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3.4.1 Header Protocol Each module placed into protected memory will incorporate a facsimile of the following code at the start of the module: ENTRY JR BEGIN ;Branch around linkage STUFHI DW $-$ ;To contain last byte used DB MODBGN-ENTRY-5 ;Calculate length of 'NAME' DB 'MODNAME' ;Name of this module MODDCB DW $-$ ;To contain DCB pointer for module SPARE DW 0 ;Reserved by the DOS . . ;Any data area needed BEGIN EQU $ ;Followed by module code Chapter 8, the appendix, is another source of information concerning the header protocol. It is sufficient for the illustration of device chains to understand that the MODDCB will contain a pointer that points to the Device Control Block established for the module during the execution of the SET library command. This pointer is passed in register pair DE to the module's initialization code by SET. The programmer writing the module code adds a routine which loads this valuue into MODDCB. 3.4.2 Sample DCB Structure For the purpose of this illustration, let's imagine three active DCBs. The first DCB is associated with the printer driver and has device specification of "*PR" (its devspec). We have also installed a filter via the SET command that performs a backspace followed by the output of a slash when it detects an ASCII zero (0). This filter has a devspec of "*S0". Lastly, we have a filter that toggles a boldface mode for a printer. This filter has a devspec of "*BF". To avoid confusion in the illustration, the devspec will be used to reference the DCB and the module names PRINTER, SLASH0, and BOLDFACE will be used to identify the entry point of the driver or filter module. We can now show this arrangement of DCB contents and module MODDCB contents as follows: ============================================== | TYPE VECTOR NAME MODULE/MODDCB | | ---- ------ ---- ________________ | | | | | | 06 PRINTER PR | PRINTER/*PR | | | |________________| | | | | | | 47 SLASH0 S0 | SLASH0/*S0 | | | |________________| | | | | | | 47 BOLDFACE BF | BOLDFACE/*BF | | | |________________| | | | ============================================== Figure 3-2: Initial DCB Table Note that the DCBs in figure 3-2 associated with the filters have bit-6 of the TYPE byte set to indicate that they are filters. Also note that the MODDCB pointer points to the DCB which points to the module. Where a filter's MODDCB is pointing to the DCB of the filter, this is indicative of an inactive filter. 3.4.3 Filtering Filters are written (as you will later learn) to perform all I/O via the @CHNIO SuperVisor Call. This SVC uses the contents of MODDCB within the filter invoking the SVC. Thus, the filter I/O is independent of any address by being handled completely through the SVC. If you perform a system command such as: 3-4
FILTER *PR USING *S0 the operating system will swap the first three bytes of the *PR DCB with the *S0 DCB. This arrangement will establish that shown in figure 3-3. ============================================== | TYPE VECTOR NAME MODULE/MODDCB | | ---- ------- ---- ________________ | | | | | | 47 SLASH0 PR | PRINTER/*PR | | | |________________| | | | | | | 06 PRINTER S0 | SLASH0/*S0 | | | |________________| | | | | | | 47 BOLDFACE BF | BOLDFACE/*BF | | | |________________| | ============================================== Figure3-3: DCB Table Modified Let's follow what happens to an @PUT which references the *PR device. The system passes control to SLASH0 (which is pointed to by the *PR vector). This filter performs its character transformation, as required, and sends characters down the chain by picking up the pointer contained in its MODDCB (a pointer to the *S0 DCB) then issuing the @CHNIO SVC. The SVC handles the call by passing control to PRINTER which is the pointer now stored in the VECTOR field of *S0. If we now try to issue the command: FILTER *PR USING *S0 the system will prohibit it since the *S0 Device Control Block does not show up as a filter (bit-6 of the TYPE byte is reset!). However, if we filter *PR using the *BF device, we achieve the arrangement in figure 3-4 after the system swaps the first three bytes of *PR with the first three bytes of *BF. Examine the arrangement in figure 3-4 closely. Note that the contents of MODDCB for each module are exactly what they were initialized to. Even though the *PR device has been twice filtered, the module itself needs absolutely no change whatsoever. An *PUT to the *PR device (say with an *PRT SVC) may be a little more complicated now, but functions perfectly well. The system first passes control to BOLDFACE (which is pointed to by the *PR vector). This filter performs its necessary device stream massaging and sends characters down the chain by picking up the pointer contained in its MODDCB (a pointer to the *BF DCB) then issuing the @CHNIO SVC. The SVC handles the call by passing control to SLASH0 which is the pointer now stored in the VECTOR field of *BF. The SLASH0 filter performs its character transformation, as required, and sends characters down the chain by picking up the pointer contained in its MODDCB (a pointer to the *S0 DCB) then issuing the @CHNIO SVC. The SVC handles the call by passing control to PRINTER which is the pointer now stored in the VECTOR field of *S0. Upon completion, a series of RET instructions pass the return code back through the modules making up the chain. 3-5
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: 3.2.2 VECTOR Field - This field in
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 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
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does exist, @RENAME should return e
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Next, it would be very useful if th
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LD HL,FCBX+9 ;Point to the LRL fiel
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operating system for such a purpose
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position where the last character i
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ecord. If a file has more than four
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DE Contains a pointer to the File C
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@RENAM SVC-56 Rename a file on disk
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@FNAME SVC-80 Obtain filespec given
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@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
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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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