11/13/01CS-550 Presentation - Overview of Microsoft disk operating system. 1 An Overview of Microsoft Disk Operating System

11/13/01CS-550 Presentation - Overview of Microsoft disk operating system. 2 Introduction to MS-DOS An operating system is a set of interrelated programs that manage and control computer processing.The Microsoft Disk Operating System, MS-DOS, is a traditional microcomputer operating system that consists of four major components.  The Operating-system loader - It brings the operating system from the startup into RAM. - Bootstrapping  The MS-DOS BIOS - BIOS stands for Basic Input/Output systems - Loaded from the file IO.SYS during initialization. - Layer that sits between operating system kernel and hardware.  The User Interface (shell) - Conventional program that allows the user to interact with the operating system. - Default MS – DOS user interface is a shell program called Command.com.  The MS-DOS Kernel  - Heart of the operating system  - it is a proprietary program supplied by Microsoft corporation.

11/13/01CS-550 Presentation - Overview of Microsoft disk operating system. 3 MS-DOS Kernel The kernel provides a collection of hardware-independent services called system functions. - File Management - Memory Management - Device Input and Output - Process control

11/13/01CS-550 Presentation - Overview of Microsoft disk operating system. 4 File Management Block Devices are accessed on a sector basis. The MS- DOS kernel through the device driver sees a block as a logical fixed size array of sectors. the device driver in turn translates the logical sector requests from the MS – DOS into physical locations on the block device. MS – DOS file system is divided into four parts - Boot Sector - Always at the beginning of a partition. - Contains OEM identification, a loader routine, and a BIOS parameter block. - File Allocation Tables (FAT) - Provides a map to the storage locations of files on the disk by indicating which clusters are allocated to each file. - Second copy of FAT as back up. - Free cluster is found by scanning the FAT for a zero value. - Root directory - Root directory entries are 32 bytes long. - Each entry includes a filename and extension, size, starting FAT entry, the time and date the file was created and the files attributes. - Files Area - Contains subdirectories, file data and unallocated clusters. - The area is divided into fixed size clusters and the use for a particular cluster is specified by the corresponding FAT entry

11/13/01CS-550 Presentation - Overview of Microsoft disk operating system. 5 Memory Management Based on a pool of variable sized memory blocks. Two basic functions are to allocate a block from the pool and to return the allocated block to the pool. MS-DOS compatible personal computers can be fitted with three kinds of RAMs –Conventional Memory –Expanded Memory –Extended Memory Conventional Memory - Up to 1MB of Memory is directly addressable. - Physical addresses for references are generated by a 16-bit segment register combined with 16-bit offset. - Out of 1MB MS-DOS occupies 640KB of the conventional memory. - This 640KB is divided into three zones - The interrupt vector table - Occupies lowest 1024 bytes of memory. - Its address and length are hardwired into the processor and cannot be changed. - The operating system area - Begins immediately above the interrupt vector table. - Holds the operating system, its tables and buffers, and device drivers. - Amount of memory occupied varies from version to version of MS-DOS. - The transient program area - Remainder of the 640KB area. - Organized into a structure called Memory Arena, which is divided into arena entries (memory blocks). - Each arena entry has a arena entry header.

11/13/01CS-550 Presentation - Overview of Microsoft disk operating system. 6 How MS-DOS supports Conventional Memory Management The MS-DOS kernel supports three memory management functions, invoked with interrupt 21H, which operate on the TPA: - Function 48H (Allocate Memory block) - Function 49H (Free Memory block) - Function 4AH (Resize Memory block) The memory manager can use any of three allocation strategies: - First fit – the arena entry at the lowest address that is large enough to satisfy the request. - Best fit – the smallest available arena entry that satisfies the request, regardless of its position. - Last fit – the arena entry at the highest address that is large enough to satisfy the request Default approach used by MS-DOS is First-fit approach. Expanded Memory - To circumvent the 1MB limit of conventional memory, expanded memory was designed. - As much as 8MB of expanded memory can be installed in a single machine. - Made available to the application software in 16KB pages. Expanded Memory Manager - Provides an interface between application programs and expanded memory. - Divided into Driver and Manager. - Manager controls the status, allocation, mapping and deallocation of expanded memory. Extended Memory - Storage at addresses above 1MB that can be accessed in Protected mode. - Extended memory is linearly addressable, so no manager required.

11/13/01CS-550 Presentation - Overview of Microsoft disk operating system. 7 Device Input and Output MS-DOS recognizes two types of devices: block devices e.g. floppy disk or fixed disk drives; and character devices, e.g keyboard, display, printer, and communication ports. MS-DOS identifies each block device by a drive letter assigned when the devices controlling software, the device driver, is loaded. A character device on the other hand, is identified by a logical name built into the device driver. Distinction between character and block devices is that the MS-DOS always adds new block – device drivers to the tail of the driver chain but adds new character – device drivers to the head of the chain. Techniques to access character devices: - Handle type function calls A handle is a 16 bit number returned by the operating system whenever a device is opened or created. - Character input and output functions: Interrupt 21H functions 01H to 0CH. These functions are designed to communicate directly with the keyboard, display, printer and serial port. Process Control Process, or task, control includes program loading, task execution, task termination, task scheduling and intertask communication. MS-DOS is not a multitasking operating system,it is a single tasking operating system.