Appending character to a 'db' variable in masm assembly - windows

Question

I have just started learning assembly, and I am stuck.
I have a character in a WPARAM variable ( can also be DWORD ), and I have to append it to a db string. I have no idea as to how to do it.
Here is my code:
.386
.model flat, stdcall
option casemap: none
WinMain proto :DWORD,:DWORD,:DWORD,:DWORD
include C:\Program Files\masm32\include\windows.inc
include C:\Program Files\masm32\include\kernel32.inc
include C:\Program Files\masm32\include\user32.inc
include C:\Program Files\masm32\include\gdi32.inc
includelib "C:\Program Files\masm32\lib\kernel32.lib"
includelib "C:\Program Files\masm32\lib\user32.lib"
includelib "C:\Program Files\masm32\lib\gdi32.lib"
.data
cn db "Parth",0
an db "Priydarshi Singh",0
char WPARAM 21h
text db "A",0
ps DWORD ?
hin HINSTANCE ?
cmd LPSTR ?
.code
start:
invoke GetModuleHandle, 0
mov hin, eax
invoke GetCommandLine
mov cmd, eax
invoke WinMain, hin, 0, cmd, SW_SHOWDEFAULT
invoke ExitProcess, 0
WinMain proc inst:HINSTANCE, pinst:HINSTANCE, cml:LPSTR, show:DWORD
LOCAL wc:WNDCLASSEX
LOCAL msg:MSG
LOCAL hwnd:HWND
mov wc.cbSize, sizeof WNDCLASSEX
mov wc.style, CS_VREDRAW or CS_HREDRAW
mov wc.lpfnWndProc, offset WndProc
mov wc.cbClsExtra, 0
mov wc.cbWndExtra, 0
push hin
pop wc.hInstance
mov wc.hbrBackground,COLOR_WINDOW+1
mov wc.lpszMenuName,NULL
mov wc.lpszClassName,OFFSET cn
invoke LoadIcon,NULL,IDI_APPLICATION
mov wc.hIcon,eax
mov wc.hIconSm,eax
invoke LoadCursor,NULL,IDC_ARROW
mov wc.hCursor,eax
invoke RegisterClassEx, addr wc
invoke CreateWindowEx, NULL, addr cn, addr an, WS_OVERLAPPEDWINDOW, 0, 0, 1366, 750, NULL, NULL, inst, NULL
mov hwnd, eax
invoke ShowWindow, hwnd, show
invoke UpdateWindow, hwnd
.WHILE TRUE
invoke GetMessage, addr msg, NULL, 0, 0
.BREAK .IF(!eax)
invoke TranslateMessage, addr msg
invoke DispatchMessage, addr msg
.ENDW
mov eax, msg.wParam
ret
WinMain endp
WndProc proc hwnd:HWND, umsg:UINT, wp:WPARAM, lp:LPARAM
LOCAL hdc:HDC
LOCAL rect:RECT
.IF umsg==WM_DESTROY
invoke PostQuitMessage, 0
.ELSEIF umsg==WM_CHAR
push wp
pop eax
mov char, eax
; I need some code here to append 'char' to 'text'
invoke InvalidateRect, hwnd, NULL, TRUE
.ELSEIF umsg==WM_PAINT
invoke BeginPaint, hwnd, addr ps
mov hdc, eax
invoke TextOut, hdc, 0, 100, addr text, sizeof text
invoke EndPaint, hwnd, addr ps
.ELSE
invoke DefWindowProc, hwnd, umsg, wp, lp
ret
.ENDIF
xor eax, eax
ret
WndProc endp
end start

Answer 1

You can only append to a string if there's space available for additional characters.
You've declared text as text db "A",0, which reserves 2 bytes at text ('A' and 0), so there's no room for additional characters. If you know the maximum length that the string ever will be you can still allocate it statically; for example text db 1024 dup(0) would give you 1024 bytes of space where all bytes have the initial value 0. If you have another variable that keeps track of the current number of characters in the string you can use that to append to the string:
mov edi,text_length
mov [text + edi],al
inc dword ptr text_length
If your string can shrink as well as grow you'll have to make sure to insert a NUL terminator at right place when you "remove" characters from the string.
If you don't know the maximum length of the string in advance, or if the maximum is very large, you can allocate memory dynamically with one of the memory allocation functions provided by Windows, e.g. HeapAlloc. If the string is about to grow beyond the currently allocated size you increase the size of the allocated block with HeapReAlloc (e.g. to twice the size of the previous size).