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8.8 MEMORY BANK SWITCHING This section discusses the techniques of using the @BANK SuperVisor Call. The control of an assembly-coded application operating in a memory banked environment requires a high degree of skill in assembly language coding and should not be undertaken by the novice. The professional is advised to carefully read the information contained in this section which discusses how bank switching is supported within the operating system. The DOS can support eight multiple RAM banks of 32K each in addition to a resident 32K bank. This brings the total RAM configuration to 288K. The non-resident RAM banks are designated as banks zero through seven. The 32K of bank zero (generally considered as "high memory") and the resident 32K are considered the standard 64K of the DOS. Banks one through seven may be used for buffers or data storage. Through sophisticated techniques, they can even be used to store executable code. An entire bank is reserved for a particular function. The system maintains a pointer (HIGH$) for bank 0 only. At any one time, only one of the banks are resident. All are imaged at address X'8000' through X'FFFF'. When a bank transfer is performed, the specified bank becomes addressable and the previous bank is no longer available. Since memory refresh is performed on all banks, nothing in the previously resident bank is altered during whatever time it is not addressable (i.e. not resident). The DOS provides support in accessing this additional RAM by means of the @BANK SuperVisor Call (SVC-102). Let's take a look at how this RAM is handled. When the operating system is booted, it examines what banks of RAM are installed in the machine. The DOS maintains a byte bit-map with each bit representing one of the banks of RAM. This byte is called "Bank Available RAM" (BAR), and its information is set when the DOS is booted. BAR bit-0 corresponds to bank 0, BAR bit-1 corresponds to bank 1, and so on to BAR bit-7 corresponding to bank 7. A machine may have only one bank, bank 0. Another byte bit-map is used to indicate whether a bank is reserved or available for use. This byte is designated the "Bank Used RAM" (BUR). Again, a bit assignment corresponds one-for-one with the bank number. The management of any memory space within a particular bank of RAM (excluding bank 0) is the sole responsibility of the application program "reserving" a particular bank. The DOS I/O device handler will always enable bank 0 upon execution of any character I/O service request (@PUT, @GET, @CTL, as well as those other character I/O SVCs that use @PUT/@GET/@CTL). The DOS also enables bank 0 at the initial entry to the task processor and when a disk I/O communications function is requested. This requires that any device driver or filter that is relocated to high memory (X'8000'-X'FFFF') must reside in bank 0. The same holds true for interrupt task routines and disk drivers/filters. The system provides this restriction to make sure that any filter, driver, or task routine that control passes to will be occupying enabled RAM memory. If a RAM bank other than 0 was resident during these operations, it would be restored upon return from the device/drive/task handler. The limitation will ensure that device I/O, task processing, and disk I/O will never be impacted due to bank switching of RAM by an application. Another restriction requires that the stack pointer (SP) is not pointing to an adddress above X'7FFE' when a bank transfer is requested. This is because that stack range would have placed the stack in the memory region that is being swapped thereby making the stack contents erroneous. The @BANK SVC will inhibit the request and return an "SVC parameter error" if this condition is violated. It is acceptable for an interrupt task, filter module, or driver that is located in the bank switched address range to perform a bank transfer to another bank provided the necessary linkage and stack area is being utilized. This will be discussed later in more detail. All bank transfer requests must be performed using the @BANK SVC. This SVC provides five functions - four of which are interogatory in nature. One of the functions performs 8-21
actual bank switching. As previously discussed, the contents of banks other than 0 are managed by the application - not the DOS. Therefore, the application first needs a way of ascertaining the availability of any given bank. For instance, if an application wants to reserve use of bank 1, it must first check if bank 1 is free to use. This is achieved by using function 2 as follows: LD C,1 ;Specify the bank # LD B,2 ;Ck BUR if bank-in-use LD A,@BANK ;Identify the SVC RST 40 JR NZ,INUSE ;NZ if in use already Astute programmers will recognize that the first two instructions could be combined to form one instruction as: LD BC,2
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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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===================================
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ENTRY JR BEGIN ;Branch around linka
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DATA1 EQU $-DATA$ DB 0 ;Data storag
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Offset Contents +0 Contains the las
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image of the serial port UART statu
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called TRACKs. Each track is then d
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The DCT contains the information re
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accessed a single-sided diskette, t
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containing the disk's directory. Th
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Command Purpose RESTORE Recalibrate
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available from Logical Systems, Inc
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exceeds this capacity, then it must
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is two physical cylinders. Although
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5. The DOS Directory Structure 5.1
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===================================
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5.2.8 PACK NAME - This field conta
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The position of a file's hash code
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5.4 THE DIRECTORY RECORD STRUCTURE
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Bit 3 Specifies the visibility; if
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the first granule in use is the sec
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travels the turnpike. In this manne
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The @FSPEC SVC will transfer the fi
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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
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
Page 125 and 126: ===================================
Page 127 and 128: The TYPE=0A indicates that it is th
Page 129 and 130: Error 08: Device not available A re
Page 131 and 132: Error 28: End of file encountered T
Page 133 and 134: 8.5 HEADER PROTOCOL OF MEMORY MODUL
Page 135 and 136: This is obviously an extremely usel
Page 137 and 138: LD (IX+2),15 JR TASKA By firmly und
Page 139: 8.7.3 Details of Low Memory Page 2
Page 143 and 144: set. Thus, the Z-flag will be indic
Page 145 and 146: 8.10 SYSTEM DISK BOOT TRACK The ope
Page 147 and 148: 14 DEBUG 72 PURGE 1B VERIFY 71 DUMP
Page 149 and 150: 8.13 USING @PARAM The @PARAM SuperV
Page 151 and 152: DW LPARM+1 DB 'FEED ' ;Line feed pa
Page 153 and 154: ;*=*=* PPARM LD BC,0 ;Zero because
Page 155 and 156: 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
Page 161 and 162: NOMEM$ DB 'High memory is not avail
Page 163 and 164: @ @ABORT..........7-2, 7-4, 7-8, 7-
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