTENT. If not, then the system searches the extent QUADs. If the record is located in one of the QUADS, then the data contained in all QUADS to the left of the "desired" QUAD is shifted right by one QUAD and the data from the "desired" QUAD is placed in the first extent QUAD field. This action is undertaken so that extent QUADs that contain records recently accessed will be searched first. If a record in a file is accessed which is not contained in any FCB extent QUAD field, then the DOS must access the directory entries for the file and locate that extent which contains the needed granule (and hence the needed record). Once the extent is located, the data in extent QUAD fields 1-3 will be shifted to occupy fields 2-4 and the new data will be placed into extent QUAD field 1. If the desired record cannot be located in any extent of the file, the system will attempt to allocate additional space necessary to position the record. Although the operating system can handle a file of any number of extents, it is wise to keep the total number of extents small. If the file has more than five extents, additional directory accessing must be done to locate the extent containing the desired 6-17
ecord. If a file has more than four extents, then it will occupy more than one directory entry record and thereby reduce the number of file slots available. The most efficient file is one with a single extent although the file can be at most 32 granules in size. The number of extents can be reduced by copying the file to a diskette containing a great deal of free space. 6-18
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MISOSYS, INC. The Programmer's Guid
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1. Introduction Many thousands of u
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language (i.e. BASIC, C, PASCAL, ..
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NAME START END DESCRIPTION LOWCORE
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ackup utility provides exceptional
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3. Device Input/Output Interfacing
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3.2.2 VECTOR Field - This field in
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FILTER *PR USING *S0 the operating
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===================================
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ENTRY JR BEGIN ;Branch around linka
Page 23 and 24: DATA1 EQU $-DATA$ DB 0 ;Data storag
Page 25 and 26: Offset Contents +0 Contains the las
Page 27 and 28: image of the serial port UART statu
Page 29 and 30: called TRACKs. Each track is then d
Page 31 and 32: The DCT contains the information re
Page 33 and 34: accessed a single-sided diskette, t
Page 35 and 36: containing the disk's directory. Th
Page 37 and 38: Command Purpose RESTORE Recalibrate
Page 39 and 40: available from Logical Systems, Inc
Page 41 and 42: exceeds this capacity, then it must
Page 43 and 44: is two physical cylinders. Although
Page 45 and 46: 5. The DOS Directory Structure 5.1
Page 47 and 48: ===================================
Page 49 and 50: 5.2.8 PACK NAME - This field conta
Page 51 and 52: The position of a file's hash code
Page 53 and 54: 5.4 THE DIRECTORY RECORD STRUCTURE
Page 55 and 56: Bit 3 Specifies the visibility; if
Page 57 and 58: the first granule in use is the sec
Page 59 and 60: travels the turnpike. In this manne
Page 61 and 62: The @FSPEC SVC will transfer the fi
Page 63 and 64: A high level language permits you t
Page 65 and 66: does exist, @RENAME should return e
Page 67 and 68: Next, it would be very useful if th
Page 69 and 70: LD HL,FCBX+9 ;Point to the LRL fiel
Page 71 and 72: operating system for such a purpose
Page 73: position where the last character i
Page 77 and 78: DE Contains a pointer to the File C
Page 79 and 80: @RENAM SVC-56 Rename a file on disk
Page 81 and 82: @FNAME SVC-80 Obtain filespec given
Page 83 and 84: @WRSEC SVC-53 Write a disk sector @
Page 85 and 86: 7.6 SUPERVISOR CALL DETAILS 7.6.1 @
Page 87 and 88: Note: @EXIT in SYS1 automatically r
Page 89 and 90: 7.6.10 @CLOSE SVC-60 This SVC will
Page 91 and 92: 7.6.18 @DCSTAT SVC-40 This SVC pass
Page 93 and 94: Directory to Buffer Entry: B 1; Fun
Page 95 and 96: 7.6.31 @FLAGS$ SVC-101 This SVC wil
Page 97 and 98: NFLAG$ This "network" flag is used
Page 99 and 100: 7.6.32 @FNAME SVC-80 This SVC will
Page 101 and 102: 7.6.40 @HEX8 SVC-98 This SVC will c
Page 103 and 104: 7.6.48 @KLTSK SVC-32 This SVC will
Page 105 and 106: 7.6.57 @OPEN SVC-59 This SVC will o
Page 107 and 108: Registers Affected: AF, BC. Entry:
Page 109 and 110: 0-14 FILENAME/EXT:D - left justifie
Page 111 and 112: 7.6.71 @REMOV SVC-57 This SVC will
Page 113 and 114: Entry: DE A pointer to the FCB cont
Page 115 and 116: Exit: A Error return code, if any.
Page 117 and 118: CURSOR CHARACTER Registers Affected
Page 119 and 120: Exit: A Will contain the error code
Page 121 and 122: esident routine into its execution
Page 123 and 124: CKPAWS LD A,@FLAGS$ ;Get Flags poin
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===================================
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The TYPE=0A indicates that it is th
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Error 08: Device not available A re
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Error 28: End of file encountered T
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8.5 HEADER PROTOCOL OF MEMORY MODUL
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This is obviously an extremely usel
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LD (IX+2),15 JR TASKA By firmly und
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8.7.3 Details of Low Memory Page 2
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actual bank switching. As previousl
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set. Thus, the Z-flag will be indic
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8.10 SYSTEM DISK BOOT TRACK The ope
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14 DEBUG 72 PURGE 1B VERIFY 71 DUMP
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8.13 USING @PARAM The @PARAM SuperV
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DW LPARM+1 DB 'FEED ' ;Line feed pa
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;*=*=* PPARM LD BC,0 ;Zero because
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8.14 TRAP Filter Illustrated ;TRAP/
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8.15 SLASH0 Filter Illustrated ;SLA
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8.16 DMP-500 BOLDFACE Filter Illust
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NOMEM$ DB 'High memory is not avail
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@ @ABORT..........7-2, 7-4, 7-8, 7-
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