Intel CPU Modes:

• Real Mode: The real mode places the processor in an emulation of an 8088 CPU, assuming of course that it is a high level processor. In real mode, the CPU only uses the first 20 address lines and is limited to 1MB of address space.
  
  
• Protected Mode: The protected mode of CPU operation was introduced with the 80286 to utilize its 24 address lines. Protected mode allows the use of up to 16MB of RAM. DOS cannot be run in protected mode, but Windows 3.x and up can.
  
  
• Enhanced Mode: The virtual mode allows the processor to pretend that it is a multiple of 8086 or 8088 processors. Each version of the multiple processors is called a virtual processor and has the same restrictions of a real 8088 processor, but each virtual processor can run seperate copies of programs.
  
  

MS-DOS:

• CONFIG.SYS - loads device drivers as TSR programs.
  
  
• HIMEM.SYS - the device driver for memory above 640K, which includes expanded (upper memory area) and extended memory areas.
  
  
• EMM386.EXE - enables expanded memory and the use of upper memory as system memory.
  
  

Windows 3.x:

• Windows 3.x requires a 286+ processor, an existing MS-DOS OS version 5.0+ or PC-DOS OS 3.1+, 2-11MB hard disk space, at least 640KB conventional memory and 1024KB of extended memory for 386-Enhanced mode.
  
  
• Icons are arranged in group files (.GRP) with one group file representing one window.
  
  
• The WIN.INI file controls how the desktop looks and how files are associated.
  
  
• Running Windows 3.x on a 286 computer in standard mode requires the file DOSX.EXE
  
  
• If Windows 3.x hangs during installation, it is probably during the hardware detection phase. To complete the installation, run setup with the /I switch to bypass this phase.
  
  
• Windows 3.x manages memory in three ways, 1) by creating temporary files so that Windows applications can extend their temporary storage requirements outside of memory, 2) by creating application swap files in the temporary directory, or 3) by creating virtual memory in 386 enhanced mode, which writes memory blocks to either a temporary virtual swap file (WIN386.SWP) or a permanent swap space (386SPART.PAR).
  
  

     Windows manages memory in allocated memory segments called heaps. The amount of memory assigned to a heap is limited by DOS memory management, which is why on a PC with 128MB of RAM, you can still get "out of memory" error messages from Windows. This happens when applications fail to release their memory allocations back to the heap when they terminate - a situation called a memory leak. The five heaps in Windows are as follows:

• The system heap includes the memory assigned to applications and system utilities. Virtual memory is included in this heap.
  
  
• The GDI heap is the memory used to display the GUI (Graphical User Interface) and its elements, including icons, fonts, and the mouse pointer. GDI stands for graphic display interface.
  
  
• The user heap holds the information on elements with which the user is interacting. These elements include open and minimized windows, dialog boxes, printer events, and the desktop layout.
  
  
• The menu heap hold all desktop and application menus and any associated data.
  
  
• The text heap holds all text elements of the Windows desktop and user interfaces, including Windows help functions.
  
  

Windows 95:

• Windows 95 requires a minimum of a 386DX+ processor, 4MB of RAM, 55MB hard disk space, and VGA or better display.
  
  
• Windows 95 supports the 16-bit code of the DOS / Windows 3.x environment as well as 32-bit programming.
  
  
• Winodws 95 supports PnP devices. Windows 95 refers to all non-PnP cards as legacy cards.
  
  
• Windows 95 has three core components that share the processing load:
  
  
  • Kernel (KERNEL.DLL) contains the essential operating system functions, including memory, file, and I/O managment, and application support.
    
    
  • User (USER32.DLL / USER.EXE) controls all elements of user interface including the mouse, keyboard, I/O ports, and the desktop layout.
    
    
  • GDI (GDI32.DLL / GDI.EXE) creates the graphical user interface and controls printing.
    
    
• Windows 95 implements the concept of the virtual machine, which allows it to run applications from other operating systems (mainly DOS) in a nearly native environment.
  
  
• When setting up Windows 95, the setup application records information in a series of log files: DETLOG.TXT records the hardware detected during installation, DETCRASH.LOG records the devices that that failed to respond during the detection phase, SETUPLOG.TXT records the activities of the setup process.
  
  
• If Setup hangs up or if the system crashes during installation, turn off the power and then back on. Setup looks at DETCRASH.LOG and SETUPLOG.TXT to determine what has already installed and what was attempting to install when the system crashed. Setup skips the step that caused it to crash and continues the installation.
  
  
• Windows 95 has a midversion update called "OSR2" (Operations Service Release 2), which is also known as Windows 95b. This update fixed operational problems and added FAT32 support.
  
  
• The registry is kept in two files, SYSTEM.DAT and USER.DAT. Each time the system boots, backup copies (with a .DA0 extention) are made of these files.
  
  
• Windows 95 starts in real mode so that it loads any legacy and 16-bit drivers and then switches to protected mode for 32-bit operations. The 32-bit drivers are called virtual device drivers and usually have a .VXD or .386 extention.
  
  
• If all devices are 32-bit, Windows 95 doesn't need CONFIG.SYS
  
  
• Windows 95 doesn't user SMARTDRV.EXE or SHARE.EXE, which free up conventional and upper memory space. Instead, the features provided by those files are built into Windows 95 directly.
  
  
• FAT is the DOS 16-bit File Allocation Table, VFAT (Virtual FAT) is what allows Windows 95 to use LFN (long file names), FAT32 is also supported after OSR2.
  
  

Major Keys of the Windows 95 Registry
Key Name	Contents
HKEY_CLASSES_ROOT	File associations and OLE data.
HKEY_USERS	User preferences, including desktop setup and network connections.
HKEY_CURRENT_USER	On a PC with only a single user, this key is a duplicate of the HKEY_USERS key. However, on a PC with multiple logins, it contains the preferences of the currently logged-in user.
HKEY_LOCAL_MACHINE	The hardware and software installed on the system.
HKEY_CURRENT_CONFIG	In addition to duplicating the HKEY_LOCAL_MACHINE key when running, this key also contains any configuration changes made in the current session and information on the printers and fonts installed.
HKEY_DYN_DATA	Records system performance information and keeps information on PnP devices.