CODEWITHSUNDEEP
assemblydata-structuresx86masmbootloader
vijay tiwari
vijay tiwari

Reputation: 39

MASM: how to resolve error "immediate operand not allowed"

My assembly program has following structure:

BPB_FAT16   STRUCT                     ; Base BPB_FAT16. Size=0x19=25.
BytesPerSector      DW  0x0200 ; Size of HW sector, usualy 512.
SectorsPerCluster   DB  0x01   ; Valid values 1,2,4,8,16,32,64,128.
ReservedSectors     DW  0x0001 ; NrOfSectors preceding FAT.
NumberOfFats        DB  0x02   ;
RootEntries         DW  0x00E0 ; Max number of YWORD entries in the 
                               ;root dir.
SmallSectors        DW  0x0B40 ; See also .LargeSectors.
MediaDescriptor     DB  0xF0   ; 0xF0 floppy disk, 0xF8 hard disk.
SectorsPerFat       DW  0x0009 ;
SectorsPerTrack     DW  0x0012 ;
NumberOfHeads       DW  0x0002 ;
HiddenSectors       DW 0x00000000
LargeSectors        DW 0x00000000
                               ; Extended BPB_FAT16. Size=0x1A=26.
PhysicalDriveNumber DB 0x00   ; 0x00 floppy disk, 0x80 hard disk.
Reserved            DB 0x00   ;
ExtBootSignature    DB 0x29   ; Or 0x28.
VolumeSerialNumber  DW 1212121 ; Randomly generated number.
VolumeLabel         DB "NO NAME    " ; Space-padded to size=11.
FileSystemType      DB "FAT12   "    ; Space-padded to size=8.
BPB_FAT16 ENDS  ;   Total BPB_FAT16. Size=0x33=51.

My program is showing two errors for only one line of code given below:

Bpb   BPB_FAT16{}

and those two errors are:

error1: missing operator in expression
error2: initializer magnitude too large for specified size

What should I do? Please guide me.

Upvotes: 0

Views: 1323

Answers (1)

Michael Petch
Michael Petch

Reputation: 47573

The error message missing operator in expression is likely occurring on every line where you use 0x prefix for hexadecimal values. MASM doesn't support that and it use the h suffix to denote hexadecimal values.

The error initializer magnitude too large for specified size is because the value 1212121 can't fit in the DW (16-bit WORD) you specified for VolumeSerialNumber. VolumeSerialNumber in a BPB needs to be a DD (32-bit DWORD).

Although it didn't generate an error, the fields HiddenSectors and LargeSectors in a BPB are a 32-bit DWORD so they should be using the type DD instead of DW.

Your structure could have been defined as:

BPB_FAT16   STRUCT                   ; Base BPB_FAT16. Size=0x19=25.
BytesPerSector      DW  200h         ; Size of HW sector, usualy 512.
SectorsPerCluster   DB  1h           ; Valid values 1,2,4,8,16,32,64,128.
ReservedSectors     DW  1h           ; NrOfSectors preceding FAT.
NumberOfFats        DB  2h           ;
RootEntries         DW  0E0h         ; Max number of YWORD entries in the
                                     ; root dir.
SmallSectors        DW  0B40h        ; See also .LargeSectors.
MediaDescriptor     DB  0F0h         ; 0xF0 floppy disk, 0xF8 hard disk.
SectorsPerFat       DW  9h           ;
SectorsPerTrack     DW  12h          ;
NumberOfHeads       DW  2h           ;
HiddenSectors       DD  0h
LargeSectors        DD  0h
                                     ; Extended BPB_FAT16. Size=0x1A=26.
PhysicalDriveNumber DB  0h           ; 0x00 floppy disk, 0x80 hard disk.
Reserved            DB  0h           ;
ExtBootSignature    DB  29h          ; Or 0x28.
VolumeSerialNumber  DD 1212121       ; Randomly generated number.
VolumeLabel         DB "NO NAME    " ; Space-padded to size=11.
FileSystemType      DB "FAT12   "    ; Space-padded to size=8.
BPB_FAT16 ENDS  ;   Total BPB_FAT16. Size=0x33=51.

With these changes your structure is now exactly 51 bytes which is what you'd expect for a FAT16 DOS 4.0 EBPB.

Upvotes: 6

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