4-23-2003, Wednesday

1. 4-23-2003, Wednesday Presentations CS8422 Computing SystemsDr. Ken Hoganson • Class • Will • Start • Momentarily…

2. History/Development Richard Iwezulu CS 8422, Spring 2003 And Demise of CP/M

3. What is CP/M? Developed in 1973 by Gary Kildall Invented as a workaround Based on Intel 8080 processor CP/M is an acronym for Control Program for Microcomputers. It is an operating system for 8 bit computers. Kildall needed an OS for a personal computer to test the compiler he was developing for Intel. He did not like the long commute to use the mini computer at Intel. History of CP/M

4. Originally developed for machines with only floppy drives and 64K memory Intel saw no potential in an OS for personal computers and declined to market it Intel allowed Kildall to keep the rights to the new software Kildall gave Intel the OS he developed for them but they were not convinced that people would want little computers for themselves. Kildall kept the rights to his software and decided to market it himself. He formed Digital Research which was bought by Novell soon after Kildall died in 1994. History of CP/M

5. Entire OS took only 8K of computer’s memory 3 main versions – 1.4, 2.2 and 3.1 8 character file names with 3 character extensions The first 36 MS-DOS system calls mirror CP/M CP/M proved to be very powerful with little hardware demands. CP/M bears a lot of familiarity with MS-DOS, they both have the same 8 character file names with 3 character extensions. History of CP/M

6. Does not require a disk, monitor or keyboard to operate Created a BIOS module for compatibility OS was separated into 3 logical pieces: Console Command Processor (CCP) Basic Disk Operating System (BDOS) Basic Input/Output System (BIOS) Kildall separated parts of his OS that addressed the specific format of diskettes and placed them in a separate module called the BIOS, thereby achieving cross-platform acceptance for his OS. Development of CP/M

7. Version 2.2 added support for up to 8 drives at 8MB per drive Version 2.2 was the basis of MSDOS and version 3.1 evolved into DRDOS Version 2.2 dominated micro computing 16 bit version was CP/M-86 based on Intel 8086 processors In 1978 alone, version 2.2 was sold on more 500,000 computers and by 1980 it seemed hardly conceivable that any other OS would ever be used on Intel-based computers. Development of CP/M

8. Users could modify OS themselves OS was very cheap Available software included Calc, WordStar and dBASE II 15 second boot up time Users could modify OS to suit themselves or meet their company’s needs. Both OS and hardware was relatively cheaper than available alternatives. Boot up was very fast. Development of CP/M

9. BIOS allowed programmers to write their routines to a set of subprograms at a time when industry standards did not exist for different machines CP/M was very stable and bug-free Drive letters could be assigned according to the user’s preference BIOS solved the headache of writing different machine code for all kinds of hardware when writing software programs for the OS. User’s had the flexibility of assigning drive letters up to “P” on any drive as they wished. Development of CP/M

10. Programs and private data had to be carried around on 5-1/4” floppies CP/M had no network capabilities Digital Research was slow to incorporate 16 bit technology CP/M did not harness the advantages a hard drive gave a computer until they had lost most of their market share. Digital Research felt that 1 MB of memory - which Intel’s new 16 bit chips were capable of - was more than anyone could figure out a use for on a PC. Demise of CP/M

11. OS depended on utilities to do some basic operations like file copying CP/M did not have a full screen editor Arcane command line syntax MSDOS commands were more user friendly CP/M needed utilities for operations like long directory listings and file copying. CP/M did not address screen formatting lacked a full screen editor, it was so bad that users started using WordStar to do the text editing. Demise of CP/M

12. 16 bit 8088 processor made it easier to write better programs in less time IBM used an open architecture like CP/M, creating a new standard based on 16 bit technology with MS-DOS Running programs on 16 bit CP/M meant transferring control back and forth from 16 to it’s native 8 bit machine language, making it slow Many hardware manufacturers jumped on the IBM bandwagon creating a new 16 bit standard and leaving CP/M behind Using 16 bit CP/M was like having two computers that transferred requests and information back and forth Demise of CP/M

13. Kildall missed his opportunity to sign up with IBM and they went to Bill Gates for an OS for their new “PC” By the time CP/M-86 was released, MS-DOS was firmly entrenched on IBM PC’s Why is the string in function 9 terminated by a dollar sign? IBM felt snubbed when they approached Kildall for an OS and he was not there to meet them Whenever approached about the rumor that MS-DOS was copied from CP/M, Kildall always had interviewers ask Bill Gates that question “…because he [Gates] can’t answer, only I know that!” Demise of CP/M

14. C/pm history Wei-Hsiang Peng CS 8422, Spring 2003

15. What is CP/M ? Control Program for Microcomputers alternatively Control Program / Monitor Control Program for Microcomputers. An operating system that manages computer resources and provides a standard systems interface to software written for a large variety of microprocessor-based computer systems. It's an operating system for 8-bit computers. It looks rather like DOS to use (only not so user-friendly); CP/M?

16. The creator of CP/M- Gary Kildall Kildall & Intel How Kildall created CP/M Gary was the first person to interface a disk system to a microcomputer and create an operating system for it. He changed what had previously been a circuit designed for process control applications into a fully functional computer. The world changed dramatically because of his work. in 1975 ,Gary Kildall was working as a consultant to Intel (inventors of the 8080 chip which at that time powered the majority of non-Apple microcomputers). Kildall's task at Intel that year was to design and develop a language called PL/M ( Programming anguage / Microcomputers ) for the 8080 chip, to be used as a systems development language. At the time, the chips themselves barely existed and Intel was just then starting to design a computer system that used the 8080. The plan was for Gary to use the 8080 emulator Intel had running on their big PDP-10 minicomputer. The only 8080-based computer Intel had available was called "Intellec-8", but it didn't have any software or disk storage attached to it. So Kildall obtained a used test floppy drive free from Shugart Associates, attached it to the Intellec-8 with a controller designed by his friend John Torode, and wrote a primitive operating system for it which he called CP/M. BIRTH-1

17. 1975 - CP/M version 1.0 1976 – The growth of “8080 chips” business “IMSAI” & Kildall CP/M was developed on Intel's 8080 emulator under DEC's TOPS-10 operating system, so naturally many parts of CP/M were inspired by it, including the eight character filenames with a three-character extension that every MS-DOS/Windows 3.X user still lives with today. By the end of 1975, Kildall at last had CP/M version 1.0 ready and had started on PL/M, but Intel was no longer interested in the systems development language by that time. Gary offered CP/M to them, but the company saw no potential in it and declined to market it. By 1976, the world was moving onward and upward. Intel was so busy selling bucketloads of 8080 chips to the many small and growing computer manufacturers that it didn't miss a dubious opportunity to get into the software business one bit. Many of these new companies had neither the time nor expertise to write their own proprietary operating systems A company called IMSAI, which built a very successful 8080-based system, had just started marketing their own floppy disk system, and needed some software to use it -- fast. They heard about Kildall and his CP/M, and contacted him. BIRTH-2

18. Kidall solved the “incompatible ”problem with “BIOS” CP/M Kildall formed the company “DR” IMSAI wanted CP/M, and Kildall wanted to sell it, but the trouble was CP/M was written to only use "standard" IBM-compatable soft-sectored floppy disks. They had a different, incompatible flavor of floppy. At this point, most ordinary programmers would simply have changed the operating system to use the different format and sold it, but Gary Kildall had a better idea. What he did was separate out the parts of CP/M version 1 that addressed the specific format of the diskettes, and placed them in a separate module he called the BIOS, for Basic Input/Output System. That way, the system could easily be adapted to new hardware without having to rewrite or even revise the complex heart of the software. Other new hardware startups need not write their own software, they could buy CP/M and adapt it themselves to their unique hardware. Further, because CP/M operated the same way on every 8080-based computer, other software developers were also relieved of the necessity of adapting their software products to each new machine -- they could write to be compatable with CP/M, and let it take care of the details of doing the I/O. Kildall was an engineer, not a businessman, but he could recognize a gold mine when he saw one. He rapidly formed his own company, called Intergalactic Digital Research, to market and further develop CP/M and other products such as his abandoned PL/M. BIRTH-3

19. CP/M 2.0 was consisted by 3 parts -CCP -BDOS -BIOS CP/M 2.0  CP/M 2.1  CP/M 2.2 The DR company's seminal product was CP/M 2.0, which fully separated the three components of the operating system into logical pieces: the CCP (console command processor); the BDOS (Basic Disk Operating System); and the BIOS. Only the BIOS need be provided by anyone to get CP/M running on a new machine, the CCP and BDOS would be unchanged. CP/M 2.0 was quite buggy, and was quickly followed by 2.1 as a fix-up release. However, 2.1 was limited in its internal capacity to small floppy drives, and by 1977, hard drives were coming on the scene. CP/M version 2.2 added expanded disk formatting tables which could allow access to up to 8 (eight) megabytes per drive in up to 8 (eight) total drives. It was version 2.2 that became the megahit that dominated microcomputing almost from its outset. LIFE-1

20. Reasons why CP/M succeed Popular programs of CP/M How many computers installed CP/M by 1980? It was CP/M's adaptability that gave it appeal and launched it on the road to success, but any operating system that had that characteristic might have succeeded in a similar way, given the right timing and some luck. But CP/M was more than just lucky -- it was good. It packed a surprizing amount of power in a tiny package, and did so in a simple, clean logical way. The first real "killer app" for CP/M was probably WordStar, a word processing program that became very widely used. Also famous was the first microcomputer database application, dBASE II. These and many, many other applications and utilities eventually made CP/M a useful tool for a wide range of ordinary people. By 1978, CP/M 2.2 had been ported to nearly every 8080 and Z80 based microcomputer built. In the end, more that 500,000 computers would be sold with CP/M as their operating system. It so dominated the microcomputer world by 1980 that it seemed hardly conceivable that any other operating system would ever be used on Intel-based computers. LIFE-2

21. Crossroad of the computer history: The direction to R&D of CP/M—8080 chips, in 1981 The computer giant-- IBM’s choice—8088 chips, in1981 By 1981, the apogee of the CP/M orbit, a new generation of Intel microprocessors was on the horizon -- the 8086 and 8088 16-bit chips, which could address an incredible 1 megabyte of memory. This seemed at the time more than anyone could ever figure out a use for, so Digital Research focused much of their attention on producing CP/M 3.0 for the dominant 8080/Z80 platform. There were plans of course to port CP/M to the new 16-bit chips with a version called CP/M-86, but it was not a priority at the time. But IBM planned on entering the microcomputer market in a serious way in 1981, and the computer giant had its own priorities and a great deal of money and marketing power to back them up. IBM chose the new 8088 chip as the heart of their new PC, and of course they needed an operating system to run on it. Instead of writing their own, as they generally did with a new machine, time and the demands of the market led them to Digital Research's door to license the ubiquitous CP/M for the new IBM-PC. DEATH-1

22. Kildall missed the contract of IBM IBM’s another company choice The IBM & Microsoft joined As the now-famous story goes, Gary Kildall was not there to open that door when IBM came calling -- an avid amateur pilot, he was flying his private plane on a business trip to the Bay area. His wife and business partner, confronted with IBM's imposing code of secrecy and nondisclouse agreements, refused to sign even enough for talks to begin. Rebuffed by what they considered arrogance, the IBMer's went elsewhere -- to Seattle and another small young software firm called Microsoft. A deal was struck there, and as they say, the rest is history. DEATH-2

23. CP/M came in three main versions : - v1.4 - v2.2 was the basis of MSDOS - v3.1 evolved into DRDOS and OpenDOS / DR-DOS. There were also 8086 and 68000 versions of CP/M. CP/M-86 evolved into DOS Plus, Concurrent DOS and REAL/32. In 1979, DR released version 2.0, which soon was followed by CP/M 2.2. This version was installed most worldwide, as it offered high performance together with little memory requirements. CP/M version 3.0, also called CP/M Plus, was not released before 1982. Unfortunately this was too late, since one year before the IBM-PC had begun its triumphant advance. Besides the AMSTRAD CPC 6128 and their PCW (both using CP/M 3.1*) you can find CP/M Plus also on the C-128 ( CP/M 3.0*). Versions of CP/M

24. CP/M is logically divided into four parts: BIOS - the basic I/0 system for serial peripheral control BDOS - the basic disk operating system primitives CCP - the console command processor TPA - the transient program area The BIOS and BDOS are combined into a single program with a common entry point and referred to as the FDOS. The CCP is a distinct program which uses the FDOS to provide a human-oriented interface to the information which is cataloged on the diskette. The TPA is an area of memory (i.e, the portion which is not used by the FDOS and CCP) where various non- resident operating system commands are executed. User programs also execute in the TPA. CP/M Memory Organization-1

25. The BIOS and BDOS are logically combined into a single module with a common entry point and referred to as the FDOS. The CCP is a distinct program that uses the FDOS to provide a human-oriented interface with the information that is cataloged on the back-up storage device. The TPA is an area of memory, not used by the FDOS and CCP, where various nonresident operating system commands and user programs are executed. The lower portion of memory is reserved for system information and is detailed in later sections. CP/M Memory Organization-2 fbase: cbase: tbase: boot:

26. There are some CP/M Plus commands listed. TYPE DIR SHOW DATE ERASE RENAME DISCKIT PIP Type-shows a named file on the screen, waiting for a key between pages. DIR-find the names of disk files. SHOW-tells you the amount of space in a drive. DATE-lets you read and set the date and time, used to mark files. ERASE-remove files from a disk. RENAME-change names of files. DISKIT- copies disks. PIP-copy files back and forth between disks, the screen, serial and parallel ports. CP/M Plus COMMANDS

27. CP/M implements a named file structure on each disk, providing a logical organization that allows any particular file to contain any number of records from completely empty to the full capacity of the drive. CP/M File types

28. The Origins of MS-DOS Sui-Tung Ng CS 8422, Spring 2003 Advanced Computing Systems

29. Tim Paterson - Original author of MS-DOS - graduated from U of Washington in 1978 - worked as an engineer in Seattle Computer Products - designed an 8086 CPU card for S-100 Bus in May 1979 - began designing DOS in 1980 after IBM had released their new 8086 microprocessor (16-bit) objectives in design of DOS - as simple as possible - make it fast and efficient - written in 8086 assembly language In college, Paterson wrote a multi-tasking operating system for the Z80 microprocessor as a term project. Therefore, Paterson had confidence in writing an OS for 8086 computer. Patterson spent half of his time working on Qdos from April to July, 1980. QDOS was completed in July, 1980. Tim Paterson

30. took two man-months to create Quick and Dirty Operating System (QDOS) used Microsoft Stand-Alone Disk BASIC to handle file organization was hired in Microsoft in 1981 in and out of Microsoft in 1980s joined Microsoft again in 1990 QDOS could work with Microsoft Stand-Alone Disk BASIC that used a File Allocation Table (FAT). Tim Paterson

31. located in Tukwila of Washington manugacturer of S-100 memory cards a hardware company, not software company, in 1980 worked with Microsoft and Digital Research shipped its first 8086 cards with Microsoft Stand-Alone Disk BASIC in Nov 1979 gave up to use Digital Research CP/M-86 as its operating system in 1980 decided to create its own DOS Seattle Computer Products (SCP) gave up to use CP/M-86 as Digital Research could not promise when it would be completed. Finally, SCP decided to develop its own DOS. Seattle Computer Products

32. QDOS - completed in July 1980 QDOS 0.10 - launched in Aug 1980 86-DOS version 0.3 - released at the end of 1980 sold non-exclusive rights to Microsoft to market 86-DOS released 86-DOS version 1.00 in Apr 1981 similar to the MS-DOS of today Seattle Computer Products (SCP)was a hardware company, therefore it was willing to sell the non-exclusive right for Microsoft to market its 86-DOS at that time. Seattle Computer Products

33. Microsoft had been working with IBM on their personal computer project and advised IBM to Digital Research for the operating system. However Digital Research refused to sign a non-disclosure agreement. Finally, Microsoft offered IBM with DOS by buying the rights to market DOS from SCP. Microsoft • paid SCP $10,000 for the right to market 86-DOS and • paid SCP $15,000 for each OEM customer in 1981 • got a secret customer for DOS - IBM • bought DOS from Seattle Computer by paying $50,000 • plus a license to include DOS in their PCs in July 1981 • named DOS as MS-DOS • provided 86-DOS1.14 as OS in IBM’s PCs • released MS-DOS version 1.24 to IBM • released MS-DOS version 1.25 to the public in Mar 1982

34. Microsoft kept improving and evolving DOS after buying the products completely from SCP. MS-DOS became a very popular Operating System in the PC markets. Microsoft • released MS-DOS version 2.0 to IBM new XT computer in Feb 1983 • settled the license dispute by buying the license back from SCP for $975,000 in 1986 • the total price for DOS ended up $1,050,000 • launched MS-DOS to its OEM in 1982 • kept improving and evolving DOS for 15 years • stopped updating DOS until Windows95 was released in 1995

35. Year DOS Version 12/1980 QDOS 07/1981 86-DOS 09/1982 MS-DOS 1.25 03/1983 MS-DOS 2.0 11/1983 MS-DOS 2.01 12/1983 MS-DOS 2.11 09/1984 MS-DOS 3.0 11/1984 MS-DOS 3.1 04/1986 MS-DOS 3.2 MS-DOS Timeline

36. 04/1987 MS-DOS 3.3 04/1988 MS-DOS 3.31 07/1988 MS-DOS 4.0 11/1988 MS-DOS 4.01 04/1989 MS-DOS 4.01a 04/1991 MS-DOS 5.0 03/1993 MS-DOS 6.0 11/1993 MS-DOS 6.2 04/1994 MS-DOS 6.22 08/1995 MS-DOS 7.0 Above timeline shows that Microsoft spent time to develop and market the MS-DOS from 1980 to 1995. The development cost of MS-DOS was high over the past 15 years. MS-DOS Timeline

37. Tim Paterson was the original author in DOS MS-DOS was an easy-to-use operating system Microsoft successfully launched MS-DOS to IBM and to the PC market Microsoft kept improving and evolving the MS-DOS over the past years Microsoft integrated MS-DOS with its windows operating system in 1995 Without the efforts of Microsoft, DOS will not be the popular operating system in 1980s and 1990s. MS-DOS now is integrated in Windows 95, 98, NT, and 2000. Conclusion

38. References A short history of MS-DOS. (2003). Retrieved February 21, 2003, from http://www.patersontech.com/Dos/Byte/History.html. Conner, D. (1998). Father of DOS still having fun at Microsoft. Retrieved February 21, 2003, from http://www.patersontech.com/Dos/Micronews/paterson04_10_98.htm. DOS timeline. (2003). Retrieved February 21, 2003, from http://www.powerload.fsnet.co.uk/timeline.htm. Hunter, D. (2003). Retrieved February 21, 2003, from http://www.patersontech.com/Dos/Softalk/Softalk.html.

39. References MS-DOS review. (2003). Retrieved February 21, 2003, from http://www.michaelh.com/comp/msdos.shtml. Paterson, T. (2003). DOS. Retrieved February 21, 2003, from http://www.patersontech.com/Dos/Encyclo.htm. The Dross of the DOS. (1997). Retrieved February 21, 2003, from http://www.forbes.com/asap/1997/1201/070_print.html.

40. History of VAX/VMS – Quality by Design Jerry Samples CS 8422, Spring 2003 (photo from “VAX OpenVMS at 20”, Digital Equipment Corp., 1997)

41. VAX and VMS were products of Digital Equipment Corporation (DEC was acquired by Compaq in 1998) VAX refers to the hardware architecture VMS operating system co-designed with VAX hardware First VAX/VMS computer was VAX 11/780, rolled out October 1977. Digital Equipment Corporation (DEC) was founded in 1957 and produced the very successful PDP family of minicomputers. However, for different markets, DEC had created different, and incompatible, operating systems for the PDP hardware. In the mid-1970’s, DEC began work to develop a family of computers that would be priced over a wide range, from very affordable workstations, up to mainframe-class, but which would all be software-compatible and have a consistent user interface. Overview

42. VAX is acronym for Virtual Address eXtension VMS stands for Virtual Memory System “11” in VAX 11/780 indicates compatibility with PDP-11, DEC’s previous-generation workhorse minicomputer One of the driving forces in the creation of VAX and VMS was the recognition that 16-bit addressing, which dominated all computing until the mid-1970’s, was limiting program size and complexity. VAX/VMS was designed to extend the 16-bit addressing of PDP computers to 32-bits. Also, main memories generally were not very large, ranging from 10’s of kilobytes up to perhaps 4 megabytes. With VMS’s virtual memory techniques, very large programs could be created and managed transparently, rather than forcing programmers to break their code into a number of separate, small programs. Naming

43. Multi-user High-performance 32-bit addressing High reliability 15 – 20 year architecture life span “Culturally compatible” with PDP-11 (look and feel) Compatible with PDP-11 software Quick to market Multiprocessing Network support and distributed computing Excellent documentation The vision of DEC cofounder Ken Olsen was affordable computers for the masses. Once the success of the VAX project was assured, DEC adopted a strategy of “one platform, one operating system, one network” and focused most effort on the VAX/VMS family. In designing VAX and VMS, hardware and software teams worked closely, each influencing the other, to achieve an outstanding final product. It is generally acknowledged that all the design goals were fully met, with first product shipping only 31 months after project start. Design Goals – A Tall Order

44. 1977 - first VAX architecture, 11/780, based on existing PDP technology (boards, bus, I/O, etc.) 1980 – VAX 11/750 uses LSI ICs to shrink size, boost performance 1985 – MicroVAX chip implements subset of VAX architecture on a single microprocessor 1986 – V-11 chip implements complete VAX architec-ture; used in mid-range VAX 8200 and 8300 models DEC always struck a healthy balance between hardware and software. VAX and VMS were co-designed from the beginning. As electronics technology advanced, the architecture was ported again and again to smaller and faster hardware implementations, but never losing VMS compatibility. Eventually, DEC products ranged from under $5000 for a VAXstation 2000, up to $5.2M for the top-of-the-line VAX 8978. VAX on a chip

45. Dave Cutler was one of VAX/VMS chief architects, later went to Microsoft and led development of Windows NT Dave Cutler, outlined by the red oval, was one of three chief software engineers who, along with three hard-ware engineers, originally designed the VAX/VMS system. He also led the design effort for DEC’s 64-bit RISC chip, Alpha. Dave Cutler later went to work for Microsoft, leading development of Windows NT. Many of NT’s features, such as the virtual memory manager, were descendents of VAX/VMS technologies. (photo from “VAX OpenVMS at 20” by Digital Equipment Corporation, 1997) Dave Cutler

46. Paging – moving portions of memory for a process from physical (main) memory to a disk file (“page file”) Swapping – moving all memory related to a process from physical memory to a disk file (“swap file”) Two-level paging system 512 byte page size For improved performance of a process currently in the queue for scheduling, paging is performed. A process will generally be allocated less physical (main) memory than it logically requires. VMS automatically moves the necessary 512-byte pages between physical memory and the process’s page file. Each process has its own page file. There is also a swap file for the entire system. When a process is to be taken off the scheduling list, all of the memory related to the process is placed in the swap file. VMS Virtual Memory

47. Digital Command Language (DCL) is the command-shell language of VMS Provides dozens of commands with English-like names, not cryptic mnemonics, as in Unix. Most commands support numerous qualifiers May be grouped inside command files (.COM) for automated execution Digital Command Language (DCL) is the shell language of VMS, similar to MS DOS’s command line or Unix shells, such as the Korn or Bourne shells. Most DCL commands, and there are dozens, can be executed from a command prompt (a dollar-sign by default). Command files, similar to shell scripts or batch files, are also supported. These are invoked by giving the command file’s name prefixed with the “@” sign. Digital Command Language

48. The DCL commands may be fully spelled out for clarity, but they may also be abbreviated to as few characters as will uniquely identify the command. The types of functions that can be performed with DCL are similar to Unix or DOS. These include file and directory creation and management; input and output, including magnetic tapes, printers, terminals, keyboards, etc.; and obtaining various kinds of system and user status information. (table taken from OpenVMS Documentation website: http://www.openvms.compaq.com/doc/ ) Common DCL Commands

49. Creation of .COM file, followed by execution The figure illustrates creation of a DCL command file, named DOFOR.COM. It is opened for writing by the CREATE command, then each of the next six lines are added literally to the file. [Ctrl/Z] represents a keyboard command which closes the file. The final command, @DOFOR AVERAGE, executes the command file and passes it the argument AVERAGE. (cont’d) (figure taken from OpenVMS Documentation website: http://www.openvms.compaq.com/doc/ ) Example DCL Usage (1)

50. Creation of .COM file, followed by execution (cont’d) The argument is represented inside the command file by the replaceable parameter, P1. When executed, DOFOR.COM calls a FORTRAN compiler, then links and runs the program. Finally, it prints the listing file (.LIS). Note that, if no source file name is provided as an argument, the user is prompted (INQUIRE) for a file name. Command files may be nested (i.e. may call other command files) by using the “@” sign within the command file. Comments in a command file are indicated by lines beginning with “$!”. Example DCL Usage (2)