811 byte windows executable with graphics update.

output.md

Win32.c

// Compile like this:
// cl Win32.c /O1 /c /GS- && crinkler Win32.obj user32.lib kernel32.lib gdi32.lib /SUBSYSTEM:WINDOWS /NODEFAULTLIB /UNSAFEIMPORT /TINYHEADER

#include <windows.h>

#define WN_TITLE  "Unnamed"
#define WN_CLASS  "WindowClassName"
#define WN_WIDTH  800
#define WN_HEIGHT 600

int ScrollX = 0;

static void BlitToWindow(HWND Window)
{
    RECT Rect;
    GetWindowRect(Window, &Rect);

    int Width = Rect.right-Rect.left;
    int Height = Rect.bottom-Rect.top;

    DWORD32 *Buffer = VirtualAlloc(NULL, Width*Height*4, MEM_COMMIT, PAGE_READWRITE);

    for (int i = 0; i < Height; i++)
    {
        for (int j = 0; j < Width; j++)
        {
            Buffer[i*Width+j] = i^j+ScrollX;
        }
    }

    ScrollX++;

    HDC DC = GetDC(Window);
    BITMAPINFOHEADER BmpInfo = { 0 };
    BmpInfo.biSize = sizeof BmpInfo;
    BmpInfo.biPlanes = 1;
    BmpInfo.biBitCount = 32;
    BmpInfo.biWidth = Width;
    BmpInfo.biHeight = -Height;

    SetDIBitsToDevice(DC, 0, 0, Width, Height, 0, 0, 0, Height, Buffer, (BITMAPINFO*)&BmpInfo, DIB_RGB_COLORS);

    ReleaseDC(Window, DC);

    VirtualFree(Buffer, 0, MEM_RELEASE);
}

UINT_PTR FrameTimer = 1;

static LRESULT CALLBACK WndProc(
    HWND   hWnd,
    UINT   Msg,
    WPARAM wParam,
    LPARAM lParam
)
{
    switch (Msg) {
    case WM_TIMER:
        if (wParam == FrameTimer)
        {
            BlitToWindow(hWnd);
        }
        break;
    case WM_CLOSE:
        DestroyWindow(hWnd);
        break;
    case WM_DESTROY: 
        PostQuitMessage(0);
        break;
    }

    return DefWindowProcA(hWnd, Msg, wParam, lParam);
}


int WinMainCRTStartup()
{
    WNDCLASSA WindowClass = { 0 };
    WindowClass.lpszClassName = WN_CLASS;
    WindowClass.lpfnWndProc = WndProc;

    // NOTE(ske): No error handling, but not really necessary.
    RegisterClassA(&WindowClass);

    // NOTE(ske): No error handling, but not really necessary.
    HWND Window = CreateWindowA(
        WindowClass.lpszClassName,    
        WN_TITLE,                 
        WS_OVERLAPPEDWINDOW,          
        CW_USEDEFAULT, CW_USEDEFAULT,
        WN_WIDTH, WN_HEIGHT,                     
        NULL,                        
        NULL,                       
        NULL,                      
        NULL                      
    );

    ShowWindow(Window, TRUE);

    SetTimer(Window, FrameTimer, 16, NULL);

    MSG Msg = { 0 };
    while (GetMessageA(&Msg, NULL, 0, 0)) {
        TranslateMessage(&Msg); 
        DispatchMessage(&Msg); 
    }

    ExitProcess(0);
}