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@SKIP, @PEOF, or @SEEKSC SuperVisor Call. It is important for the system to know whether or not the disk file buffer has been changed since it was read from the file. If the buffer has been altered, it is necessary to write the buffer back to the file prior to any movement of the file pointer. This flag conveys such status. If it is set to a "1", it indicates that the buffer contents have been changed since the buffer was read from the file. If it is set to a "0", the indication is that the buffer has not been modified. The system will set this bit whenever a WRITE operation is performed on the buffer by either a PUT or the write of a record (of LRL < 256). The bit is reset by the system when the buffer is physically written to the disk via the @WRSEC SuperVisor Call request. Bit 3 The normal method to reflect changes in a file's directory entry record data is to update the directory entry only when the file is closed. Thus, the FCB contains all of the information pertinent to the modifications. This keeps the directory accesses to a minimum and results in faster file throughput. However, it is important to note that if the system crashes after extensive file updating (specifically where the file has been extended), the added information will be unrecoverable without manual corrections to the file's directory entry record. It is possible to force the system to always update the directory whenever the system extends the file by writing another sector. Unattended operation may utilize this extra measure of file protection. It is specified by appending an exclamation mark "!" to the end of a file specification when the filespec is requested at open time. This bit will then be set by the system. It is used to specify that the directory record is to be updated every time that the NRN exceeds the ERN. Bits 2-0 These bits will contain the access protection level as retrieved from the directory entry record of the file when the file is first opened. The specific bit pattern will be adjusted to the protection level granted according to the password (OWNER vs USER) entered at file open time. 6.4.3 PDS Member Origin Offset - When a Partitioned Data Set (PDS) has been opened for individual member access (a sector origin member), the PDS linkage routines will adjust the EOF contained in the FCB to be the logical EOF of the member. The member origin offset is the number of relative sectors between the logical ERN of the member and the first relative sector of the member. This byte will contain that forward offset so that the linkage routines may be able to calculate the logical beginning of the member. The calculation is required for linkage to all SuperVisor Calls that reference file positioning forward of the NRN (@BKSP, @REWIND, @POSN, @SEEKSC). 6.4.4 Disk File Buffer Pointer - This is a pointer to the disk file buffer that is used for all disk I/O associated with the file. The pointer is a 16-bit address stored in normal low-order - high-order format. This pointer is the buffer address specified in register pair HL at open time. 6.4.5 Next Record Number Byte Offset - When a file is accessed with either character I/O or record I/O of Logical Record Length less than 256, requests for I/O may not necessarily require the transfer of a physical sector from/to the disk. Therefore, the system needs a pointer to the byte position within the buffer that is to be used for the next I/O operation. This field contains that position - it is termed an OFFSET within the sector pointed to by the NRN. If this offset is a zero value, then the next byte to be transferred during an I/O operation is dependent on whether or not the buffer contains the current sector as noted by FCB+1, bit 5. The system automatically maintains this OFFSET byte during record and character I/O. If your application is performing full sector I/O for writing data while it is maintaining its own character buffering, then it is important for it to maintain this byte when the file is closed if the true end-of-file offset is not at a sector boundary. Remember, this offset is a pointer to the next available buffer position and not to the 6-15
position where the last character is placed. For instance, after writing three bytes into positions 0, 1, and 2 of the buffer, the offset must be incremented to "3" since the next available buffer position is byte 3. 6.4.6 Logical Drive Number - This contains the logical drive number in binary of the drive containing the file. It is absolutely essential that this byte be left undisturbed. It is used by the system's file access routines to obtain the logical disk drive number that physical I/O is to reference. It, and the Directory Entry Code contained in FCB+7 are the only links to the directory information for the file. Since the operating system supports a maximmum of eight logical drives, the logical drive number is contained in a 3-bit field. The remaining bits are reserved for future use in large disk segmentation. Bits 7-3 This field is reserved by the DOS for future use. Bits 2-0 This field contains the logical drive number where the file is stored. 6.4.7 Directory Entry Code - This field contains the Directory Entry Code (DEC) which points to the file's primary directory entry. This code is the relative position in the Hash Index Table where the hash code for the file's directory entry appears. Whenever the system needs to access the directory for the open file, it must use both this DEC and the logical DRIVE to uniquely specify the proper directory record. Do not tamper with this byte. It may be interesting to note that the device name, which uniquely identifies a device, and the DEC-DRIVE, which uniquely identifies a file, are contained in the same fields of their respective control blocks. 6.4.8 Ending Record Number Byte Offset - This field contains the byte offset in the Ending Record Number which points to one byte past the end-of-file. This byte is similar to FCB+5 except it pertains to the ERN rather than the NRN. If a file has been extended during the time it was open, then the NRN byte offset and NRN become the new ERN byte offset and ERN when the file is closed. 6.4.9 Logical Record Length - This field contains the logical record length in effect when the file was opened. This may not be the same LRL that exists in the directory. The directory LRL is generated at the file creation and will never change unless another file is cloned to it. 6.4.10 Next Record Number This field contains the Next-Record-Number (NRN), which is a pointer to the relative sector for the next I/O operation. When a file is opened, NRN is set to zero indicating a pointer to the beginning of the file. Each physical sector I/O advances NRN by one. An @REWIND SuperVisor Call request will reset the NRN to zero. 6.4.11 Ending Record Number This field is a pointer to the last sector of the file regardless of whether the sector is a full sector (i.e. all bytes occupied and EOF-OFFSET has a zero value) or a partial sector (i.e. EOF-OFFSET is not equal to zero). In a null file (one with no records), ERN will be equal to zero. If one sector had been written, ERN would be equal to one. 6.4.12 Starting Extent - This field contains the same information as the first extent of the directory. This represents the starting cylinder of the file (FCB+14) and the starting relative granule within the starting cylinder (FCB+15). FCB+15 also contains the number of contiguous granules allocated in the extent. This can always be used as a pointer to the beginning of the file referenced by the FCB. During any file access, this field will be searched first to see if it contains the granule which stores the physical sector that is being referenced. 6-16
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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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===================================
Page 21 and 22: 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: operating system for such a purpose
Page 75 and 76: ecord. If a file has more than four
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
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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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