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
In summary, a lot! Version 3.0 fixes one bug from Version 2.2 (February 2006) and adds a number of instructions and features.
Bug Fix
The SLTI and SLTIU basic instructions were incorrectly coded as R-format (register) rather than I-format (immediate) instructions. This is now corrected.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:
- 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.
- 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).
- 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.
- 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.
- 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.