[M3devel] what does it mean to read a UINT8?

[Rodney M. Bates]

As in my other response, I think you are left entirely too much
out in the wind by the language/compiler as to what your expectations
should be. In this particular example, it would help to know the type
UINT8, and those of 'used', 'info', 'RegionSize', and maybe 'b'.  I
could make some guesses, but that could turn out a snipe hunt.

Do you mean it copies a full INTEGER into b, or just fetches
an INTEGER, then extracts from it?  What is the alignment of
the actual address in the failing case?

On 09/21/2013 10:58 PM, Jay K wrote:
> It appears that this code:
>
>
> b := LOOPHOLE(used + info.RegionSize - 1, UNTRACED REF UINT8)^;
>
>
> generates a read of a full INTEGER, in this case 8 bytes.
>
>
> Now, I know, I could change it to:
>
>
> b := LOOPHOLE(used + info.RegionSize - BYTESIZE(INTEGER), UNTRACED REF INTEGER)^;
>
>
> What were my expectations wrong in the first place?
>
>
> This code was part of getting stack bounds and it'd read
> the end of the stack to try to ensure it was correct.
>
>
> I removed it.
>
>
> 0:000> g
> (15bc.116c): Access violation - code c0000005 (first chance)
> First chance exceptions are reported before any exception handling.
> This exception may be expected and handled.
> *** WARNING: Unable to verify checksum for cm3.exe
> cm3!ThreadWin32__GetStackBounds+0x1e8:
> 00000001`3fdf1a78 488b48ff        mov     rcx,qword ptr [rax-1] ds:00000000`0028
> ffff=????????????????
> 0:000> r rax
> rax=0000000000290000
> 0:000> r rsp
> rsp=000000000028fb60
> 0:000> dv
>      start_L_275 = 0x00000000`00338de0
>        end_L_276 = 0x00000000`00338de8
>      L_501_L_502 = 0n48
>       used_L_272 = 0x00000000`0028f000
> available_L_273 = 0x00000000`00190000 "--- memory read error at address 0x000000
> 00`00190000 ---"
>          b_L_274 = 0x30 '0'
>          a_L_271 = 0n48
>       info_L_270 = struct TA669C7A1
>           _frame = struct ThreadWin32__GetStackBounds_Frame_t
> 0:000> ?? info_L_270
> struct TA669C7A1
>     +0x000 BaseAddress      : 0x00000000`0028f000  "0???"
>     +0x008 AllocationBase   : 0x00000000`00190000  "--- memory read error at addr
> ess 0x00000000`00190000 ---"
>     +0x010 AllocationProtect : 4
>     +0x014 L_7              : [4]  ""
>     +0x018 RegionSize       : 0n4096
>     +0x020 State            : 0x1000
>     +0x024 Protect          : 4
>     +0x028 Type             : 0x20000
>     +0x02c L_8              : [4]  ""
> 0:000> q
> quit:
>
>
>   - Jay