A2070: invalid instruction operands - but why?

[cobold]

Hello,

seems that I still don't understand how the assembler figures out operand sizes. I thought that
f.ex.

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mov al,00f34h ; ----> invalid both operands do not have same size (al = 8bit / 0f34h = 16 bit right?


on the other hand:

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cmp al,0 ; immed value 0 is unspecified, but the assembler "knows" that it's a byte bec dest is register al, which is byte, correct?


So I assumed:
If you have an opcode with dest = regXXX and src = immediate value, the assembler "assumes" src to be of size XXX. It seems my assumption was wrong because:

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.if [esi] == 1 ;--------> error A2070: invalid instruction operands
; but
.if dword ptr[esi] == 1 ;works


.if //CONTENT of address that esi is pointing to == 1 // now, what should 1 be, a byte, a word, a dword? How should the assembler know?
Well, because you told him before:
lea esi,Something  and Something is defined as DWORD in .data
You see esi contains the ADDRESS of "Something", and the assembler knows
- the address of the symbol (Something) plus
- knows that Something is DWORD somenum dup(whatyouwant)

So am I stupid, or is it the assembler - because it should or at least could "know" what to "assume"?

[jj2007]

Well, because you told him before:
lea esi,Something  and Something is defined as DWORD in .data

You may have defined the address as Xword, but the assembler sees only [esi], and the assembler has no idea where esi will stand at runtime. So that could be a string/byte, a dword etc... that's why in this case you have to tell the stupid machine which size you mean.

[hutch--]

cobold,

Its a quirk leftover of MASM's shorthand notation. If you fully specified the line you would see the problem.

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mov al,00f34h
becomes
mov al, WORD PTR 00f34h

cmp al,0
becomes
cmp al, BYTE PTR 0


It just happens to be that the assembler can identify the operand size from one side if the instruction is not ambiguous and this only occurs when a memory operand is used where you don't know its size.

[raymond]

mov al, WORD PTR 00f34h

And therefore, the assembler would also return the message "error A2070: invalid instruction operands", because the assembler now knows the size of both sides and realizes they are different.

[cobold]

Thanks to all,

cobold