1. 2003 Dominic Swayne1 Microsoft Disk Operating System and PC DOS CS-550-1: Operating Systems Fall 2003 Dominic Swayne

2. 2003 Dominic Swayne2 Introduction Understand the Operating System Cover some of its important features Improve your understanding of the operating system

3. 2003 Dominic Swayne3 Brief History 8-bit processors had been the norm in Personal Computers (PC) Digital Research’s Control Program for Microcomputers CP/M-80 most popular 8-bit OS Intel’s 8086 – first 16 bit processor for PCs OS developed by Seattle Computer Products (SCP) for their own 8086 based system IBM to launch 8086 based PC – needed an OS Digital Research’s CP/M-86 and Softech Microsoft licenses the software from SCP The rest is history

4. 2003 Dominic Swayne4 Design Goals Easy to translate CP/M-80 applications – backwards compatible with 8-bit applications Speed and efficiency – reduce disk transfers, improve their speed and decrease OS compute time Written in Assembly Language – due to development tools –CP/M-80 based assembler –2k-byte monitor/debugger

5. 2003 Dominic Swayne5 Limitations Hardware could only address 1MB Programs couldn’t be larger than 640kb Single user – single processor – no hard drives

6. 2003 Dominic Swayne6 DOS Modules Basic Input – Output System (BIOS) –Loaded in low memory – just above Interrupt Tables –EPROM based –Additional drivers loaded from CONFIG.SYS Kernel –Loaded above BIOS –MSDOS.SYS file –No default error handling COMMAND.COM –User interface –Resident and Transient portions –Handles all error traps

7. 2003 Dominic Swayne7 Memory Map Interrupt vector starts at 0 I/O System - from manufacturer DOS kernel starts at 1k Resident portion of COMMAND.COM Above 640kb is 384kb of additional system memory – not directly addressable 1MB upper limit Reserved for BIOS1MB Unused 640k Command Transient Program memory Command Resident DOS Kernel 1k I/O System Interrupt Vectors 0k

8. 2003 Dominic Swayne8 Synchronization No true multi-tasking - Re-entrant code not supported by DOS Interrupt driven – no scheduler or CPU sharing Three types of interrupts: –Internal, External, Software CPU senses interrupt –Places Program Status Word, code segment register and instruction pointer on a stack –Disables interrupt system –Starts processing based at 8-bit address from interrupt table (interrupt vector) –Re-enables interrupt system

9. 2003 Dominic Swayne9 Discrimination Memory allocated at program load – allocation based on header file for.EXE files (header file specified min/max).COM software gets all available program area (aka Transient Program Area).BAT files – text files that call programs in sequence Dynamic memory allocation/recovery available starting with version 3.3 Only works with Assembly language and C programs

10. 2003 Dominic Swayne10 Memory Block Free blocks in linked list 32 bit descriptor – 16 bit segment/16 bit offset, 64k direct 16-byte – minimum size Maximum size – as much as is free Free adjacent blocks on the list merge automatically “Bug” permits access to 64k of Upper Memory

11. 2003 Dominic Swayne11 Processor Modes Real mode – compatible with 8086 limits Protected mode – selector/table/offset – memory system –Four 16-bit words used as memory descriptor –First available on the 80286 based systems (required hard reboot to switch modes) –Four privilege levels (0 – 3, with 0 having most privileges) –Additional table indicator to distinguish between local and global memory

12. 2003 Dominic Swayne12 Conclusion Revolutionary design – no chance Revolutionary impact – absolutely Simple, expandable OS Standardized user interface and applications Lasted from 1981 until 1994 as a separate product – still in use by Windows 98 (and in ME though better hidden)