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.

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".

Mars 3.5 was released in August 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.

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.

Mars 3.4 was released in January 2008. Major enhancements are:

• 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 occurred 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.

MARS 3.3 was released in July 2007. Major enhancements are:

• 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.

MARS 3.2.1 was released in January 2007. It is a bug fix release that addresses the following bug in 3.2: a NullPointerException occurs when MIPS program execution terminates by "dropping off the bottom" of the program rather than by using one of the Exit system calls.

MARS 3.2 was released in December 2006. Major enhancements are:

• It fixes several minor bugs, including one that could cause incorrect file sequencing in the Project feature.
• It includes the AbstractMarsToolAndApplication abstract class to serve as a framework for easily constructing "tools" and equivalent free-standing applications that use the MARS assembler and simulator engines (kudos to the SIGCSE 2006 audience member who suggested this capability!). A subclass of this abstract class can be used both ways (tool or application).
• The floating point and data cache tools were elaborated in this release and a new tool for animating and visualizing memory references was developed. All are AbstractMarsToolAndApplication subclasses.
• This release includes support for exception handlers: the kernel data and text segments (.kdata and .ktext directives), the MIPS trap-related instructions, and the ability to automatically include a selected exception (trap) handler with each assemble operation.
• Items in the Settings menu became persistent with this release.
• Added default assembly file extensions "asm" and "s" to the Config.properties file and used those not only to filter files for the File Open dialog but also to filter them for the "assemble all" setting.
• Implemented a limit to the amount of text scrollable in the Mars Messages and Run I/O message tabs - default 1 MB is set in the Config.properties file.
• For programmer convenience, labels can now be referenced in the operand field of integer data directives (.word, .half, .byte). The assembler will substitute the label's address (low order half for .half, low order byte for .byte).
• For programmer convenience, character literals (e.g. 'b', '\n', '\377') can be used anywhere that integer literals are permitted. The assembler converts them directly to their equivalent 8 bit integer value. Unicode is not supported and octal values must be exactly 3 digits ranging from '\000' to '\377'.
• Replaced buttons for selecting Data Segment display base addresses with a combo box and added more base addresses: MMIO (0xFFFF0000), .kdata (0x90000000), .extern (0x10000000).

MARS 3.1 was released in October 2006. The major issues and features are listed here:

• It addressed several minor limits (Tools menu items could not be run from the JAR file, non-standard shortcuts for Mac users, inflexible and sometimes inappropriate sizing of GUI components).
• It changed the way SYSCALLs are implemented, to allow anyone to define new customized syscall services without modifying MARS.
• It added a primitive Project capability through the "Assemble operation applies to all files in current directory." setting (also available as "p" option in command mode). The command mode also allows you to list several file names not necessarily in the same directory to be assembled and linked.
• Multi-file assembly also required implementing the ".globl" and ".extern" directives.
• And although "Mars tools" are not officially part of MARS releases, MARS 3.1 includes the initial versions of two tools: one for learning about floating point representation and another for simulating data caching capability.

MARS 3.0 was released in August 2006, with one bug fix and two major additions.

• The bug fix was corrected instruction format for the slti and sltiu instructions.
• One major addition is a greatly expanded MIPS-32 instruction set (trap instructions are the only significant ones to remain unimplemented). This includes, via pseudo-instructions, all reasonable memory addressing modes for the load/store instructions.
• The second major addition is ability to interactively step "backward" through program execution one instruction at a time to "undo" execution steps. It will buffer up to 2000 of the most recent execution steps (this limit is stored in a properties file and can be changed). It will undo changes made to MIPS memory, registers or condition flags, but not console or file I/O. This should be a great debugging aid. It is available anytime execution is paused and at termination (even if terminated due to exception).
• A number of IDE settings, described above, are now available through the Settings menu.

MARS 2.2 was released in March 2006 with additional bug fixes and implemented command line options (run MARS from command line with h option for command line help). This also coincides with our SIGCSE 2006 paper "MARS: An Education-Oriented MIPS Assembly Language Simulator".

MARS 2.1 was released in October 2005 with some bug fixes.

MARS 2.0 was released in September 2005. It incorporated significant modifications to both the GUI and the assembler, floating point registers and instructions most notably.

MARS 1.0 was released in January 2005 and publicized during a poster presentation at SIGCSE 2005.

Dr. Ken Vollmar initiated MARS development in 2002 at Missouri State University. In 2003, Dr. Pete Sanderson of Otterbein College and his student Jason Bumgarner continued implementation. Sanderson implemented the assembler and simulator that summer, and the basic GUI the following summer, 2004.

The development of Releases 3.1 and 3.2 during fall 2006 was supported by the Otterbein College sabbatical leave program. The development of Release 3.7 during summer 2009 was supported by a SIGCSE Special Projects Grant.

This document is available for printing on the MARS home page http://www.cs.missouristate.edu/MARS/.