Contributor: SWAG SUPPORT TEAM
{ Ok here it is.. I have disasembled the following TP Program to
show you the inner workings of TP (well at least 6.0). The
Folloing Program was Compiled in the IDE With RANGE, I/O, STACK
checking turned off. Look at the code close and see if you can
find a nasty little bug in it beFore I show you the Asm that TP
Created on disk.
}
Program TstFiles;
Type MyRec = Record
LInt : LongInt;
Hi : Word;
Lo : Word;
B1 : Byte;
B2 : Byte;
B3 : Byte;
B4 : Byte;
end; {Record Size 12 Bytes}
Const MaxRecs = 100;
Var MyTypedFile : File of MyRec;
MyUnTypedFile : File;
Rec : MyRec;
RecCnt : Word;
Procedure FillRec (RecSeed : LongInt);
begin
Rec.Lint := RecSeed;
Rec.Hi := Hi (Rec.Lint);
Rec.Lo := Lo (Rec.Lint);
Rec.B1 := Lo (Rec.Lo);
Rec.B2 := Hi (Rec.Lo);
Rec.B3 := Lo (Rec.Hi);
Rec.B4 := Hi (Rec.Hi);
end;
begin
Assign (MyTypedFile, 'Type.Dat');
Assign (MyUnTypedFile, 'UnTyped.Dat');
ReWrite (MyTypedFile);
ReWrite (MyUnTypedFile);
For RecCnt := 1 to MaxRecs do
begin
FillRec (RecCnt);
Write (MyTypedFile , Rec);
{ Write (MyUnTypedFile, Rec);} {Illegal can't do this}
FillRec (RecCnt + $FFFF);
{ BlockWrite (MyTypedFile, Rec, 1);} {Illegal Can't do this eather}
BlockWrite (MyUnTypedFile, Rec, Sizeof (MyRec));
end;
end.
The Asm Break down is in the next two messages...
TSTFileS.38: begin
cs:0051 9A0000262D call 2D26:0000 <-------TP Start Up Code
cs:0056 55 push bp
cs:0057 89E5 mov bp,sp
TSTFileS.39: Assign (MyTypedFile, 'Type.Dat');
cs:0059 BF4400 mov di,0044
cs:005C 1E push ds
cs:005D 57 push di
cs:005E BF3C00 mov di,003C
cs:0061 0E push cs
cs:0062 57 push di
cs:0063 9AC004262D call 2D26:04C0 <-------TP's Routine to set
up File Records.
TSTFileS.40: Assign (MyUnTypedFile, 'UnTyped.Dat');
cs:0068 BFC400 mov di,00C4
cs:006B 1E push ds
cs:006C 57 push di
cs:006D BF4500 mov di,0045
cs:0070 0E push cs
cs:0071 57 push di
cs:0072 9AC004262D call 2D26:04C0 <-------TP's Routine to set
up File Records.
TSTFileS.41: ReWrite (MyTypedFile);
cs:0077 BF4400 mov di,0044
cs:007A 1E push ds
cs:007B 57 push di
cs:007C B80C00 mov ax,000C
cs:007F 50 push ax
cs:0080 9AF704262D call 2D26:04F7 <-------TP's Routine to
Create File.
TSTFileS.42: ReWrite (MyUnTypedFile);
cs:0085 BFC400 mov di,00C4
cs:0088 1E push ds
cs:0089 57 push di
cs:008A B88000 mov ax,0080
cs:008D 50 push ax
cs:008E 9AF704262D call 2D26:04F7 <-------TP's Routine to
Create File.
TSTFileS.44: For RecCnt := 1 to MaxRecs do
cs:0093 C70650010100 mov Word ptr [TSTFileS.RECCNT],00
*** Clear the loop counter For first loop
cs:0099 EB04 jmp TSTFileS.46 (009F)
*** Jump to the start of the Loop
cs:009B FF065001 inc Word ptr [TSTFileS.RECCNT]
*** The Loop returns to here to inC the loop counter
TSTFileS.46: FillRec (RecCnt);
cs:009F A15001 mov ax,[TSTFileS.RECCNT]
*** Move our RecCnt Var into AX register
cs:00A2 31D2 xor dx,dx
*** Clear the DX Register
cs:00A4 52 push dx
cs:00A5 50 push ax
*** Push the DX and AX Registers on the stack. Remember our
FillRec Routine expects a LongInt to be passed and RecCnt
is only a Word. So it Pushes the DX as the 0 Upper Word
of the LongInt.
cs:00A6 0E push cs
*** Push the code segment For some reasion.
cs:00A7 E856FF call TSTFileS.FILLREC
*** Call our FillRec Routine
TSTFileS.48: Write (MyTypedFile , Rec);
cs:00AA BF4400 mov di,0044
cs:00AD 1E push ds
cs:00AE 57 push di
*** These instructions push the address of MyTypedFile Record
on the stack. The first paramiter
cs:00AF BF4401 mov di,0144
cs:00B2 1E push ds
cs:00B3 57 push di
*** These instructions push the address of Rec Record
on the stack. The second paramiter
cs:00B4 9AAA05262D call 2D26:05AA
*** Call the System Function to Write a Typed File. (In next msg)
cs:00B9 83C404 add sp,0004
*** Remove our passed parameters from the stack
TSTFileS.51: FillRec (RecCnt + $FFFF);
cs:00BC A15001 mov ax,[TSTFileS.RECCNT]
cs:00BF 05FFFF add ax,FFFF
cs:00C2 31D2 xor dx,dx
cs:00C4 52 push dx
cs:00C5 50 push ax
cs:00C6 0E push cs
cs:00C7 E836FF call TSTFileS.FILLREC
*** Now heres a NASTY littel bug With the code!!! Look at the
above routine. We wanted to pass a LongInt $FFFF + rec cnt
But we wound up adding the $FFFF to a Word then passing a
LongInt. if you Compile the sample pas File you'll be able
to see this bug in action.. Good reasion to use a Debugger.
TSTFileS.55: BlockWrite (MyUnTypedFile, Rec, Sizeof (MyRec))
cs:00CA BFC400 mov di,00C4
cs:00CD 1E push ds
cs:00CE 57 push di
*** These instructions push the address of MyUnTypeFile Record
on the stack. The First paramiter
cs:00CF BF4401 mov di,0144
cs:00D2 1E push ds
cs:00D3 57 push di
cs:0594 26817D02B3D7 cmp es:Word ptr [di+02],D7B3
*** Armed With the address of the File Record in ES:DI
Check the File mode For a In/Out operation. See Dos
Unit Constant definitions.
cs:059A 7406 je 05A2
*** if that Compare was equal then jump to return
cs:059C C7063C006700 mov Word ptr [SYSTEM.inOUTRES],0069
*** if we didn't jump then put File not oopen For output in
Ioresult.
cs:05A2 C3 ret
*** Go back to where we were called
cs:05A3 B43F mov ah,3F
cs:05A5 BA6400 mov dx,0064
cs:05A8 EB05 jmp 05AF
*** The Write instruction entered the system Unit here
cs:05AA B440 mov ah,40
*** Load Dos Function in AH
cs:05AC BA6500 mov dx,0065
*** Default error code 101 disk Write error load in DX
cs:05AF 55 push bp
*** Save the BP register
cs:05B0 8BEC mov bp,sp
*** Load the BP Register With the stack Pointer
cs:05B2 C47E0A les di,[bp+0A]
*** Load Address of MyTypeFile Rec in ES:SI
cs:05B5 E8DCFF call 0594
*** Call check For File mode. See top of message
cs:05B8 751B jne 05D5
*** if error jump out of this
cs:05BA 1E push ds
cs:05BB 52 push dx
*** Save These Registers as we'er going to use them
cs:05BC C55606 lds dx,[bp+06]
*** Load the address of our Rec in DS:DX Registers
cs:05BF 268B4D04 mov cx,es:[di+04]
*** Look up Record structure For a File Rec and you'll see
that RecSize is Byte # 4. Move that value to CX
cs:05C3 268B1D mov bx,es:[di]
*** First Byte of a File Rec is the Handel. Move into BX
cs:05C6 CD21 int 21
*** Make the Dos CALL to Write. AH = 40
BX = File Handel
CX = # of Bytes to Write.
DS:DX = Address of Buffer
Returns Error In AX if Carry flag set or
if good CF = 0 number of Bytes written in AX
cs:05C8 5A pop dx
cs:05C9 1F pop ds
*** Restore the Registers
cs:05CA 7206 jb 05D2
*** Jump if there was an error (if Carry flag Set)
cs:05CC 3BC1 cmp ax,cx
*** Comp Bytes requested to what was written
cs:05CE 7405 je 05D5
*** if equal then jump out we'r just about done
cs:05D0 8BC2 mov ax,dx
*** Move default errorcode 101 to AX
cs:05D2 A33C00 mov [SYSTEM.inOUTRES],ax <--Set Ioresult
*** Store 101 to Ioresult
cs:05D5 5D pop bp
*** Restore BP register
cs:05D6 CA0400 retf 0004
*** We'r out of here
cs:05D9 B33F mov bl,3F
cs:05DB B96400 mov cx,0064
cs:05DE EB05 jmp 05E5
*** The BlockWrite instruction entered the system Unit here
cs:05E0 B340 mov bl,40
*** Move Dos Function in BL
cs:05E2 B96500 mov cx,0065
*** Default error 101 Write error in CX
cs:05E5 55 push bp
*** Save BP Register
cs:05E6 8BEC mov bp,sp
*** Move Stack Pointer to BP
cs:05E8 C47E10 les di,[bp+10]
*** Load Address of MyUnTypedFile Record in ES:DI
cs:05EB E8A6FF call 0594
*** Check For Open in Write Mode See top of message
cs:05EE 753F jne 062F
*** Jump if not in Write mode
cs:05F0 8B460A mov ax,[bp+0A] ]
*** Move File Record cnt in to ax
cs:05F3 0BC0 or ax,ax
*** Check For 0 Record request
cs:05F5 741C je 0613
*** Jump if 0 rec requested
cs:05F7 1E push ds
cs:05F8 51 push cx
*** Save them we'er going to use them
cs:05F9 26F76504 mul es:Word ptr [di+04]
*** Multiply Record size With RecCnt in AX result in DX & AX
cs:05FD 8BC8 mov cx,ax