64 bit variables

[zedd151]

Why does this not work: ?

Code Select Expand

mov var, 7FFFFFFFFFFFFFFDh

error A2084:constant value too large

But this does work: ???

Code Select Expand

mov rax, 7FFFFFFFFFFFFFFDh
mov var, rax
In both cases 'var' is a qword variable. Using ml64 to assemble.

     

[sinsi]

Pretty sure that mov mem,imm is restricted to a 32-bit value only.

[zedd151]

Pretty sure that mov mem,imm is restricted to a 32-bit value only.

Really? Even for qwords in 64 bit code?? That's rather odd. 
It seems I ought to do some more research.  ... ... grumble, grumble.

Okay, thanks.

[sinsi]

According to the Intel docs, the only mov is MOV r64,imm64, there's no equivalent MOV r/m64,imm64 

restricted to a 32-bit value only

Of course I meant "restricted to a maximum 32-bit value only" 

[zedd151]

Yes, I was just now finding this out... and the reasons why.   
Makes me want to stick to 32 bit.   

[sinsi]

Yes, I was just now finding this out... and the reasons why.   
Makes me want to stick to 32 bit.   

One little thing like that? You wuss

[zedd151]

What good are 64 bit variables if I cannot use them to their full potential?   

Anyway I have converted "dwtoa" from the masm32 library to "qwtoa" for use in 64 bit code. I was testing that function when I came across this issue.

So far "qwtoa" is working as expected, it converts signed qword values to ascii decimal string. Will post it when my testing is finished.

[NoCforMe]

Pretty sure that mov mem,imm is restricted to a 32-bit value only.

Is that an Intel restriction or an assembler one?
According to Felix Cloutier, the largest valid MOV of an immediate value to memory uses a 32-bit value (which can be sign-extended to 64 bits).

[zedd151]

It's a limitation of the ISA. (Intel/AMD)
We are actually running an x86 in 64 bit mode, if I correctly understood what I have read just a little while ago.

x86_64 This is not where I read about it, but explains some of the details.

[sinsi]

Pretty sure that mov mem,imm is restricted to a 32-bit value only.

Is that an Intel restriction or an assembler one?

It's a limitation of the CPU, there's no encoding for it.

[sinsi]

We are actually running an x86 in 64 bit mode

32-bit code runs in compatibility mode under long mode if that makes sense

[zedd151]

Now I am totally confused. There seems to be a lot of conflicting information around, or maybe I simply don't understand it.   

[daydreamer]

Write 64 bit version of m2m macro as solution ?
It short for memory2memory

[NoCforMe]

Nah: just do

Code Select Expand

  MOV  [register], [64-bit value]
  MOV  [memory], [register]


[zedd151]

The only good thing about this, is that Microsoft is off the hook for this oddity.   
64 bit CPU (but only sometimes).