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wsim (Formerly rexsim)

This project is a full simulator of the WRAMP (Waikato RISC Architecture MicroProcessor) CPU used at the University of Waikato for teaching computer architecture concepts, along with the Basys 3 FPGA that contains the CPU.

Prior versions of this software, written by Paul Monigatti, simulated the Rex board which held an older version of the hardware, while this version simulates the reimplementation. It was updated in tandem with the creation of the reimplementation, so it should accurately reflect the behaviour of the hardware.

Usage

Running wim under Linux (and probably Mac OS, though it hasn't been tested) requires mono to be installed. There have been no compatibility issues reported, so any reasonably recent version should work. It should also work on Windows.

Either double-clicking RexSimulatorGui/bin/Debug/RexSimulatorGui.exe or running $ mono RexSimulatorGui.exe will launch the program.

On first open, two windows will appear: The main board, and the first serial port. This serial port is used for communication with WRAMPmon. Type ? for help, or simply run load to upload a program file. Dragging an .srec into the serial port window will send it, or you can press CTRL-A to open the Send File dialog and browse for your .srec file. Typing go after a file is loaded will run it.

The board's physical interfaces can be interacted with using the main window, and the tick boxes at the bottom of the window will open other useful windows. See the Debugging section below for more details.

Building

To build wsim on Linux, xbuild should be used from the root directory. If you want to build a release copy, use xbuild /p:Configuration=Release. From Windows, the project can be opened in Visual Studio.

Changelog

A changelog can be found in CHANGELOG.md.

Debugging

wsim contains a number of useful debugging features that can help when writing programs for WRAMP. On top of the usual commands included with WRAMPmon (type help in the WRAMPmon prompt for information), wsim offers full read access to all memory locations and registers, as well as breakpoints, single-step mode, and the ability to view the buses and interrupts.

Viewing Registers

To view the contents of the registers, click the check box at the bottom of the form corresponding to the set of registers you are interested in. The General Purpose Registers are those numbered $0~$13 (plus $sp and $ra), while the Special Purpose Registers are the named ones that must be interacted with via the movgs and movsg instructions.

You can also view the registers of each of the I/O devices, using the check boxes in the rightmost column. These can be useful to ensure that the devices actually behave as you expect them to.

All these forms offer decimal, hexadecimal, and binary representations of the numbers contained in the registers.

Viewing Memory

To view the memory, click the check box labelled Memory (RAM). This form has a number of features, but the first one you will notice is that any data contained in the user-writeable memory space (0x0000~0x3FFF) is viewable. The form will show the hexadecimal value, as well as a disassembly of that value. If the data in memory is an instruction, this disassembly will reflect the original assembly code which was used to generate that instruction. As such, this form is useful to trace how your program executes its code.

The leftmost column will show the locations of any special registers that currently point at memory addresses in range, as well as the program counter. To jump to the location of a particular register, click Go To > $regname. Particularly useful here are $pc, $ra, $evec, and $ear. $sp will generally be near the bottom of memory, and can be useful to help visualise your stack frame. Be aware that there are no additional amenities, so you must figure out where your stack frame begins and ends on your own. $rbase and $ptable can generally be ignored, unless you are using the experimental user mode features.

Viewing Interrupts and Buses

Below the picture of the Basys board are a number of words in grey and black. These represent the current state of the WRAMP system.

  • The Address Bus shows which memory address is currently being interacted with.
  • The Data Bus shows what data is being sent to another place in the system (such as a register, or a memory location).
  • The Program Counter is a pointer to the next instruction that will be executed. If this number is larger than 0x80000, then the system is currently executing code that is part of WRAMPmon.

The second row shows the currently active interrupt lines. The word corresponding to a particular interrupt type will turn black when that interrupt is both active and unmasked in $cctrl. The word Interrupts: will be black if any interrupt is active, regardless of the value of $cctrl. This is useful to see if there are any active interrupts which are unmasked, which normally indicates misconfiguration of an I/O device.

Stopping Execution

Stepping slowly through code is one of the most useful methods of debugging WRAMP code. The most basic method to begin this is to click the large green Stop button on the main form. This will pause execution and change the button to a bright red colour, with its text changing to Run. While the system is paused, you can browse the register and memory contents however you desire. You can also interact with the parallel and serial ports, but you should be aware that some actions will not get through to the system while it is paused.

  • Flipping the switches will be persisted, since they stay flipped.
  • Pressing a button will not be persisted, and the system will not notice they were ever pressed.
  • This does not apply to the user interrupt button (green) or either of the reset buttons (red). You will see their effects once you unpause the system.
  • Typing into either of the serial ports will still work, but only the first character typed will make it to the system.

The Single Step button beneath the Stop/Run button will cause the emulated WRAMP CPU to execute a single instruction. The Memory (RAM) form shows the location of the program counter, which is also the next instruction that will execute. This is useful to see the exact environment that a particular instruction is executed in: For example, you can see what the two input registers hold and check that the output is what you expected.

You might also notice the Full Speed check-box. This will cause the program to stop attempting to match the real clock rate of the Basys implementation of WRAMP, and instead just run as fast as your computer will allow. This might come in useful while waiting for long messages to send, large files to load into WRAMPmon, or during other long-running computations.

Breakpoints

If you want to pause execution at a particular instruction in your program, you can set a breakpoint on that instruction. With the Memory (RAM) form open, double-click on any memory address. The Address field will change to show a [B] label next to the address of the instruction, indicating the presence of a breakpoint. When the program counter reaches this instruction, execution will stop as though you pressed the green Stop button. You can then continue as you did by pressing the button yourself, viewing memory and register contents and otherwise interacting with the halted board. You can choose to restart execution, or use the Single Step functionality to continue debugging.