Error A2070 - puzzling

[carlottagp]

I am trying to get data from a file into my program, which of course involves
interpreting ASCII codes.

The line in my code which is problematic is:

sub [BX], 30h

I get an A2070 error for this line, referring to
"invalid instruction operands."  (Some earlier lines,
without errors, deal with letters A through F.)

But this line looks like a simple subtraction to me.

Any advice appreciated!

Michael

[sinsi]

Too ambiguous - you can subtract from a byte in memory as well as a word (dword, qword). Use a size override

Code Select Expand

sub BYTE PTR [BX],'0'

[carlottagp]

Thank you for the suggestion, although I don't fully understand it, since
in a previous line I had set BX to the offset of a buffer.  Presumably you
are suggesting that I insert your line ahead of mine.

Meanwhile, experimenting, I have copied [BX] to AL, and then subtracted
30h from AL; no error messages after this change.

Thanks again,

Michael

[Neil]

What sinsi means is replace your line:-

                             sub [BX], 30h

with his line:-           sub BYTE PTR [BX],'0' 

There is no need to move it into al, all you have to do as sinsi says is use a size override i.e.

                             BYTE PTR   or  WORD PTR   etc.

otherwise the assembler doesn't know what you are subtracting from.

[MichaelW]

Michael,

Given this instruction:

sub [BX], 30h

ML has no way of knowing what the size of the destination operand is, and this information is essential to correctly encode the instruction, because the destination operand could be a BYTE, a WORD, or a DWORD. For example, here are the three possible forms:

Code Select Expand


    sub BYTE PTR [bx], 30h
    sub WORD PTR [bx], 30h
    sub DWORD PTR [bx], 30h


And here is the assembled machine code (in hex) for each instruction, followed by a disassembly of the instruction:

Code Select Expand


802F30    sub BYTE PTR [bx], 30h
832F30    sub WORD PTR [bx], 30h
66832F30  sub DWORD PTR [bx], 30h


As you can see each form has a different encoding. If instead of a constant the source operand were a register, ML would assume that the size of the destination operand matched the size of the source operand:

Code Select Expand


    sub [bx], al
    sub [bx], ax
    sub [bx], eax


Code Select Expand


2807      sub BYTE PTR [bx], al
2907      sub WORD PTR [bx], ax
662907    sub DWORD PTR [bx], eax