The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

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Sometimes, the apparent difference in access speed across different systems stems from a poor selection of the sector interleave. The Version 6 DOS uses the method of applying the interleave during the formatting process. The sectors in each track are therefore numbered in a staggered order. [Most CP/M systems format sequential sector numbers and use a sector interleave translation table to translate sequential access requests to the staggered number when the access is made]. One other attempt at increasing the sequential access of sectors is to examine the time between transferring the last sector number of a track and sector zero of the next higher track [for the moment let's not compound the situation of two sided diskettes where the sectors on the second side rotate in an order reverse of the obverse side]. The time lag will include the sector interleave plus the track-to-track step time. Thus it might make sense to not start each track with sector number zero, but to optimize the starting number so that the position of sector zero will have its ID FIELD just coming up to the head by the time that the drive has stepped and is ready to scan for the ID FIELD. This staggering is termed TRACK SKEW. The DOS introduces such a skew during the formatting process; however, such a skew is probably optimum for only one track-to-track stepping rate. With all of this, we still can state that each track contains like numbered sectors - regardless of track number or surface. Therefore, each sector on a disk is designated unique by its respective sector, surface, and track numbers. When the operating system formats a diskette (or hard disk), all of the parameters associated with the diskette are predetermined. Thus the number of sectors per track, number of sectors per granule and thus the granules per track, number of sides (or surfaces), and number of cylinders are all designated as well as the density of the media in the case of floppy diskettes. Some of these figures (density, sides, granules per track) are written to fields in the Granule Allocation Table which is part of the directory (see chapter 5). Others (sectors per track, sectors per granule, in addition to the former quantities) are part of the DCT fields. When the system attempts to open a file on a disk, it uses the @CKDRV SuperVisor Call function to ascertain the availability of the disk and then logs the disk once it finds it available. The function of "logging" will update the DIRCYL field (providing the driver returns proper system sector error codes), then update the DBLBIT field and the MAXCYL field based on information stored in the GAT. It is up to the driver to sense the density of the floppy media [the "data record not found" controller error is the usual indication that the driver must toggle to the alternate density. If a data record ID FIELD is not readable under both single density and double density, then the assumption is that the corresponding sector is not on the disk and the error is passed back to the system]. The toggling function of the driver includes the updating of the CONFIGURATION FIELD in the DCT appropriate to the density being selected. The SVC disk primitives are funneled through a common system routine that establishes a linkage protocol between the operating system and the disk device driver(s). When an I/O request is invoked by a higher level SVC, such as a request to READ a file record, the request is translated to that disk primitive needed to satisfy the function. The linkage protocol is uniform across all disk devices that are connected to the system. This makes the access of files transparent to size or nature of the disk device within the scope of the DCT parameters acceptable to the system. 4.2 DRIVE CONTROL TABLE (DCT) The Drive Control Table (DCT) is the way in which the DOS interfaces the operating system with specific disk driver routines. This table is one of the examples of the versatility of the system as it embodies within it the method of customizing the parameters of a drive so that each disk drive may incorporate a unique set of parameters. For instance, one drive may be a 35-track single headed drive. Another may be an 80-track dual headed. While a third may yet be a 5 megabyte hard drive. Ingenuity and oddball hardware will mix well to provide an easy interface. 4-3
The DCT contains the information relating to the granule size. In the case of floppies, granule sizes are standardized by the system according to the disk size and density. Chapter 5 contains more information on granule allocation sizes. Data on the number of sectors per track, number of heads, number of partitions, and maximum number of cylinders is also contained in the DCT for each drive. This data is an essential ingredient in the allocation and accessibility of file records and therefore must be accurately introduced. The table contains a maximum of eight DCT records - one record for each logical drive designated 0-7. Each DCT record is fielded as follows: 4.2.1 DCT VECTOR - This three-byte field specifies whether the logical drive position is enabled or disabled. The system will not attempt to communicate with a logical drive number whose DCT position is considered disabled. If the position is enabled, then the field will also contain the address vector of the disk driver module that communicates with the controller interfacing the disk drive. The first byte of the DCT VECTOR would contain an X'C3' value if the drive position is enabled (an X'C3' represents an absolute jump [JP nnnn] instruction in Z-80 machine code). If the drive is disabled, this byte will be an X'C9' value (an X'C9' represents an absolute return [RET] from subroutine instruction in Z-80 machine code). The second and third bytes of the field will contain the vector transfer address of the disk driver module that communicates with the controller. The operating system typically places the disk drivers in the low memory driver region. A "stock" system has available in this region, memory sufficient to store additional drivers that are not supplied by the system. The DOS will dynamically use this low memory region based on requests to invoke system drivers and filters (such as the COM/DVR or FORMS/FLT). A retrievable pointer to the first available memory address in this region can be used to locate the origin of a user-supplied driver or filter (if sufficient space is available). This will be discussed in a later section. 4.2.2 DCT FLAG-1 - This field contains a series of sub-field parameters associated with the disk drive specifications. The field is encoded as follows: Bit 7 Set to 1 will indicate the disk device is "software" write protected. It is the responsibility of the disk driver to check this bit on any disk primitive that references a WRITE operation (i.e. write sector, write system sector, format track, or format device) and return a "Write protected disk" error code (error 15) if set. Bit 6 If set to a "1", it indicates that the floppy diskette currently being accessed is formatted in double density. If set to a "0" it indicates that the diskette is single density. The disk driver is responsible for maintaining this bit by recognizing the density of the disk it is accessing. The bit is used both by the driver in the drive selection process and by the system in informative messages by such things as DEVICE displays, DIRectory displays, and FREE displays. This bit is not referenced by the system if the DCT is associated with a hard drive (see bit 3 of this field). Bit 5 If this bit is set to a "1", the drive associated with the DCT position is an 8" drive. This bit will be a "0" if the drive associated with the DCT position is a 5-1/4" drive. This bit is initially set by whatever installs the disk driver (see the FLOPPY/DCT utility). In the installation of a hard disk driver, this bit should be set according to the size of the hard drive - 5" or 8". In the case of floppy drives, the system formatter will use this bit to adjust its formatting data to 5" or 8". It is also used to adjust informative messages as mentioned under bit-6. 4-4
Page 1 and 2: MISOSYS, INC. The Programmer's Guid
Page 3 and 4: 1. Introduction Many thousands of u
Page 5 and 6: language (i.e. BASIC, C, PASCAL, ..
Page 7 and 8: NAME START END DESCRIPTION LOWCORE
Page 9 and 10: ackup utility provides exceptional
Page 11 and 12: 3. Device Input/Output Interfacing
Page 13 and 14: 3.2.2 VECTOR Field - This field in
Page 15 and 16: FILTER *PR USING *S0 the operating
Page 17 and 18: ===================================
Page 19 and 20: ===================================
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: called TRACKs. Each track is then d
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 and 74: position where the last character i
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 @
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7.6 SUPERVISOR CALL DETAILS 7.6.1 @
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Note: @EXIT in SYS1 automatically r
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7.6.10 @CLOSE SVC-60 This SVC will
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7.6.18 @DCSTAT SVC-40 This SVC pass
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Directory to Buffer Entry: B 1; Fun
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7.6.31 @FLAGS$ SVC-101 This SVC wil
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NFLAG$ This "network" flag is used
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7.6.32 @FNAME SVC-80 This SVC will
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7.6.40 @HEX8 SVC-98 This SVC will c
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7.6.48 @KLTSK SVC-32 This SVC will
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7.6.57 @OPEN SVC-59 This SVC will o
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Registers Affected: AF, BC. Entry:
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0-14 FILENAME/EXT:D - left justifie
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7.6.71 @REMOV SVC-57 This SVC will
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Entry: DE A pointer to the FCB cont
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Exit: A Error return code, if any.
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CURSOR CHARACTER Registers Affected
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Exit: A Will contain the error code
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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
Page 139 and 140:
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
Page 149 and 150:
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/
Page 157 and 158:
8.15 SLASH0 Filter Illustrated ;SLA
Page 159 and 160:
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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The Programmer's Guide to TRSDOS Version 6 - Tim Mann's Home ...

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