Older Operating Systems Class Notes # 14 Computer Configuration November 20, 2003 The default configuration of DOS, which may be adequate for many users, is intended to work with the greatest number of systems. DOS also allows users to take full advantage of their computers features. We could configure our computer to operate more efficiently. There are two general ways of making our computers efficient: • Add or replace hardware • Use software to attain an optimal configuration for existing hardware resources DOS falls in the second category. With DOS we can manage our memory more efficiently and we can also use special device drivers and utility programs to enhance the performance of our hardware and software. Device Drivers Device drivers are software programs, which is used by DOS to control hardware devices. Most device drivers are loaded at boot time by including the device driver in your CONFIG.SYS. The DEVICE command tells DOS to load a device driver and uses the following syntax: DEVICE = driver /switches The driver is the actual software for the device, and the /switches are any switches that the device driver software needs. For example, the mouse device driver (MOUSE.SYS) is loaded in the CONFIG.SYS file with this command: DEVICE = C:\DRIVERS\MOUSE.SYS The DEVICE = command tells DOS that the program file name MOUSE.SYS stored in the \DRIVERS directory on drive C is a TSR needed to drive the mouse device. You may also load certain device drivers in AUTOEXEC.BAT. For example, the mouse device driver can also be loaded from the AUTOEXEC.BAT file. To load the mouse device driver add the following command in your AUTOEXEC.BAT: C:\DRIVERS\MOUSE.COM Table one lists the device driver files included with DOS 6.2. Table one: DOS 6.2 device drivers Device Driver Description ANSI.SYS Enables control of display by using ANSI control sequences. CHKSTATE.SYS Used by memmaker when optimizing your system. DBLSPACE.BIN Enables disk compression, increases the size of your hard disk. DISPLAY.SYS Provides support for code page switching to the screen. 1 DMDRVR.BIN Provides support for Ontrack disk manager. DRIVER.SYS Sets parameters for physical and logical disk drives. EGA.SYS Saves and restores an EGA screen when using DOSSHELL and the task swapper. EMM386.SYS Uses XMS memory to emulate EMS memory and provide upper memory blocks. HIMEM.SYS Manages extended memory. INTERLNK.EXE Interlnk network client control program and device driver. MONOUMB.386 Device driver used by Windows which allows EMM386 access to the monochrome video region, treating it as UMB area. MOUSE.COM Support for a Microsoft mouse. POWER.EXE Support for computer utilizing APM (Advanced Power Management) hardware. PRINTER.SYS Support for international characters and code page switching on certain printers. RAMDRIVE.SYS Uses a portion of RAM to simulate a hard disk (RAM disk). SETVER.EXE Establishes a version table that lists the version number DOS reports to named programs. SMARTDRV.EXE Uses extended or expanded memory to buffer disk reads. SSTOR.SYS Support for SpeedStor hard disk compression. VFINTD.386 Used by Windows to give MWBACKUP access to tape drives. Memory Management In the early 1980s, when IBM and Microsoft were designing the original PC, they decide to make 640 kbytes of memory addresses available to the user, thinking this was plenty. The 640 kbytes of address was intended to hold the OS, the application software, and the data being processed. The video BIOS and its data was placed in the very first part of upper memory (640k to 768k). All video ROM written for DOS based computers assumes that these programs and data are stored in this area. Also, many DOS and Windows applications interact directly with video ROM and RAM in this address range. The major reason that applications have 640k memory limit is that video ROM begins at 640k. If DOS and Windows 9x allowed applications into these upper memory addresses, all DOS compatible video ROM would need to be rewritten, many existing video boards would be obsolete and many DOS applications that access these video addresses would not work. The memory map is shown in figure one. The first 64k of extended memory is called the high memory area, and exists because a bug in the programming for the older 286 CPU produced this small pocket of unused memory addresses. The OS capitalized on this bug by storing portions of itself in the high memory area, thus freeing some conventional memory where DOS had been stored. This process of storing DOS in high memory area is called loading DOS high. To load DOS high add DOS = HIGH in CONFIG.SYS. 2 Extended Memory 1024k + 64 kbytes System BIOS F0000 - FFFFF Upper Memory 640k to 1024k Video ROM C0000 - CFFFF Video RAM A0000 - BFFFF Conventional Memory 00000 - 9FFFF 0 to 640k Figure 1: DOS Memory map Extended Memory Extended memory is actually managed by the OS as a device that is controlled by a device driver called a memory manager. DOS provides the memory device driver called HIMEM.SYS to access the extended memory region. HIMEM.SYS manages memory according to the rules set out in the Extended Memory Specification (XMS). Three areas of memory above the conventional 640k barrier can be made available for programs to use: • UMA (Upper Memory Area). This consists of 384kbytes of memory above 640kbyte up to 1Mbyte and is divided into many different sized blocks, called UMBs (Upper Memory Blocks). • HMA (High Memory Area). This is a single block of memory essentially consisting of the first 64kbytes of extended memory. HMA slightly overlaps the upper memory area – the last 16 bytes of the upper memory area are the first 16 bytes of the high memory area. • XMS (eXtended Memory Blocks). This portion of memory includes all memory above 1024kbytes. When extended memory is managed by an extended memory manager, you refer to the memory as XMS memory. The syntax for using HIMEM.SYS is: DEVICE = HIMEM.SYS /A20Control:ON|OFF /numhandles = n /EISA /HMAMIN = m /int15 = xxx /machine:xx /shadowram: ON|OFF /cpuclock: ON|OFF /quiet 3 Most of the switches are irrelevant for today’s PCs. The switches are described in table two. Table two: HIMEM.SYS switches Switch Description A20CONTROL HIMEM can detect how the address line A20 is used by the computer NUMHANDLES=n Extended memory manager assigns one or more extended memory block handles to a program. Indicates the maximum number of handles available. The value of n can be assigned 1 to 128. EISA Use this switch to tell HIMEM to automatically allocate all available extended memory if your machine is using EISA (Extended Industry Standard Architecture) bus with more than 16Mbytes of memory. HMAMIN=n Sets the minimum amount of memory that must be requested by an application before the application is permitted to use HMA (High Memory Area). INT15=xxx Some programs address extended memory by the interrupt 15 interface. You add the INT15=xxx where the xxx indicates the amount of extended memory you want HIMEM to assign to the INT15 interface. MACHINE: xx You can specify the type of A20 handler your PC uses by inserting for xx the code that matches your computer. CPUCLOCK Turn on to ensure HIMEM does not slow your computer’s clock speed. QUIET HIMEM loads without the usual status message when QUIET is on. SHADOWRAM Turn on to uses some of your upper memory blocks. All PCs store some of their basic operating instructions in read only memory (ROM). Computers cannot access instructions from ROM as fast as they can access instructions from RAM. If you have plenty of memory, you can increase the performance of your computer by asking the computer to copy the instructions from ROM into an upper memory block (i.e.RAM). This technique is known as shadow RAM. Expanded Memory Expanded memory was introduced before extended memory, therefore many PC applications were written to take advantage of expanded memory, not extended memory. Extended memory is far more common. The DOS device driver EMM386.EXE enables applications to use expanded memory. The EMM386.EXE emulates expanded memory by using extended memory. Therefore, you must load HIMEM.SYS before EMM386.EXE. The syntax for EMM386.EXE is: DEVICE=EMM386.EXE ON|OFF RAM NOEMS MIN=n There are more switch options available. The OFF switch starts the computer with EMM386.EXE loaded in memory but inactive. The OFF switch is not compatible with 4 NOEMS or RAM switches. The expanded memory emulator allocates 256kbytes of extended memory to be used as expanded memory. The RAM switch enables upper memory. Use the NOEMS switch to free the maximum amount of upper memory and to provide no EMS memory (i.e. do not allocate extended memory for use as expanded memory). Terminate and Stay Resident (TSR) To load device drivers or memory resident programs (TSR), into upper memory, the following conditions must be met: • HIMEM.SYS is loaded as a device driver. • EMM386.EXE is loaded as a device driver with the RAM or NOEMS switch. • The command DOS=UMB appears in the CONFIG.SYS If you also want to use the command DOS=HIGH to load DOS into high memory, you can combine the two commands as: DOS = HIGH, UMB You can load two types of programs into upper memory: device drivers and TSRs. Normally, device drivers are loaded using the DEVICE command. When you want to load a device driver into upper memory, use the DEVICEHIGH command. To start a memory resident program into upper memory, precede the program’s start-up command with LOADHIGH (you can also use LH).