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1. following a successful assembly The word Assembler stops blinking the smiley face symbol appears at the top right corner of the screen and the hexadecimal code listing is found in Window 2 Figure 3 is an example of a typical hex listing Window 2 Window 2 contains the following three programs 1 In addition to being a simple text viewer Window 2 contains the AK68 Stepper Tracer The Stepper operates when a successful assembly has been performed in Window 1 resulting in a hex listing in Window 2 When the above conditions have been met then the Stepper is the default program in Window 2 09 2 abed A HiLite bar will appear across the current program listing line Hitting any key which does not designate a menu item causes the program to execute one instruction of the 68000 listing If a 68000 Exception occurs or if the program attempts to execute code outside of the assembled program listing then the HiLite bar will disappear and a half second beep will be produced The Stepper Tracer is useful for tracking expected program flow expected loop iterations and in determining the instruction which generates an unexpected Exception Error The Stepper can be operated concurrently with the Simulator described below That is a program can be stepped to a desired position and then Window 3 described below can be accessed to continue program flow or simply determine CPU register and memory values at that time Not2. insert delete home end page up and page down keys It also recognizes the control sequence lt Ctrl gt Y which deletes the entire line at the cursor position The maximum file size is 64000 characters including white space which translates to approximately 800 lines of assembly code The text editor is the default condition while in Window 1 that is when not performing any menu selected operation then the text editor is active Figure 1 is a DOS Print Screen output of a typical 68000 code fragment Note that the original is in DOS Mode Color 80 allowing the top and bottom menu lines and their various menu items to be discriminated from the text editor by color 2 The 68000 assembler The assembler operates from Window 1 and is activated by mouse clicking on the highlighted word Assemble or alternately by typing lt Alt gt A See Figure 1 top line The word Assembler blinks red during the two pass assembly operation An error free compilation is then indicated by the bracketed block at the top left corner of the screen turning into the ubiquitous smiley face symbol An incorrect assembly is halted at the first occurrence of an error This was found superior given the design objectives to generating a typical plethora of errors and overwhelming the student programmer When the first error is encountered the program scrolls the screen to the line which contains the error and then either highlights the entire l
3. of this cross assembler simulator i an exterior or program flow view and ii an interior or CPU register view Two selectable windows were created to fulfill this requirement The assembler s output a hexadecimal object code listing is piped to the assembler s text editor where the line addressed by the simulated CPU program counter instruction fetch is highlighted The interior CPU view contains all of the Motorola M68000 internal registers stack pointers and the status register The Motorola M68000 simulator does not emulate an internal multi phase CPU clock Multiple simultaneous exceptions more than one exception occurring during the execution of a single instruction result in exception processing which may or may not be equivalent to the order of execution which would occur in a real Motorola M68000 hardware environment For this reason when simultaneous exception errors are detected the program halts and a message is displayed During two years of use at Penn State Altoona this limitation has never been observed to restrict the primary utility of the program that is to visually illustrate the operation of the Motorola M68000 microprocessor This paper provides a detailed description of the cross assembler integrated CPU simulator titled AK68 Design considerations are described and details of AK68 operation are discussed INTRODUCTION While teaching a sophomore level introductory course on microprocessors to the Penn State Alto
4. 000 s trace mode stacking and vector table addresses that is it traces the trace mode To track current operation the last memory location and the last central processor unit s register acted upon is always highlighted The last instruction performed is displayed on the next to the bottom line on the screen see figure 4 Last gt MOVE B DO D2 The next instruction to be executed is displayed on the bottom line of the screen see figure 4 Next gt AND L 0000007 D2 FUTURE ENHANCEMENTS A 6820 PIA software emulator acting through the personal computer parallel printer port will be added The I O emulation will add a limited hardware ability to the simulator so that lab exercises can be operated with external inputs and outputs up to the parallel port s size limit REFERENCES 1 M68000 Family Programmer s Reference Manual M68000PM Ad Motorola Inc 1989 2 M68000 8 16 32 Bit Microprocessors User s Manual Eighth Edition Prentice Hall Inc Englewood Cliffs NJ 1990 3 The 68000 Principles and Programming Blacksburg Continuing Education Series by Leo J Scalon Howard W Sams amp Co Inc Indianapolis Indiana 1981 4 Microprocessor and Microcomputer Data Digest by Walter H Buchsbaum and Gina Weissenberg Reston Publishing company Inc Reston Virginia 1983 5 Turbo Pascal Version 6 0 Scotts Valley Calf Borland International BIOGRAPHY SOHAIL ANWAR Sohail Anwar obtained a Ph D in In
5. Session 1532 AK68 A New Cross Assembler and Integrated CPU Simulator For The Motorola M68000 Microprocessor Dr Sohail Anwar Durel Hauser Roy Sheehan Penn State Altoona US Postal Service Sheehan Electronics ABSTRACT This paper describes a cross assembler and integrated CPU simulator for the Motorola M68000 microprocessor developed by the authors for use in an undergraduate introductory course on microprocessors The principal design objective of the authors was to create a less intrusive or more user friendly assembler combined with a dynamic model of program execution to encourage deductive problem solving in place of undesirable trial and error activity The design of the cross assembler centered on three goals 1 The assembler must discriminate between labels operators and operands without any strict set of identification rules 2 The assembler must produce a broad set of error messages which provide corrective guidance as opposed to a vague error identification That is the error messages must distinguish assembler errors from the Motorola M68000 assembly language syntax errors and the assembler must graphically highlight the line element label operator or operand which caused the error Furthermore the error message must indicate as possible the needed corrective action 3 The assembler must include a mouse operated pop up hexadecimal binary decimal calculator The design goals listed above required two main views
6. ability of the assembler to recognize a label unless it followed an unnecessarily strict set of identification rules 3 The lack of a default data size parameter 4 A limited set of cryptic error messages which failed to adequately inform the user if errors were generated as the result of 68000 language syntax requirements or arbitrary assembler rules of operation Additionally it seemed desirable that the laboratory software should assist in a deductive resolution of correctly assembled but faultily executing code This desire propelled the integration of a 68000 assembler text editor and simulator as described below THE AK68 ASSEMBLER SIMULATOR THEORY OF DESIGN The AK68 assembler simulator was designed to assemble and then simulate error free code until that point at which an error occurs There is an important distinction to be made between this mode of operation and the more exotic design mode which is intended to continue to execute beyond executable code The difference between the two design philosophies is that AK68 informs the operator that the program has crashed the exotic design informs the operator after some time given the random bit patterns that RAM has been reduced to exactly how the CPU continues to trash the remaining memory Although the latter is a real consideration in the design of 68000 based computers it is not necessary for the more modest requirements needed to learn how to write a 68000 assembly program In o
7. al and the status register is always displayed and modified in binary In addition a pop up screen is available which translates the current status register s contents in terms of 68000 branch instructions in both signed and unsigned formats see figure 5 line 2 Any part of the 64K memory bank can be selected for viewing or modification in hex The memory display sub window contains the 128 byte block most recently read or written by the program being executed A different block may be recalled by mouse clicking on any of the block addresses and entering a new address in hex From the header menu a number of stepping operations may be chosen Selecting Run to Count will set the number of instructions to execute before the next break Setting the count number to 0 will turn the function off that is the simulator will run continuously Selecting Run to Address will set an address break Selecting a negative v 09 2 bed number or hitting lt Esc gt will turn the Run to Address function off The Run to Count and Run to Address functions may be set to operate concurrently Setting Run to Address off the default condition and setting Run to Count to 1 also the default offers simple single stepping operation Since the simulator is effectually running the 68000 in a pseudo trace mode turning the simulator s status register s Trace Mode bit On causes the simulator to trace through the 68
8. cation program The communication program is not a standardized function of AK68 It is a custom include file added to AK68 which enables communication between AK68 and a specific small board computer When the communications module is present then hex listings from Window 2 may be directly down loaded to the intended computer Other functions such as reading the small board computer s memory contents are subject to the availability of command response procedures present in the small board computer s basic input output routines 3 S record writer Selecting Write HEX from the File Menu in Window 2 generates a standard S record formatted file in the current directory Most small board computers are supplied with a download program which can communicate an S record file to the small board computer A successful Assembly must have been accomplished before the S record writer can operate Window 3 Window 3 contains the following major program Window 3 is the 68000 Small Board Simulator see figure 4 This simulator visually represents all of the 68000 internal registers stack pointers and the status register Each register is modifiable during program execution Data register display and modification may be selected to be either in decimal hexadecimal standard and IBM extended code ASCII Binary Coded Decimal or 32 bit position binary The program counter and address registers are always displayed and modified in hexadecim
9. dustrial and Vocational Education from The Pennsylvania State University in December 1995 and an M S degree in Electrical Engineering from the University of Texas at Arlington in May 1982 He completed additional graduate coursework in control theory and applied mathematical sciences at the University of Texas at Arlington during 1982 1984 Since August 1992 Sohail has been working as an assistant professor of engineering technology at Penn State Altoona DUREL HAUSER Durel Hauser obtained Associate of Applied Science degrees in electrical engineering technology and in computer engineering technology from The Pennsylvania State University in 1995 Durel is currently working for the US Postal Service in Dubois Previous experience includes service in the Navy as a Fire Control Technician aboard the Nuclear submarine U S S Dallas where he operated and maintained a complicated mainframe computer system ROY SHEEHAN Roy Sheehan is a graduate of the US Army Electronics School Ft Gordon GA with a specialty in cryptographic machines He has additional training in pencil beam radar ranging equipment with the Air National Guard where he was the chief enlisted maintenance sergeant for the mobile unit In his civilian life for the past twenty two years he has been involved in the design installation and maintenance of electronic systems for local industry and in his owner operated business Sheehan Electronics Ashville PA Roy has a number of computer p
10. e that although the simulator described below can execute any random memory contents limited by the discussion above see THE AK68 ASSEMBLER SIMULATOR THEORY OF DESIGN the Stepper can only operate upon a successfully assembled hex listing The Stepper which is an alternate view of the simulator described below is an independent program from the assembler Some simulators for ease of construction simulate code by operating upon the mnemonics and plane text contained in the original assembled listing This mode of operation never allows a self check of the assembler simulator integration Once an error enters the assembly stream as a part of its hex output file it is perpetuated by the simulator and until compared to an actual hardware run may never be noticed by the operator Separately constructed and separately operating programs will immediately indicate to the operator that the assembler simulator program itself is the cause of the error by the observation that a particular assembler command is at variance with the corresponding mnemonic or symbol contained in the hex output file Rather than operating on the mnemonics and plane text output of the assembler the simulator converts the assembled hex output into a binary stream and then simulates the central processor unit by performing Boolean mathematical operations on that binary data in a manner similar to the action of a digital circuit 2 Window 2 also contains an RS232 serial communi
11. ine or when possible highlights the specific element label operator operand or sized operation which generated the error An error message including corrective action when such action can be anticipated is displayed on the bottom line of the screen The Help program which is larger than the 68000 assembler is included in the assembler Its objective is to find and highlight a given error and then display a useful error message The Help program also includes a menu driven reference file which lists all 68000 commands indexed by Math Logic Shifts Bit Diddles Big Diddles Compares Jumps Branch_On Addr Stack Moves Flags BCD and Other Stuff sized operations divided into Inherently Sized Multiply Sized Index Sized Illegal Size and Sized Assembler Commands effective addressing modes registers number formats assembler commands flags down loads labels the simulator and a generic Help Me file To obtain an error free assembly the only assembler command required to be in every text listing is the END command All commands after the END statement are ignored Labels are identified by the assembler by being the first word of a line and not being a legal operator No special symbol or column placement is required In addition to 68000 commands some labels are made illegal simply to avoid confusion when such labels are subsequently used as operand constants or offsets e g D8 The assembler routes its output to Window 2
12. ona electrical engineering technology students the authors of this paper observed that a significant amount of students laboratory time was spent on an aspect of the exercises considered to be extraneous to the fulfillment of the laboratory objectives The laboratory experience centered on elemental computer programming with a hardware emphasis To accomplish this exercises were devised involving a Motorola M68000 based small board computer and a corresponding commercially available cross assembler Although a major aspect of the laboratory experience involved CPU programming the actual experience was observed to be highly skewed toward another activity A significant amount of time was spent learning the environment in which the programming was performed Although learning the programming environment as L 09 2 abeg opposed to learning programming is a necessary activity the authors deemed that it should be a minor activity and quickly mastered The development of the Motorola M68000 assembler simulator software described in this paper was an attempt to reduce the time spent learning a particular programming environment thereby increasing the time spent learning the 68000 programming language The excessive time required to master the assembler environment was tied to four main factors 1 The lack of an integrated text editor resulting in a large number of separate program executions in order to obtain one error free assembly 2 The in
13. rograms and micro processor systems in current use in industry and university settings G 09 2 ebeY
14. sence of TRACE exception generation and privileged instructions which change the SR register result in exception processing which may or may not be equivalent to the order of execution which would occur in a real 68000 hardware environment The lack of the ability to prioritize single instruction multiple exceptions and the resultant inability to track the convolutions of multiple errors is a reasonable trade off for the benefits derived A small and reasonably fast program simulator that faithfully runs all programs as long as they run When they stop running the simulator indicates a failure and resets the program counter PC In some environs this may be a limiting factor but for learning how to write 68000 programs this may in fact be an advantage Z 09 2 bed SOFTWARE DESIGN AK68 consists of six major programs and a pop up hexadecimal binary decimal calculator Each major program operates from within one of three windows Each window is identified by a bracketed number found at the top right corner of the screen as seen in figures 1 through 5 The program is written in Borland Turbo Pascal TM version 6 0 and is based on Borland s Turbo Vision object oriented library which has been placed in the public domain by Borland Window 1 This window contains the following two programs 1 The text editor The text editor is a standard ASCII text editor which recognizes pointer operations such as a mouse and all alpha numeric keys the
15. ther words AK68 is designed for writing 68000 programs it is not designed to identify hardware software failures In particular the more modest AK68 design executes multiple EXCEPTIONS in a non 68000 manner This difference is not visible when running normal 68000 assembly language programs but becomes apparent if an attempt by design or otherwise is made to execute exceptions inside exceptions At some point a non 68000 simulator generated DOUBLE EXCEPTION error will occur and the program counter PC will be set to the value contained at Memory address 0004 the Motorola RESET INITIAL PC value This probably would not be the way the same code would act when running on a 68000 based board even with the BIOS EPROMS disabled The result is that simple EXCEPTION processing routines can be written and executed within the simulator but in depth testing of these routines cannot be performed The EXCEPTION processing is not different from standard 68000 operation in the manner in which memory is manipulated The vector table is emulated the status register is stacked at the SSP location in accordance with 68000 rules of operation The non 68000 nature of the simulator s operation appears within the exact time at which the exception is processed AK68 does not emulate an internal multi phase CPU clock Multiple simultaneous exceptions more than one exception occurring during the execution of a single instruction particularly in the pre
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