The Calyx Interactive Debugger

The Calyx Interactive Debugger is a prototype debugging tool built on top of the Calyx Interpreter which exposes a gdb-like interface for debugging Calyx programs.

Getting Started

If you are using fud, getting started with the debugger is easy.

First, follow these instructions to build the interpreter and register it as a fud stage.

Assuming you are trying to debug a program called my_program.futil with data file, invoke the debugger with the following command:

fud e --to debugger -q my_program.futil -s

This will open the target program in the interactive debugger. Note that we use fud's quiet flag, -q, here. This avoids clashes between fud's outputs and the debugger's outputs, since both tools interact with stdout.

The interpreter does not currently support ref cells. To run the debugger on a program that uses these, you must first compile them away by running the compile-invoke pass:

fud e --to debugger -s calyx.flags '-p compile-invoke' -q my_program.futil -s

Advancing Program execution


The simplest way to advance the program is via the step command which causes time to advance by a clock tick. It also has a shortcode: s.

 > step
 > s

The above snippet advances the program by two steps.


Another way to advance the program is via the step-over command. Unlike the step command, this command requires a second argument which is the name of the group to advance over. The step-over command then advances the program until the given group is no longer running.

If you want to use the command to advance the program past a group group_1, do the following:

 > step-over group_1

Note that the step-over command will do nothing if the given group is not running.

 > step-over other_group
 Group is not running


Finally, the continue command will run the program until either a breakpoint is hit or the program terminates. This command is used in conjunction with breakpoints and watchpoints to provide more targeted inspection. It may also be accessed with the shortcode c.

 > continue
Main component has finished executing. Debugger is now in inspection mode.


CIDR supports breakpoints on group definitions. This helps focus attention on suspect portions of the code.

Setting a breakpoint

Breakpoints may be set on the main component by simple specifying the group of interest.

 > break group_1

This is identical to

 > break main::group_1

For sub-components, the name of the sub-component must be included with the double colon separating the group name. To break on the do_mul group inside the pow sub-component:

 > break pow::do_mul

Managing breakpoints

To see a list of breakpoints:

 > info break


 > ib

This produces output like this:

 > ib
     Current breakpoints:
    1.  main::group_1  enabled
    2.  pow::do_mul enabled

All breakpoints have a number associated with them and they may be managed with this number or the group name.

To enable or disable a breakpoint:

 > disable group_1 2
 > enable 1 pow::do_mul

Note that this is equivalent to:

 > disable group_1
 > disable 2
 > enable 1
 > enable pow::do_mul

To delete a breakpoint:

 > delete 1
 > del pow::do_mul

Deleted breakpoints will be entirely removed while disabled breakpoints will remain until they are either enabled again or subsequently deleted. Disabled breakpoints will not cause program execution to halt when continue-ing.

Inspecting State


The display command dumps the full state of the main component without formatting. Use the print and print-state commands for targeted inspection with formatting.

Formatting codes

CIDR supports several different formatting codes which do the hard work of interpreting the data in human readable ways.

binaryThe default, a bit vector with the msb on the left
unsigned\uUnsigned bit-num formatting
signed\sTwo's Complement formatting
unsigned fixedpoint\u.NFor N >=1. Unsigned Fixed-point with N fractional bits. The remaining bits are for the integral component.
signed fixedpoint\s.NFor N >=1. Signed Fixed-point with N fractional bits. The remaining bits are for the integral component.

These commands allow inspecting instance state with optional formatting. Note that this is different from breakpoints which operate on definitions. For example to print the ports of the std_mul instance named mul in the pow instance pow_1 attached to the main component:

 > print main.pow_1.mul

as with breakpoints, the leading main may be elided:

 > print pow_1.mul

This will print all the ports attached to this multiplier instance with binary formatting.

Formatting codes may be supplied as the first argument.

 > print \u pow_1.mul

The print may also target specific ports on cells, rather than just the cell itself. To see only the output of the multiplier (with unsigned formatting):

 > print \u pow_1.mul.out

The print-state command works in the same way as the print command, except it displays the internal state of a cell, rather than port values. As such, it can only target cells and only those with some internal state, such as registers or memories. For example, if the main component has a memory named out_mem its contents may be viewed via:

 > print-state main.out_mem

or just

 > print-state out_mem

As with print, print-state supports formatting codes as an optional first argument. So to view the contents of out_mem with a signed interpretation:

 > print-state \s out_mem


Watchpoints are like breakpoints but rather than stop the execution when they are passed, they instead print out some information. Like breakpoints, they are set on group definitions, such as main::group_1 or pow::do_mul

Setting watchpoints

The general form of watchpoints looks like



  • GROUP is the group definition to be watched
  • PRINT-COMMAND is a full print or print-state command to be run by the watchpoint

The optional POSITION argument may either be before or after. This specifies whether the watchpoint should run when the group first becomes active (before) or when the group finishes running (after). This defaults to before if not set.

Managing watchpoints

Watchpoint management is similar to breakpoints. However there may be multiple watchpoints for a single group definition, so deleting watchpoints via the group name will delete all the watchpoints associated with the group. Watchpoints do not currently have an enable/disable state.

To view all the watchpoint definitions:

 > info watch


 > iw

To delete watchpoints:

 > delete-watch 1
 > del-watch main::group_1

Viewing the program counter

There where command (alias pc) displays the currently running portion of the control tree including active subcomponents. This can be used to more easily determine the currently active portion of the design as well as visualize how much of the execution is occurring in parallel at any given point.

Exiting the debugger

Use help to see all commands. Use exit to exit the debugger.