About and eax,0FFFFh

[Force]

I usally see in some dialogbox codes

Code Select Expand


elseif eax==WM_COMMAND
mov eax,wParam
and eax,0FFFFh
.if eax==IDM_FILE_EXIT
invoke SendMessage,hWin,WM_CLOSE,0,0
.elseif eax==IDM_HELP_ABOUT
invoke ShellAbout,hWin,addr AppName,addr AboutMsg,NULL
.endif
or

Code Select Expand


mov eax, uMsg
.if eax==WM_CHAR
mov eax,wParam
and eax,0FFFFh
Can you explain it to  me What it means

[jj2007]

The API documentation often says "the ID is in the loword of wParam" or similar. A dword like wParam or lParam contains two words called hiword and loword. You can load the loword with

Code Select Expand

mov eax, wParam
and eax, 0ffffh
; and eax, 00000000000000001111111111111111b ; same in binary notation - mask out the lower 16 bits
or, more elegant and shorter:

Code Select Expand

movzx eax, word ptr wParam

The hiword can be loaded with

Code Select Expand

mov eax, wParam
shr eax, 16
or, more elegant and shorter:

Code Select Expand

movzx eax, word ptr wParam+2

[dedndave]

sometimes, the loword or hiword are signed values
if so, you can use MOVSX to get a sign-extended dword   :U
mouse movement is one example that comes to mind

[jj2007]

sometimes, the loword or hiword are signed values
if so, you can use MOVSX to get a sign-extended dword   :U
mouse movement is one example that comes to mind

Negative mouse positions? Sure?

[dedndave]

mouse wheel   :P

WM_MOUSEWHEEL
WM_MOUSEHWHEEL

Code Select Expand

wParam
The high-order word indicates the distance the wheel is rotated, expressed in multiples or divisions
of WHEEL_DELTA, which is 120. A positive value indicates that the wheel was rotated forward, away
from the user; a negative value indicates that the wheel was rotated backward, toward the user.

[Force]

if I understand true

value is in loword of wParam that means that value is 16 bits
after transfering it to eax we need to fill other 16 bits with zeros
its important for mouse movement

when i change it "movzx eax word ptr wParam"
it works same again

and

i erased and eax,0FFFFh

it works without problem   also :toothy

[dedndave]

MOVZX Zero eXtends the source into the destination register
so...

Code Select Expand

        movzx     eax,word ptr wParam
takes the low word of wParam and places it in AX
the high word of EAX is set to 0

it can also be used to zero extend a byte

Code Select Expand

        movzx     eax,byte ptr [esi]
takes the byte pointed to by ESI and places it in AL
the upper 3 bytes of EAX are set to 0

MOVSX is similar, except that it Sign eXtends

Code Select Expand

        movsx     eax,word ptr wParam
if the low word of wParam is positive (i.e., bit 15 is 0), it will clear all the upper bits in EAX to 0
if the low word of wParam is negative (i.e., bit 15 is 1), it will set all the upper bits in EAX to 1

[Force]

Thanks Dave

You told it clearly I understand now