REG command / CALL INTERRUPT

[Heinz Salomon]

I have recently converted a QuickBasic program to PB/DOS 3.50.
This program reads a disk's serial number and media type using interrupt &h21.

Obviously, something went wrong, as the results contain nothing but chr$(0).
But when I replaced the REG and CALL INTERRUPT commands by their ASM equivalents the program worked fine.

Can anybody tell me what went wrong?
Thanks a lot for your help!

Heinz Salomon

' Gets a disk's media descriptor and also its serial number.
' Based on an idea by Ethan Winer (SERIAL 1.0).
'

$ERROR ALL ON

%UsePureAsm = 1
%UseCallInterrupt = 0
%CompileVar = %UseCallInterrupt

%AX = 1
%BX = 2
%CX = 3
%DX = 4

TYPE SNType
  Zero  AS INTEGER
  SerLo AS INTEGER
  SerHi AS INTEGER
  Label AS STRING * 11
  Media AS STRING * 8
END TYPE
DIM SN AS SNType

CLS
PRINT
PRINT "Get a disk's media descriptor and serial number"
PRINT "(based on Ethan Winer's SERIAL 1.0)"
PRINT
INPUT "Enter drive letter: ", DriveLetter$

Drive% = ASCII(UCASE$(DriveLetter$)) - 64
If Drive% < 1 THEN END

SNoffset?? = VARPTR(SN)

$IF %CompileVar
  ! MOV AX, &H6900
  ! MOV BX, Drive%
  ! MOV DX, SNoffset??
  ! INT &H21
$ELSE
  REG %AX, &H6900
  REG %BX, Drive%
  REG %DX, SNoffset??
  CALL INTERRUPT &H21
$ENDIF

PRINT
PRINT "Media descriptor: "; RTRIM$(SN.Media, CHR$(0))
PRINT "   Serial number: "; padHex$(SN.SerHi%); "-"; padHex$(SN.SerLo%)

END


FUNCTION padHex$(rInt%)
  padHex$ = RIGHT$("000" + HEX$(rInt%), 4)
END FUNCTION

[This message has been edited by Heinz Salomon (edited May 25, 2005).]

[Michael Mattias]

>DS X = buffer to contain extended BIOS Parameter Block (~BPB~, AL=0)

You didn't set DS to equal the stack segment, which is where it is when LOCAL.

you can use this:

dsdx&     = VARPTR32(SN)
REG %DS, dsDx \ &h1000
REG %DX, dsDx MOD &h1000

I think you also need to save and restore DS if you put this into a function

FWIW, that call is clearly marked "undocumented DOS 4+"

[Heinz Salomon]

Michael,

my program works fine if I change the critical code portion according to your suggestion:

   REG %AX, &H6900
   REG %BX, Drive%
     dsdx& = VARPTR32(SN)
   REG %DS, dsDx& \ &h10000
   REG %DX, dsDx& MOD &h10000
   CALL INTERRUPT &H21

Conclusion:
I erroneously assumed REG(%DS) contains the data segment by default.
Instead, REG(%DS) must always be assigned explicitly before using CALL INTERRUPT, otherwise any junk it contains at that time gets filled into the 'real' DS register.

Thank you!

But why do you think the variable SN is stored on the stack?
After all, it is declared in the main program and not within a sub or function.

Heinz



[This message has been edited by Heinz Salomon (edited May 26, 2005).]

[Michael Mattias]

>But why do you think the variable SN is stored on the stack?

I don't know that it is.. I guess I am now thinking purely in "windows mode" where I use essentially nothing but LOCAL vars which are stored on stack.

All I know is you can always get segffset of any var using high and low 16 bits of VARPTR32 regardless of where that var is stored. I also never assume anything like "where is DS".. if doc says DSX needs to be set, I set DS and DX.