XMS Memory Manager

Unit: XMS

Extended memory (XMS) access via HIMEM.SYS (1992, KIV without Co).

Types

type
  EMMstruct = record
    Size: LongInt;    { Transfer size in bytes }
    SOH: Word;        { Source handle (0=conventional memory) }
    SOO: LongInt;     { Source offset }
    DSH: Word;        { Destination handle (0=conventional memory) }
    DSO: LongInt;     { Destination offset }
  end;

Globals

var
  XMSerror: Byte;   { Last error code }

Functions

Detection

function XMSinstalled: Boolean;

Memory Allocation

procedure GetXMSmem(var Total, Block: Word);
function AllocXMS(KB: Word): Word;               { Returns handle }
function ReallocXMS(Handle: Word; KB: Word): Boolean;
procedure FreeXMS(Handle: Word);

Transfer

procedure MoveXMS(var EMM: EMMstruct);
procedure Mem2Xms(var Buf; Count: Word; Handle: Word; Offset: LongInt);
procedure Xms2Mem(Handle: Word; Offset: LongInt; var Buf; Count: Word);

Locking

function LockBlock(Handle: Word; var Address: LongInt): Boolean;
function UnlockBlock(Handle: Word): Boolean;

Info

procedure GetXMShandleInfo(Handle: Word; var LockCount, FreeHandles, Size: Word);
function XMSerrorMSG(Error: Byte): string;

HMA

function RequestHMA(Mem: Word): Boolean;
procedure ReleaseHMA;

Example (Sound Bank)

uses XMS, SBDSP;

var
  Handle: Word;
  Buffer: array[0..16383] of Byte;
begin
  if not XMSinstalled then
  begin
    WriteLn('HIMEM.SYS not loaded!');
    Exit;
  end;

  { Allocate 64KB XMS }
  Handle := AllocXMS(64);
  if Handle = 0 then
  begin
    WriteLn('XMS allocation failed: ', XMSerrorMSG(XMSerror));
    Exit;
  end;

  { Upload sound data }
  Mem2Xms(Buffer, SizeOf(Buffer), Handle, 0);

  { Download to DMA buffer }
  Xms2Mem(Handle, 0, Buffer, SizeOf(Buffer));

  { Free XMS memory }
  FreeXMS(Handle);
end;

Critical: Handle=0 Convention

When Handle=0 in EMMstruct, offset is pointer (Seg:Ofs), NOT linear address:

var
  EMM: EMMstruct;
  Buf: array[0..1023] of Byte;
begin
  EMM.Size := 1024;
  EMM.SOH := 0;                          { Source = conventional memory }
  EMM.SOO := LongInt(@Buf);              { Pointer (Seg:Ofs) }
  EMM.DSH := XMSHandle;                  { Dest = XMS memory }
  EMM.DSO := 0;                          { Linear offset }
  MoveXMS(EMM);
end;

Error Codes

Code	Message
0x80	Not implemented
0x81	VDISK detected
0x82	A20 error
0xA0	All memory allocated
0xA1	All handles allocated
0xA2	Invalid handle
0xA3	Source handle invalid
0xA4	Source offset invalid
0xA5	Destination handle invalid
0xA6	Destination offset invalid
0xA7	Length invalid
0xAB	Block locked
0xAC	Block locked

Requirements

• HIMEM.SYS in CONFIG.SYS:

  DEVICE=C:\DOS\HIMEM.SYS
  

Notes

• Used by SNDBANK for large sound buffers
• Max 286/386 extended memory (above 1MB)
• Handles DMA 64KB boundary crossing automatically in SNDBANK
• GetXMSmem returns total and largest block in KB