MARS MIPS simulator - Missouri State University

Release 4.0

Mars 4.0 was released in August 2010. Enhancements and bug fixes include:

Coming soon: new text editor (as soon as we resolve display issues on Macs).
Improved Instruction Help: All the instruction examples in the help tabs (and new popup instruction guides) now use realistic register names, e.g. $t1, $t2, instead of $1, $2. The instruction format key displayed above the MIPS help tabs has been expanded to include explanations of the various addressing modes for load and store instructions and pseudo-instructions. Descriptions have been added to every example instruction and pseudo-instruction.
Improved Assembly Error Capability: If the assemble operation results in errors, the first error message in the Mars Messages text area will be highighted and the corresponding erroneous instruction will be selected in the text editor. In addition, you can click on any error message in the Mars Messages text area to select the corresponding erroneous instruction in the text editor. The first feature does not select in every situation (such as when assemble-on-open is set) but in the situations where it doesn't work no harm is done plus the second feature, clicking on error messages, can still be used.
Console input syscalls (5, 6, 7, 8, 12) executed in the IDE now receive input keystrokes directly in the Run I/O text area instead of through a popup input dialog. Thanks to Ricardo Pascual for providing this feature! If you prefer the popup dialogs, there is a setting to restore them.
The floor, ceil, trunc and round operations now all produce the MIPS default result 2^31-1 if the value is infinity, NaN or out of 32-bit range. For consistency, the sqrt operations now produce the result NaN if the operand is negative (instead of raising an exception). These cases are all consistent with FCSR (FPU Control and Status Register) Invalid Operation flag being off. The ideal solution would be to simulate the FCSR register itself so all MIPS specs for floating point instructions can be implemented, but that hasn't happened yet.
The Basic column in the Text Segment Window now displays data and addresses in either decimal or hexadecimal, depending on the current settings. Note that the "address" in branch instructions is actually an offset relative to the PC, so is treated as data not address. Since data operands in basic instructions are no more than 16 bits long, their hexadecimal display includes only 4 digits.
The Source column in the Text Segment Window now preserves tab spacing for a cleaner appearance (tab characters were previously not rendered).
Instruction mnemonics can now be used as labels, e.g. "b:".
New syscall 36 will display an integer as an unsigned decimal.
A new tool, Digital Lab Sim, contributed by Didier Teifreto (dteifreto@lifc.univ-fcomte.fr). This tool features two seven-segment displays, a hexadecimal keypad, and a counter. It uses MMIO to explore interrupt-driven I/O in an engaging setting. More information is available from its Help feature. Many thanks!
MARS 4.0 requires Java 1.5 (5.0) instead of 1.4. If this is an issue for you, let me know.

Release 3.8

Mars 3.8 was released in January 2010. Enhancements and bug fixes include:

A new feature to temporarily suspend breakpoints you have previously set. Use it when you feel confident enough to run your program without the breakpoints but not confident enough to clear them! Use the Toggle Breakpoints item in the Run menu, or simply click on the "Bkpt" column header in the Text Segment window. Repeat, to re-activate.
Two new Tools contributed by Ingo Kofler of Klagenfurt University in Austria. One generates instruction statistics and the other simulates branch prediction using a Branch History Table.
Two new print syscalls. Syscall 34 prints an integer in hexadecimal format. Syscall 35 prints an integer in binary format. Suggested by Bernardo Cunha of Portugal.
A new Setting to control whether or not the MIPS program counter will be initialized to the statement with global label 'main' if such a statement exists. If the setting is unchecked or if checked and there is no 'main', the program counter will be initialized to the default starting address. Release 3.7 was programmed to automatically initialize it to the statement labeled 'main'. This led to problems with programs that use the standard SPIM exception handler exceptions.s because it includes a short statement sequence at the default starting address to do some initialization then branch to 'main'. Under 3.7 the initialization sequence was being bypassed. By default this setting is unchecked. This option can be specified in command mode using the 'sm' (Start at Main) option.
Mars Tools that exist outside of Mars can now be included in the Tools menu by placing them in a JAR and including it in a command that launches the Mars IDE. For example: java -cp plugin.jar;Mars.jar Mars Thanks to Ingo Kofler for thinking of this technique and providing the patch to implement it.
Corrections and general improvements to the MIDI syscalls. Thanks to Max Hailperin of Gustavus Adolphus College for supplying them.
Correction to an assembler bug that flagged misidentified invalid MIPS instructions as directives.

Release 3.7

Mars 3.7 was released in August 2009. Enhancements and bug fixes include:

A new feature for changing the address space configuration of the simulated MIPS machine. The 32-bit address space configuration used by all previous releases remains the default. We have defined two alternative configurations for a compact 32KB address space. One starts the text segment at address 0 and the other starts the data segment at address 0. A 32KB address space permits commonly-used load/store pseudo-instructions using labels, such as lw $t0,increment, to expand to a single basic instruction since the label's full address will fit into the 16-bit address offset field without sign-extending to a negative value. This was done in response to several requests over the years for smaller addresses and simplified expansions to make assembly programs easier to comprehend. This release does not include the ability to define your own customized configuration, although we anticipate adding it in the future. It is available both through the command mode (option mc) and the IDE. See Memory Configuration... at the bottom of the Settings menu.
Related to the previous item: load and store pseudo-instructions of the form lw $t0,label and lw $t0,label($t1) will expand to a single instruction (addi for these examples) if the current memory configuration assures the label's full address will fit into the low-order 15 bits. Instructions for which this was implemented are: la, lw, lh, lb, lhu, lbu, lwl, lwr, ll, lwc1, ldc1, l.s, l.d, sw, sh, sb, swl, swr, sc, swc1, sdc1, s.s, and s.d.
If a file contains a global statement label "main" (without quotes, case-sensitive), then execution will begin at that statement regardless of its address. Previously, program execution always started at the base address of the text segment. This will be handy for multi-file projects because you no longer need to have the "main file" opened in the editor in order to run the project. Note that main has to be declared global using the .globl directive.
We have added a Find/Replace feature to the editor. This has been another frequent request. Access it through the Edit menu or Ctrl-F. Look for major enhancements to the editor in 2010!
The syscalls for Open File (13), Read from File (14), and Write to File (15) all now place their return value into register $v0 instead of $a0. The table in Computer Organization and Design's Appendix B on SPIM specifies $a0 but SPIM itself consistently uses $v0 for the return values.
Pseudo-instructions for div, divu, mulo, mulou, rem, remu, seq, sne, sge, sgeu, sgt, sgtu, sle, sleu now accept a 16- or 32-bit immediate as their third operand. Previously the third operand had to be a register.
Existing Tools were tested using reconfigured memory address space (see first item). Made some adaptations to the Keyboard and Display Simulator Tool that allow it to be used for Memory Mapped I/O (MMIO) even under the compact memory model, where the MMIO base address is 0x00007f00 instead of 0xffff0000. Highlighting is not perfect in this scenario.
Bug Fix: The syscall for Open File (13) reversed the meanings of the terms mode and flag. Flags are used to indicate the intended use of the file (read/write). Mode is used to set file permissions in specific situations. MARS implements selected flags as supported by Java file streams, and ignores the mode if specified. For more details, see the Syscalls tab under Help. The file example in that tab has been corrected.
Bug Fix: The assembler incorrectly generated an error on Jump instructions located in the kernel text segment.
Bug Fix: The project (p) option in the command interface worked incorrectly when MARS was invoked within the directory containing the files to be assembled.
Acknowledgement: The development of Release 3.7 was supported by a SIGCSE Special Projects Grant.

What's New in MARS Version 3.6

Mars 3.6 was released in January 2009. Enhancements and bug fixes include:

We've finally implemented the most requested new feature: memory and register cells will be highlighted when written to during timed or stepped simulation! The highlighted memory/register cell thus represents the result of the instruction just completed. During timed or stepped execution, this is NOT the highlighted instruction. During back-stepping, this IS the highlighted instruction. The highlighted instruction is the next one to be executed in the normal (forward) execution sequence.
In conjunction with cell highlighting, we've added the ability to customize the highlighting color scheme and font. Select Highlighting in the Settings menu. In the resulting dialog, you can select highlight background color, text color, and font for the different runtime tables (Text segment, Data segment, Registers). You can also select them for normal, not just highlighted, display by even- and odd-numbered row but not by table.
Cool new Labels Window feature: the table can be sorted in either ascending or descending order based on either the Label (alphanumeric) or the Address (numeric) column. Just click on the column heading to select and toggle between ascending (upright triangle) or descending (inverted triangle). Addresses are sorted based on unsigned 32 bit values. The setting persists across sessions.
The Messages panel, which includes the Mars Messages and Run I/O tabs, now displays using a mono-spaced (fixed character width) font. This facilitates text-based graphics when running from the IDE.
The Mars.jar distribution file now contains all files needed to produce a new jar file. This will make it easier for you to expand the jar, modify source files, recompile and produce a new jar for local use. CreatMarsJar.bat contains the jar instruction.
The Help window now includes a tab for Acknowledgements. This recognizes MARS contributors and correspondents.
We've added a new system call (syscall) for generating MIDI tones synchronously, syscall 33. The original MIDI call returns immediately when the tone is generated. The new one will not return until the tone output is complete regardless of its duration.
The Data Segment display now scrolls 8 rows (half a table) rather than 16 when the arrow buttons are clicked. This makes it easier to view a sequence of related cells that happen to cross a table boundary. Note you can hold down either button for rapid scrolling. The combo box with various data address boundaries also works better now.
Bug Fix: Two corrections to the Keyboard and Display Simulator Tool. Transmitter Ready bit was not being reset based on instruction count when running in the kernel text segment, and the Status register's Exception Level bit was not tested before enabling the interrupt service routine (could lead to looping if interrupts occur w/i the interrupt service routine). Thanks to Michael Clancy and Carl Hauser for bringing these to my attention and suggesting solutions.
Bug Fix: Stack segment byte addresses not on word boundaries were not being processed correctly. This applies to little-endian byte order (big-endian is not enabled or tested in MARS). Thanks to Saul Spatz for recognizing the problem and providing a patch.
Minor Bug Fixes include: Correcting a fault leading to failure when launching MARS in command mode, clarifying assembler error message for too-few or too-many operands error, and correcting the description of lhu and lbu instructions from "unaligned" to "unsigned".

What's New in MARS Version 3.5

Mars 3.5 was released on 24 July 2008. Major enhancements and bug fixes include:

A new Tool, the Keyboard and Display MMIO Simulator, that supports polled and interrupt-driven input and output operations through Memory-Mapped I/O (MMIO) memory. The MIPS program writes to memory locations which serve as registers for simulated devices. Supports keyboard input and a simulated character-oriented display. Click the tool's Help button for more details.
A new Tool, the Instruction Counter, contributed by MARS user Felipe Lessa. It will count the number of MIPS instructions executed along with percentages for R-format, I-format, and J-format instructions. Thanks, Felipe!
Program arguments can now be provided to the MIPS program at runtime, through either an IDE setting or command mode. See the command mode "pa" option for more details on command mode operation. The argument count (argc) is placed in $a0 and the address of an array of null-terminated strings containing the arguments (argv) is placed in $a1. They are also available on the runtime stack ($sp).
Two related changes permit MARS to assemble source code produced by certain compilers such as gcc. One change is to issue warnings rather than errors for unrecognized directives. MARS implements a limited number of directives. Ignore these warnings at your risk, but the assembly can continue. The second change is to allow statement labels to contain, and specifically begin with, '$'.
In command mode, final register values are displayed by giving the register name as an option. Register names begin with '$', which is intercepted by certain OS command shells. The convention for escaping it is not uniform across shells. We have enhanced the options so now you can give the register name without the '$'. For instance, you can use t0 instead of $t0 as the option. You cannot refer to registers by number in this manner, since an integer option is interpreted by the command parser as an instruction execution limit. Thanks to Lucien Chaubert for reporting this problem.
Minor enhancements: The command mode dump feature has been extended to permit memory address ranges as well as segment names. If you enter a new file extension into the Open dialog, the extension will remain available throughout the interactive session. The data segment value repetition operator ':' now works for all numeric directives (.word, .half, .byte, .float, .double). This allows you to initialize multiple consecutive memory locations to the same value. For example:
ones: .half 1 : 8 # Store the value 1 in 8 consecutive halfwords
Major change: Hexadecimal constants containing less than 8 digits will be interpreted as though the leading digits are 0's. For instance, 0xFFFF will be interpreted as 0x0000FFFF, not 0xFFFFFFFF as before. This was causing problems with immediate operands in the range 32768 through 65535, which were misinterpreted by the logical operations as signed 32 bit values rather than unsigned 16 bit values. Signed and unsigned 16 bit values are now distinguished by the tokenizer based on the prototype symbols -100 for signed and 100 for unsigned (mainly logical operations). Many thanks to Eric Shade of Missouri State University and Greg Gibeling of UC Berkeley for their extended efforts in helping me address this situation.
Minor Bug Fixes: round.w.s and round.w.d have been modified to correctly perform IEEE rounding by default. Thanks to Eric Shade for pointing this out. Syscall 12 (read character) has been changed to leave the character in $v0 rather then $a0. The original was based on a misprint in Appendix A of Computer Organization and Design. MARS would not execute from the executable Mars.jar file if it was stored in a directory path those directory names contain any non-ASCII characters. This has been corrected. Thanks to Felipe Lessa for pointing this out and offering a solution. MARS will now correctly detect the EOF condition when reading from a file using syscall 14. Thanks to David Reimann for bringing this to our attention.

What's New in MARS Version 3.4.1

Mars 3.4.1 was released on 23 January 2008. It is a bug fix release to address two bugs.

One bug shows up in pseudo-instructions in which the expansion includes branch instructions. The fixed branch offsets were no longer correct due to changes in the calculation of branch offsets in Release 3.4. At the same time, we addressed the issue of expanding such pseudo-instructions when delayed branching is enabled. Such expansions will now include a nop instruction following the branch.
We also addressed an off-by-one error that occurred in generating the lui instruction in the expansion of conditional branch pseudo-instructions whose second operand is a 32 bit immediate.
The expansions for a number of pseudo-instructions were modified to eliminate internal branches. These and other expansions were also optimized for sign-extended loading of 16-bit immediate operands by replacing the lui/ori or lui/sra sequence with addi. Pseudo-instructions affected by one or both of these modifications include: abs, bleu, bgtu, beq, bne, seq, sge, sgeu, sle, sleu, sne, li, sub and subi. These modifications were suggested by Eric Shade of Missouri State University.

What's New in MARS Version 3.4

A new syscall (32) to support pauses of specified length in milliseconds (sleep) during simulated execution.
Five new syscalls (40-44) to support the use of pseudo-random number generators. An unlimited number of these generators are available, each identified by an integer value, and for each you have the ability to: set the seed value, generate a 32 bit integer value from the Java int range, generate a 32 bit integer value between 0 (inclusive) and a specified upper bound (exclusive), generate a 32-bit float value between 0 (inclusive) and 1 (exclusive), and generate a 64-bit double value between 0 (inclusive) and 1 (exclusive). All are based on the java.util.Random class.
Ten new syscalls (50-59) to support message dialog windows and data input dialog windows. The latter are distinguished from the standard data input syscalls in that a prompting message is specified as a syscall argument and displayed in the input dialog. All are based on the javax.swing.JOptionPane class.
The capability to dump .text or .data memory contents to file in various formats. The dump can be performed before or after program execution from either the IDE (File menu and toolbar) or from command mode. It can also be performed during an execution pause from the IDE. Look for the "Dump Memory" menu item in the File menu, or the "dump" option in command mode. A .text dump will include only locations containing an instruction. A .data dump will include a multiple of 4KB "pages" starting at the segment base address and ending with the last 4KB "page" to be referenced by the program. Current dump formats include pure binary (java.io.PrintStream.write() method), hexadecimal text with one word (32 bits) per line, and binary text with one word per line. An interface, abstract class, and format loader have been developed to facilitate development and deployment of additional dump formats. This capability was prototyped by Greg Gibeling of UC Berkeley.
Changed the calculation of branch offsets when Delayed Branching setting is disabled. Branch instruction target addresses are represented by the relative number of words to branch. With Release 3.4, this value reflects delayed branching, regardless of whether the Delayed Branching setting is enabled or not. The generated binary code for branches will now match that of examples in the Computer Organization and Design textbook. This is a change from the past, and was made after extensive discussions with several MARS adopters. Previously, the branch offset was 1 lower if the Delayed Branching setting was enabled -- the instruction label: beq $0,$0,label would generate 0x1000FFFF if Delayed Branching was enabled and 0x10000000 if it was disabled. Now it will generate 0x1000FFFF in either case. The simulator will always branch to the correct location; MARS does not allow assembly under one setting and simulation under the other.
Bug fix: The mars.jar executable JAR file can now be run from a different working directory. Fix was suggested by Zachary Kurmas of Grand Valley State University.
Bug fix: The problem of MARS hanging while assembling a pseudo-instruction with a label operand that contains the substring "lab", has been fixed.
Bug fix: No Swing-related code will be executed when MARS is run in command mode. This fixes a problem that occured when MARS was run on a "headless" system (no monitor). Swing is the Java library to support programming Graphical User Interfaces. Fix was provided by Greg Gibeling of UC Berkeley.
The '\0' character is now recognized when it appears in string literals.

What's New in MARS Version 3.3

Support for MIPS delayed branching. All MIPS computers implement this but it can be confusing for programmers, so it is disabled by default. Under delayed branching, the next instruction after a branch or jump instruction will always be executed, even if the branch or jump is taken! Many programmers and assemblers deal with this by inserting a do-nothing "nop" instruction after every branch or jump. The MARS assembler does not insert a "nop". Certain pseudo-instructions expand to a sequence that includes a branch; such instructions will not work correctly under delayed branching. Delayed branching is available in command mode with the "db" option.
A new tool of interest mainly to instructors. The Screen Magnifier tool, when selected from the Tools menu, can be used to produce an enlarged static image of the pixels that lie beneath it. The image can be annotated by dragging the mouse over it to produce a scribble line. It enlarges up to 4 times original size.
You now have the ability to set and modify the text editor font family, style and size. Select "Editor..." from the Settings menu to get the dialog. Click the Apply button to see the new settings while the dialog is still open. Font settings are retained from one session to the next. The font family list begins with 6 fonts commonly used across platforms (selected from lists found at www.codestyle.org), followed by a complete list. Two of the six are monospaced fonts, two are proportional serif, and two are proportional sans serif.
The Labels window on the Execute pane, which displays symbol table information, has been enhanced. When you click on a label name or its address, the contents of that address are centered and highlighted in the Text Segment window or Data Segment window as appropriate. This makes it easier to set breakpoints based on text labels, or to find the value stored at a label's address.
If you re-order the columns in the Text Segment window by dragging a column header, the new ordering will be remembered and applied from that time forward, even from one MARS session to the next. The Text Segment window is where source code, basic code, binary code, code addresses, and breakpoints are displayed.
If a MIPS program terminates by "running off the bottom" of the program, MARS terminates, as before, without an exception, but now will display a more descriptive termination message in the Messages window. Previously, the termination message was the same as that generated after executing an Exit syscall.
A new system call (syscall) to obtain the system time is now available. It is service 30 and is not available in SPIM. Its value is obtained from the java.util.Date.getTime() method. See the Syscall tab in MIPS help for further information.
A new system call (syscall) to produce simulated MIDI sound through your sound card is now available. It is service 31 and is not available in SPIM. Its implementation is based on the javax.sound.midi package. It has been tested only under Windows. See the Syscall tab in MIPS help for further information.

What's New in MARS Version 3.0

Bug Fix

SLTI

SLTIU

Additional MIPS Instructions

Over 40 new basic instructions were added to the MIPS-32 instruction set that MARS now assembles and simulates. The total is now 141 basic instructions. It now supports nearly all instructions from Patterson and Hennessy's Computer Organization and Design textbook. Exceptions are the trap and trap-related instructions, and branch instructions that assume delayed branching ("likely").

A large number of pseudo-instructions were also added. All the pseudo-instructions from Appendix A of Patterson and Hennessy's text are now recognized and expanded. A few others were added for convenience (ADDI with 32 bit constant) or fun (SUBI).

Most of the pseudo-instruction additions, however, implement alternative memory addressing modes for the various load and store instructions. Native MIPS load and store instructions support only one addressing mode: base addressing of the form 100($1), where 100 is a 16 bit constant and $1 is a register whose contents are added to the constant to form the effective address.

MARS now recognizes and expands the following addressing modes for all load and store operations:

`($1)`	register contents plus 0 offset
`100000`	32 bit constant
`100000($1)`	register contents plus 32 bit offset
`label`	labeled address
`label+100000`	labeled address plus 32 bit offset
`label($1)`	register contents plus 32 bit labeled address
`label+100000($1)`	register contents plus sum of 32 bit constant and labeled address

A comment concerning 32 bit constants. A pseudo-instruction may expand differently depending on the value of the constant. If it cannot be contained in 16 bits then its high order 16 bits are loaded into the $at register (LUI) prior to processing. We have decided to consistently determine the 16 bit "boundary" based on signed values: -32768 to 32767. Other simulators may interpret the values differently (e.g. unsigned) depending on the context in which they are used.

New IDE features

Ability to step "backward" through simulated MIPS program execution, undoing the results one execution step at a time. Its toolbar icon is a left-pointing arrow with a "1", to bookend the step icon's right-pointing arrow with "1". It will undo as many as the most recent 1000 execution steps. Since each backstep requires a click or keystroke, this should be plenty for debugging purposes. It is also available at the time of normal or abnormal program termination. One limit: this feature will only undo writes to memory, registers or floating point condition flags. SYSCALL operations to external resources (I/O console, files) are not undone by this feature.
The Help feature has been expanded and refined somewhat. The MIPS tab now contains four tabs of its own: a list of basic instructions with descriptions, a list of pseudo instructions and instruction formats with descriptions, a list of assembler directives, and documentation for all available SYSCALL functions and their usage. And just for fun, I've added a tab containing Walter Chang's lyrics to "The MIPS Instruction Set" that can be sung to a Gilbert and Sullivan tune.
In the Text Segment display, the source and corresponding basic statements are now displayed in adjacent columns rather than being concatenated in the same column. Both are still highlighted when doing a stepped or timed run.
A Settings menu has been added, to control several true/false options. The current options and their default values are:
1. Display the Labels Window (symbol table) in the Execute tab. Default value is false. If selected, the Labels Window will be displayed to the right of the Text Segment.
2. Permit programs to use extended (pseudo) instructions and formats. Default value is true. This includes all memory addressing modes other than the MIPS native mode (16 bit constant offset added to register content).
3. Assemble a file automatically as soon as it is opened, and initialize the File Open dialog with the most-recently opened file. Default value is false. This is convenient if you use an external editor for composing your programs.
4. Display memory addresses in hexadecimal. Default value is true. If deselected, addresses will be displayed in decimal. This setting can also be toggled in a checkbox on the border of the Data Segment Window.
5. Display memory and register contents in hexadecimal. Default value is true. If deselected, vlaues will be displayed in decimal. This setting can also be toggled in a checkbox on the border of the Data Segment Window.
A menu item "Clear all breakpoints" has been added to the Run menu. It is enabled whenever one or more execution breakpoints have been set. Much more convenient than having to clear them individually. They can be cleared before, during, or after execution. Note that all breakpoints are automatically cleared each time a file is assembled.
New toolbar and menu item icons.

New Command Line features

np