用MemoryStream读取，然后按照约定的数据格式，读取指定的长度，如果数据很大，就分批发送

给个网上例子
function ReadFile(FileName:String):boolean;
var F:File;
Src:Array[0..31] of Char;
i,resu:Integer;
begin
result:=True;
if not FileExists(FileName) then
begin
result:=false;
exit;
end;
AssignFile(F,FileName);
Reset(F,1);
while not Eof(F) do
begin
BlockRead(F,Src,sizeof(Src),resu);
for i:=0 to 20 do
Form1.Memo1.Lines.add(Src[Resu]);
end;
CloseFile(F);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
if not(OpenDialog1.Execute) then exit;
ReadFile(OpenDialog1.FileName);
end;

另一个实例，自己分析一下吧。

Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, ComCtrls,StrUtils;

type
TForm1 = class(TForm)
Edit1: TEdit;
Button1: TButton;
Label1: TLabel;
ProgressBar1: TProgressBar;
Memo1: TMemo;
OpenDialog1: TOpenDialog;
Memo2: TMemo;
Button2: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
j:integer;
Form1: TForm1;
function ReadFile0(FileName:String):boolean;
function ReadFile1(FileName:String):boolean;
function ReadFile2(FileName:String):boolean;
function StrToHex(Const str: Ansistring): Ansistring;
function HexToStr(const Str: AnsiString): AnsiString;

implementation

{$R *.dfm}

//读取文件函数

function ReadFile0(FileName:String):boolean;
var
F:File;
Src:Array[0..1024-1] of Char;
i,j,numread:Integer;
begin
result:=True;
if not FileExists(FileName) then
begin
result:=false;
exit;
end;
AssignFile(F,FileName);
Reset(F,1);
if filesize(F) < sizeof(Src) then
begin //Test
CloseFile(f);
showmessage('文件大小错误');
exit;
end
else begin
numread:= sizeof(Src)-1;
repeat
BlockRead(F,Src,sizeof(Src),numread);
form1.Memo1.lines.add(Src[numread]);
Application.ProcessMessages;
until (NumRead = 0) ;
CloseFile(F);
Application.ProcessMessages;
end;
end;

//读取文件函数
function ReadFile1(FileName:String):boolean;
var
sFileName: string;
f: TfileStream;
ED_Data: array[0..2048] of char;
lRet: integer;
lOffset: Longint;
lOrigin: Word;
i,count:integer;
begin
sFileName:=form1.opendialog1.FileName;
f := TfileStream.Create(sFileName,fmOpenRead);
try
f.Seek(lOffset,lOrigin); //文件定位，从lOrigin开始移动lOffset
lRet := f.Read(f,count); //lRet为实际读到的字节数，ED_Data为读到的内容
form1.Memo2.Lines.LoadFromStream(f);
j:=f.size div 1024; //计算文件一共有多少个包
Application.ProcessMessages;
for i:=0 to j do
begin
form1.Label1.Caption:='正在读取第 '+inttostr(i)+' 包 ';
form1.Memo1.Text:=form1.Memo1.Text+ midstr(strtohex(form1.Memo2.text),2048*i,2048)+' ';
form1.ProgressBar1.Max :=j;
form1.ProgressBar1.Min:=0;
form1.ProgressBar1.Position := i;
Application.ProcessMessages;
end;
showmessage('成功读取'+inttostr(i)+'个包');
Application.ProcessMessages;
finally
FreeAndNil(f);
Application.ProcessMessages;
end;
end;

//delphi帮助文件
function ReadFile2(FileName:String):boolean;
var
iFileHandle: Integer;
iFileLength: Integer;
iBytesRead: Integer;
Buffer: Pchar;
i: Integer ;
begin
try
iFileHandle := FileOpen(form1.OpenDialog1.FileName, fmOpenRead);
iFileLength := FileSeek(iFileHandle,0,32);
FileSeek(iFileHandle,0,0);
Buffer :=pchar( AllocMem(iFileLength + 1));
iBytesRead := FileRead(iFileHandle, Buffer^, iFileLength);
FileClose(iFileHandle);
form1.Memo1.Lines.Add(buffer[i]);
finally
FreeMem(Buffer);
end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
memo1.Clear;
memo2.Clear;
if (OpenDialog1.Execute) and (Opendialog1.FileName<>'' )then
ReadFile0(Opendialog1.FileName);
end;

//20100403新增函数：
function StrToHex(Const str: Ansistring): Ansistring;
asm
push ebx
push esi
push edi
test eax,eax
jz @@Exit
mov esi,edx //保存edx值，用来产生新字符串的地址
mov edi,eax //保存原字符串
mov edx,[eax-4] //获得字符串长度
test edx,edx //检查长度
je @@Exit {Length(S) = 0}
mov ecx,edx //保存长度
Push ecx
shl edx,1
mov eax,esi
{$IFDEF VER210}
movzx ecx, word ptr [edi-12] {需要设置CodePage}
{$ENDIF}
call System.@LStrSetLength //设置新串长度
mov eax,esi //新字符串地址
Call UniqueString //产生一个唯一的新字符串，串位置在eax中
Pop ecx
@@SetHex:
xor edx,edx //清空edx
mov dl, [edi] //Str字符串字符
mov ebx,edx //保存当前的字符
shr edx,4 //右移4字节，得到高8位
mov dl,byte ptr[edx+@@HexChar] //转换成字符
mov [eax],dl //将字符串输入到新建串中存放
and ebx,$0F //获得低8位
mov dl,byte ptr[ebx+@@HexChar] //转换成字符
inc eax //移动一个字节,存放低位
mov [eax],dl
inc edi
inc eax
loop @@SetHex
@@Exit:
pop edi
pop esi
pop ebx
ret
@@HexChar: db '0123456789ABCDEF'
end;

function HexToStr(const Str: AnsiString): AnsiString;
asm
push ebx
push edi
push esi
test eax,eax //为空串
jz @@Exit
mov edi,eax
mov esi,edx
mov edx,[eax-4]
test edx,edx
je @@Exit
mov ecx,edx
push ecx
shr edx,1
mov eax,esi //开始构造字符串
{$IFDEF VER210}
movzx ecx, word ptr [edi-12] {需要设置CodePage}
{$ENDIF}
call System.@LStrSetLength //设置新串长度
mov eax,esi //新字符串地址
Call UniqueString //产生一个唯一的新字符串，串位置在eax中
Pop ecx
xor ebx,ebx
xor esi,esi
@@CharFromHex:
xor edx,edx
mov dl, [edi] //Str字符串字符
cmp dl, '0' //查看是否在0到f之间的字符
JB @@Exit //小于0，退出
cmp dl,'9' //小于=9
ja @@DoChar//CompOkNum
sub dl,'0'
jmp @@DoConvert
@@DoChar:
//先转成大写字符
and dl,$DF
cmp dl,'F'
ja @@Exit //大于F退出
add dl,10
sub dl,'A'
@@DoConvert: //转化
inc ebx
cmp ebx,2
je @@Num1
xor esi,esi
shl edx,4
mov esi,edx
jmp @@Num2
@@Num1:
add esi,edx
mov edx,esi
mov [eax],dl
xor ebx,ebx
inc eax
@@Num2:
dec ecx
inc edi
test ecx,ecx
jnz @@CharFromHex
@@Exit:
pop esi
pop edi
pop ebx
end;

//ByteToHex:

function ByteToHex(Src: Byte): String;
begin
SetLength(Result, 2);
asm
MOV EDI, [Result]
MOV EDI, [EDI]
MOV AL, Src
MOV AH, AL // Save to AH
SHR AL, 4 // Output High 4 Bits
ADD AL, '0'
CMP AL, '9'
JBE @@OutCharLo
ADD AL, 'A'-'9'-1
@@OutCharLo:
AND AH, $f
ADD AH, '0'
CMP AH, '9'
JBE @@OutChar
ADD AH, 'A'-'9'-1
@@OutChar:
STOSW
end;
end;

end.