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Summary: Low memory is the memory at the beginning of the address space. Some processors use designated low memory addresses during power on, exception processing, interrupt processing, and other hardware conditions. Some operating systems use designated low memory addresses for global system variables, global system structures, jump tables, and other system purposes.
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Addresses are hexadecimal in segment:offset form.
0000:0400h-0000:04ABh
0000:0400h-000:0407h addresses of RS232 adaptors
0000:0408h-0000:040Fh addresses of printers
0000:0410h-0000:0411h equipment flag: Also returned by interrupt 11h. MS-DOS only sets this value at startup. Programs that switch video adapters or switch modes in mulitadapter video cards should correctly set bit pair 5:4 — 00=not used, 01=CGA card 40 columns, 10=CGA card 80 columns, 11=monochrome card
0000:0412h initialization flag
0000:0413h-0000:0414h memory size: normally accessed via interrupt 12h.
0000:0415h-0000:0416h amount of memory in I/O channel
0000:0417h-0000:043Dh keyboard data area 1: keyboard status flags and 15-byte keyboard buffer. Normally read via interrupt 16h.
0000:043Eh-0000:0448h diskette data
0000:0449h-0000:0466h video control data area 1: normally accessed via interrupt 10h.
0000:00467h--0000:046Bh cassette data; in some models of PS/2 this contains the far (doubleword) address of the code that resets the computer while preserving memory.
0000:046Ch-0000:0470h time data: normally accessed via interrupt 1Ah.
0000:0471h break flag: set to 1 if the break key has been pressed.
0000:0472h-0000:0473h reset flag: if set to 1234h, the BIOS initialization routine (starting at segment FFFFh) will bypass the memory check normally performed during a cold boot. In many models of the PS/2, if the flag is set to 4321h, the system will perform a reset operation without erasing memory.
0000:04ACh-0000:04EFh
Reserved for BIOS.
0000:04F0h-0000:04FFh
May be used by any program in any way. Originally intended for communications and exchange of data between programs, but because Microsoft couldn’t figure out a standard, the area is in complete anarchy and programs should assume that other programs will act destructively on any attempts to use the area.
0000:0500h-0000:05FFh
Reserved for use by DOS and BASIC.
0000:0500 print screen status flag: one byte value — 0=print screen not active or successful termination, 1=print screen in progress, 255=error occurred during print screen.
0x0000 unassigned
0x0004 ResetSPPC
0x0008 BusError
0x000C AddrErr
0x0010 Illegal
0x0014 ZeroDiv
0x0018 ChkError
0x001C TrapVErr
0x0020 Privileg
0x0024 Trace
0x0028 Line1010
0x002C Line 1111
0x0030 unassigned
0x0034 Coproces
0x0038 FmtErrVect
0x003C Uninited
0x0040 Unassig2
0x0060 Spurious
0x0064 AutoInt1
0x0068 AutoInt2
0x006C AutoInt3
0x0070 AutoInt4
0x0074 AutoInt5
0x0078 AutoInt6
0x007C AutoInt7
0x0080 TRAPtble
0x00C0 FP-68881
0x00DC reserved
0x00E0 PMMU
0x00E0 SMgrOldCore
0x00EC reserved
0x0100 MonkeyLives
0x0100 SysCom
0x0102 ScrVRes
0x0104 ScrHRes
0x0106 ScreenRow
0x0108 MemTop
0x010C BufPtr
0x0110 StkLowPtr
0x0114 HeapEnd
0x0118 TheZone
0x011C UTableBase
0x0120 MacJmp
0x0124 DskRtnAdr
0x0128 PollRtnAdr
0x012C DskVerify
0x012D LoadTrap
0x012E MmInOK
0x012F CPUFlag
0x012F DskWr11
0x0130 ApplLimit
0x0134 SonyVars
0x0138 PWMValue
0x013A PollStack
0x013E PollProc
0x0142 DskErr
0x0144 SysEvtMask
0x0146 SysEvtBuf
0x014A EventQueue
0x0154 EvtBufCnt
0x0156 RndSeed
0x015A SysVersion
0x015C SEvtEnb
0x015D DSWndUpdate
0x015E FontFlag
0x015F IntFlag
0x0160 VBLQueue
0x016A Ticks
0x016E MBTicks
0x0172 MBState
0x0173 Tocks
0x0174 KeyMap
0x017C KeyPadMap
0x0180 unknown
0x0184 KeyLast
0x0186 KeyTime
0x018A KeyRepTime
0x018E KeyThresh
0x0190 KeyRepThresh
0x0192 Lvl1DT
0x01B2 Lvl2DT
0x01D2 UnitNtryCnt
0x01D4 VIA
0x01D8 SCCRd
0x01DC SCCWr
0x01E0 IWM
0x01E4 GetParam
0x01F8 SysParam
0x01F8 SPValid
0x01F9 SPATalkA
0x01FA SPATalkB
0x01FB SPConfig
0x01FC SPPortA
0x01FE SPPortB
0x0200 SPAlarm
0x0204 SPFont
0x0206 SPKbd
0x0207 SPPrint
0x0208 SPVolCtl
0x0209 SPClikCaret
0x020A SPMisc1
0x020B CDeskPat
0x020C Time
0x0210 BootDrive
0x0212 JShell
0x0214 SFSaveDisk
0x0216 KbdVars
0x0218 KbdLast
0x021A JKybdTask
0x021E KbdType
0x021F AlarmState
0x0220 MemErr
0x0222 JFigTrkSpd
0x0226 JDiskPrime
0x022A JRdAddr
0x022E JRdData
0x0232 JWrData
0x0236 JSeek
0x023A JSetUpPoll
0x023E JRecal
0x0242 JControl
0x0246 JWakeUp
0x024A JReSeek
0x024E JMakeSpdTbl
0x0252 JAdrDisk
0x0256 JSetSpeed
0x025A NiblTbl
0x025E FlEvtMask
0x0260 SdVolume
0x0261 Finder
0x0262 SoundVars
0x0262 SoundPtr
0x0266 SoundBase
0x026A SoundVBL
0x027A SoundDCE
0x027E SoundActive
0x027F SoundLevel
0x0280 CurPitch
0x0282 SoundLast
0x0282 Switcher
0x0286 SwitcherTPtr
0x028A RSDHndl
0x028E ROM85
0x0290 PortAUse
0x0291 PortBUse
0x0292 ScreenVars
0x029A JGNEFilter
0x029E Key1Trans
0x02A2 Key2Trans
0x02A6 SysZone
0x02AA ApplZone
0x02AE ROMBase
0x02B2 RAMBase
0x02B6 BasicGlob
0x02B6 ExpandMem
0x02BA DSAlertTab
0x02BE ExtStsDT
0x02CE SCCASts
0x02CF SCCBSts
0x02D0 SerialVars
0x02D8 ABusVars
0x02E0 FinderName
0x02F0 DoubleTime
0x02F4 CaretTime
0x02F8 ScrDmpEnb
0x02F9 ScrDmpType
0x02FA TagData
0x02FC BufTgFNum
0x0300 BufTgFFlg
0x0302 BufTgFBkNum
0x0304 BufTgDate
0x0308 DrvQHdr
0x0312 PWMBuf2
0x0316 MacPgm
0x031A Lo3Bytes
0x031E MinStack
0x0322 DefltStack
0x0326 MMDefFlags
0x0328 GZRootHnd
0x032C GZRootPtr
0x0330 GZMoveHnd
0x0334 DSDrawProc
0x0338 EjectNotify
0x033C IAZNotify
0x0340 CkdDB
0x0342 NxtDB
0x0344 MaxDB
0x0346 FlushOnly
0x0347 RegRscr
0x0348 FLckUnlck
0x0349 FrcSync
0x034A NewMount
0x034C DrMstrBlk
0x034E FCBSPtr
0x0352 DefVCBPtr
0x0356 VCBQHdr
0x0360 FSBusy
0x0362 FSQHead
0x0366 FSQTail
0x036A RgSvArea
0x0372 WDCBsPtr
0x0376 HFSVars
0x0384 DefVRefnum
0x0392 HFSDSErr
0x0398 CurDirStore
0x03A2 ErCode
0x03A4 Params
0x03D6 FSTemp8
0x03DE FSTemp4
0x03E2 FSQueueHook
0x03E6 ExtFSHook
0x03EA DskSwtchHook
0x03EE RegstVol
0x03F2 ToExtFS
0x03F6 FSFCBLen
0x03F8 DSAlertRect
0x0400 OSTable
0x0800 GrafBegin
0x0800 JHideCursor
0x0804 JShowCursor
0x0808 JShieldCursor
0x080C JScrnAddr
0x0810 JScrnSize
0x0814 JInitCursor
0x0818 JSetCrsr
0x081C JCrsrObscure
0x0820 JUpdateProc
0x0824 ScrnBase
0x0828 MTemp
0x082C RawMouse
0x0830 Mouse
0x0834 CrsrPin
0x083C CrsrRect
0x0844 TheCrsr
0x0888 CrsrAddr
0x088C JAllocCrsr
0x0890 JSetCCrsr
0x0894 JOpcodeProc
0x0898 CrsrBase
0x089C CrsrDevice
0x08A0 SrcDevice
0x08A4 MainDevice
0x08A8 DeviceList
0x08AC CrsrRow
0x08AE unknown
0x08B0 QDColors
0x08CC CrsrVis
0x08CD CrsrBusy
0x08CE CrsrNew
0x08CF CrsrCouple
0x08D0 CrsrState
0x08D2 CrsrObscure
0x08D3 CrsrScale
0x08D4 unknown
0x08D6 MouseMask
0x08DA MouseOffset
0x08DE JournalFlag
0x08E0 JSwapFont
0x08E4 WidthListHand
0x08E8 JournalRef
0x08EA unknown
0x08EC CrsrThresh
0x08EE JCrsrTask
0x08F2 WWExist
0x08F3 QDExist
0x08F4 JFetch
0x08F8 JStash
0x08FC JIODone
0x0900 CurApRefNum
0x0902 LaunchFlag
0x0903 FondState
0x0904 CurrentA5
0x0908 CurStackBase
0x090C LoadFiller
0x0910 CurApName
0x0930 SaveSegHandle
0x0934 CurJTOffset
0x0936 CurPageOption
0x0938 HiliteMode
0x0939 unknown
0x093A LoaderPBlock
0x0944 PrintErr
0x0946 ChooserBits
0x0954 CoreEditVars
0x0960 ScrapSize
0x0964 ScrapHandle
0x0968 ScrapCount
0x096A ScrapState
0x096C ScrapName
0x0970 ScrapTag
0x0980 RomFont0
0x0984 ApFontID
0x0986 SaveFondFlags
0x0987 FMDefaultSize
0x0988 CurFMInput
0x098A CurFMSize
0x098C CurFMFace
0x098D CurFMNeedBits
0x098E CurFMDevice
0x0990 CurFMNumer
0x0994 CurFMDenom
0x0998 FOutError
0x099A FOutFontHandle
0x099E FOutBold
0x099F FOutItalic
0x09A0 FOutULOffset
0x09A1 FOutULShadow
0x09A2 FOutULThick
0x09A3 FOutShadow
0x09A4 FOutExtra
0x09A5 FOutAccent
0x09A6 FOutDescent
0x09A7 FOutWidMax
0x09A8 FOutLeading
0x09A9 FOutUnused
0x09AA FOutNumer
0x09AE FOutDenom
0x09B2 FMDotsPerInch
0x09B6 FMStyleTab
0x09CE ToolScratch
0x09D6 WindowList
0x09DA SaveUpdate
0x09DC PaintWhite
0x09DE WMgrPort
0x09E2 DeskPort
0x09E6 OldStructure
0x09EA OldContent
0x09EE GrayRgn
0x09F2 SaveVisRgn
0x09F6 DragHook
0x09FA TempRect
0x0A02 OneOne
0x0A06 MinusOne
0x0A0A TopMenuItem
0x0A0C AtMenuBotom
0x0A0E IconBitMap
0x0A1C MenuList
0x0A20 MBarEnable
0x0A22 CurDeKind
0x0A24 MenuFlash
0x0A26 TheMenu
0x0A28 SavedHandle
0x0A2C MBarHook
0x0A30 MenuHook
0x0A34 DragPattern
0x0A3C DeskPattern
0x0A44 DragFlag
0x0A46 CurDragAction
0x0A4A FPState
0x0A50 TopMapHndl
0x0A54 SysMapHndl
0x0A58 SysMap
0x0A5A CurMap
0x0A5C ResReadOnly
0x0A5E ResLoad
0x0A60 ResErr
0x0A62 TaskLock
0x0A63 FScaleDisable
0x0A64 CurActivate
0x0A68 CurDeactive
0x0A6C DeskHook
0x0A70 TEDoText
0x0A74 TERecal
0x0A78 ApplScratch
0x0A84 GhostWindow
0x0A88 CloseOrnHook
0x0A8C ResumeProc
0x0A90 SaveProc
0x0A94 SaveSP
0x0A98 ANumber
0x0A9A ACount
0x0A9C DABeeper
0x0AA0 DAStrings
0x0AB0 TEScrpLength
0x0AB2 unknown
0x0AB4 TEScrpHandle
0x0AB8 AppPacks
0x0AD8 SysResName
0x0AE8 SoundGlue
0x0AEC AppParmHandle
0x0AF0 DSErrCode
0x0AF2 ResErrProc
0x0AF6 TEWdBreak
0x0AFA DlgFont
0x0AFC LastTGlobal
0x0B00 TrapAgain
0x0B04 KeyMVars
0x0B06 ROMMapHndl
0x0B0A PWMBuf1
0x0B0E BootMask
0x0B10 WidthPtr
0x0B14 AtalkHk1
0x0B18 AtalkHk2
0x0B1C FourDHack
0x0B20 unknown
0x0B22 HWCfgFlags
0x0B24 TimeSCSIDB
0x0B26 TopMenuItem
0x0B28 AtMenuBottom
0x0B2A WidthTabHandle
0x0B2E SCSIDrvrs
0x0B30 TimeVars
0x0B34 BtDskRfn
0x0B36 BootTmp8
0x0B3E unknown
0x0B3F T1Arbitrate
0x0B4C JDiskSel
0x0B44 JSendCmd
0x0B48 JDCDReset
0x0B4C LastSPExtra
0x0B50 AppleShare
0x0B54 MenuDisable
0x0B58 MBDFHndl
0x0B5C MBSaveLoc
0x0B60 BNMQHd
0x0B64 Twitcher1
0x0B68 unknown
0x0B7C Twitcher2
0x0B80 RMgrHiVars
0x0B9E RomMapInsert
0x0B9F TmpResLoad
0x0BA0 IntlSpec
0x0BA0 SMgrCore
0x0BA4 RMgrPerm
0x0BA5 WordRedraw
0x0BA6 SysFontFam
0x0BA8 SysFontSize
0x0BAA MBarHeight
0x0BAC TESysJust
0x0BAE HiHeapMask
0x0BB2 SegHiEnable
0x0BB3 FDevDisable
0x0BB4 CMVector
0x0BB8 XFSGlobs
0x0BBC ShutDownQHdr
0x0BC0 NewUnused
0x0BC2 LastFOND
0x0BC6 FONDID
0x0BC8 App2Packs
0x0BE8 MAErrProc
0x0BEC MASuperTab
0x0BF0 MimeGlobs
0x0BF4 FractEnable
0x0BF5 UsedWidths
0x0BF6 FScaleHFact
0x0BFA FScaleVFact
0x0BFE unknown
0x0C00 SCSIBase
0x0C04 SCSIDMA
0x0C08 SCSIHsk
0x0C0C SCSIGlobals
0x0C10 RGBBlack
0x0C16 RGBWhite
0x0C1C unknown
0x0C20 RowBits
0x0C22 ColLines
0x0C24 ScreenBytes
0x0C28 IOPMgrVars
0x0C2C NMIFlag
0x0C2D VidType
0x0C2E VidMode
0x0C2F SCSIPoll
0x0C30 SEVarBase
0x0CB0 MMUFlags
0x0CB1 MMUType
0x0CB2 MMU32bit
0x0CB3 WhichBox
0x0CB4 MMUTbl
0x0CB8 MMUTblSize
0x0CBC SInfoPtr
0x0CC0 ASCBase
0x0CC4 SMGlobals
0x0CC8 TheGDevice
0x0CCC CQDGlobals
0x0CD0 AuxWinHead
0x0CD4 AuxCtlHead
0x0CD8 DeskCPat
0x0CDC SetOSDefKey
0x0CE0 LastBinPat
0x0CE8 DeskPatEnable
0x0CEA unknown
0x0CF8 ADBBase
0x0CFC WarmStart
0x0D00 TimeDBRA
0x0D02 TimeSCCDB
0x0D04 SlotQDT
0x0D08 SlotPrTbl
0x0D0C SlotVBLQ
0x0D10 ScrnVBLPtr
0x0D14 SlotTICKS
0x0D18 PowerMgrVars
0x0D1C AGBHandle
0x0D20 TableSeed
0x0D24 SRsrcTblPtr
0x0D28 JVBLTask
0x0D2C WMgrCPort
0x0D30 VertRRate
0x0D32 SynListHandle
0x0D36 LastFore
0x0D3E LastMode
0x0D40 LastDepth
0x0D42 FMExist
0x0D43 SavedHilite
0x0D44 unknown
0x0D50 MenuCInfo
0x0D54 MBProcHndl
0x0D58 MBSaveLoc
0x0D58 MRect
0x0D5C MBFlash
0x0D5C MenuCInfo
0x0D60 ChunkyDepth
0x0D62 CrsrPtr
0x0D64 unknown
0x0D66 PortList
0x0D6A MickeyBytes
0x0D6E QDErr
0x0D70 VIA2DT
0x0D90 SInitFlags
0x0D92 DTQFlags
0x0D92 DTQueue
0x0D94 DtskQHdr
0x0D98 DTskQTail
0x0D9C JDTInstall
0x0DA0 HiliteRGB
0x0DA6 TimeSCSIDB
0x0DA8 DSCtrAdj
0x0DAC IconTLAddr
0x0DB0 VideoInfoOK
0x0DB4 EndSRTPtr
0x0DB8 SDMJmpTblPtr
0x0DBC JSwapMMU
0x0DC0 SdmBusErr
0x0DC4 LastTxGDevice
0x0DC8 PmgrHandle
0x0DCC LayerPalette
0x0E00 ToolTable
0x1E00 SystemHeap
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Structured Computer Organization, 4th edition; by Andrew S. Tanenbaum; Prentice Hall; October 1998; ISBN 0130959901; Paperback; 669 pages; $95.00; used by CS 308-273A (Principles of Assembly Languages) at McGill University School of Computer Science
Computers: An Introduction to Hardware and Software Design; by Larry L. Wear, James R. Pinkert (Contributor), William G. Lane (Contributor); McGraw-Hill Higher Education; February 1991; ISBN 0070686742; Hardcover; 544 pages; $98.60; used by CS 308-273A (Principles of Assembly Languages) at McGill University School of Computer Science
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Copyright © 2001 Milo
Last Updated: April 10, 2001
Created: March 28, 2001
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