C H A P T E R 6


OpenBoot provides debugging tools that include a disassembler, register display commands, and breakpoint commands.

Using the Disassembler

The built-in disassembler translates the contents of memory into equivalent SPARC assembly language.

lists commands that disassemble memory into equivalent op codes.

TABLE 6-1 Disassembler Commands


Stack Diagram



( -- )

Continue disassembling where the last disassembly left off.


( addr -- )

Begin disassembling at the specified address.

dis begins to disassemble the data content of any desired location. The system pauses when:

Disassembly can then be stopped or the +dis command can be used to continue disassembling at the location where the last disassembly stopped.

Memory addresses are normally shown in hexadecimal. However, if a symbol table is present, memory addresses are displayed symbolically whenever possible.

Displaying Registers

You can enter the User Interface from the middle of an executing program as a result of a program crash, a user abort with Stop-A , or an encountered breakpoint. (Breakpoints are discussed on Breakpoints .) In all these cases, the User Interface automatically saves all the CPU data register values in a buffer area. You can then inspect or alter these values for debugging purposes.

lists the SPARC register commands.

TABLE 6-2 SPARC Register Commands


Stack Diagram


%f0 through %f31

( -- value )

Return the value in the specified floating point register.


( -- value )

Return the value in the floating point status register.

%g0 through %g7

( -- value )

Return the value in the specified global register.

%i0 through %i7

( -- value )

Return the value in the specified input register.

%l0 through %l7

( -- value )

Return the value in the specified local register.

%o0 through %o7

( -- value )

Return the value in the specified output register.

%pc %npc %psr
%y %wim %tbr

( -- value )

Return the value in the specified register.


( -- )

Display the values in %f0 through %f31 .


( -- )

Display the values in the i , l and o registers.


( -- )

Formatted display of the program status register.


( -- )

Display values in %g0 through %g7 , plus %pc , %npc , %psr , %y , %wim , %tbr .


( window# -- )

Same as w .locals ; display the desired window.


( -- )

Display the return stack showing C subroutines.


( new-value -- )

Set %pc to new-value , and set %npc to ( new-value +4).

to regname

( new-value -- )

Change the value stored in any of the registers above.
Use in the form: new-value to regname .


( window# -- )

Set the current window for displaying %i x , %L x , or %o x .

After the values have been inspected and/or modified, program execution can be continued with the go command. The saved (and possibly modified) register values are copied back into the CPU, and execution resumes at the location specified by the saved program counter.

If you change %pc with to , you should also change %npc . (It is easier to use set-pc , which changes both registers automatically.)

For the w and .window commands, a window value of 0 usually specifies the current window--that is, the active window for the subroutine where the program was interrupted. A value of 1 specifies the window for the caller of this subroutine, 2 specifies the caller's caller, and so on, up to the number of active stack frames. The default starting value is 0.


The User Interface provides a breakpoint capability to assist in the development and debugging of stand-alone programs. (Programs that run under the operating system generally do not use this feature, but use other debuggers designed to run under the operating system.) The breakpoint feature lets you stop the test program at desired points. After program execution has stopped, registers or memory can be inspected or changed, and new breakpoints can be set or cleared. You can resume program execution with the go command.

lists the breakpoint commands that control and monitor program execution.

TABLE 6-3 Breakpoint Commands


Stack Diagram



( addr -- )

Add a breakpoint at the specified address.


( addr -- )

Remove the breakpoint at the specified address.


( -- )

Remove the most-recently-set breakpoint.


( -- )

Display all currently set breakpoints.


( -- )

Perform a specified action when a breakpoint occurs. This word can be altered to perform any desired action. For example, to display registers at every breakpoint, type: ['] .registers is .breakpoint . The default behavior is .instruction . To perform multiple behaviors, create a single definition which calls all desired behaviors, then load that word into .breakpoint .


( -- )

Display the address, opcode for the last-encountered breakpoint.


( -- )

Perform a specified action when a single step occurs (see .breakpoint ).


( -- )

Remove all breakpoints.


( -- )

Execute until the end of this loop.


( -- )

Continue from a breakpoint. This can be used to go to an arbitrary address by setting up the processor's program counter before issuing go .


( n -- )

Execute go n times.


( -- )

(Like the step command.) Treat a subroutine call as a single instruction.


( n -- )

Execute hop n times.


( -- )

Execute until the end of this subroutine.


( -- )

Execute until the end of this leaf subroutine.


( -- )

Skip (do not execute) the current instruction.


( -- )

Single-step one instruction.


( n -- )

Execute step n times.


( addr -- )

Execute until the given address is encountered. Equivalent to +bp go .

To debug a program using breakpoints, use the following procedure.

1. Load the test program into memory at location 4000 (hex).

See Chapter 5 for more information. Using dload is generally best, since the symbol table for the program is preserved. boot -h also works if the program is not available over Ethernet.

The values for %pc and all other registers are initialized automatically.

2. (Optional) Disassemble the downloaded program to verify a properly-loaded file.

3. Begin single-stepping the test program using the step command.

You can also set a breakpoint, then execute (for example, using the commands 4020 +bp and go ) or perform other variations.

The Forth Source-level Debugger

The Forth Source-level Debugger allows single-stepping and tracing of Forth programs. Each step represents the execution of one Forth word.

The debugger commands are shown in .

TABLE 6-4 Forth Source-level Debugger Commands




"Continue". Switch from stepping to tracing, thus tracing the remainder of the execution of the word being debugged.


"Down a level". Mark for debugging the word whose name was just displayed, then execute it.


Start a subordinate Forth interpreter. When that interpreter exits (with resume ), control returns to the debugger at the place where the F command was executed.


"Quit". Abort the execution of the word being debugged and all its callers and return to the command interpreter.


"Up a level". Un-mark the word being debugged, mark its caller for debugging, and finish executing the word that was previously being debugged.

debug name

Mark the specified Forth word for debugging. Enter the Forth Source-level Debugger on all subsequent attempts to execute name . After executing debug , the execution speed of the system may decrease until debugging is turned off with debug-off . (Do not debug basic Forth words such as ".".)


Turn off the Forth Source-level Debugger so that no word is being debugged.


Exit from a subordinate interpreter, and go back to the stepper (see the F command in this table).


Set "step mode" for the Forth Source-level Debugger, allowing the interactive, step-by-step execution of the word being debugged. Step mode is the default.


Set "trace mode" for the Forth Source-level Debugger. This traces the execution of the word being debugged, while showing the name and stack contents for each word called by that word.


Execute the word just displayed and proceed to the next word.

Every Forth word is defined as a series of one or more words that could be called "component" words. While debugging a specified word, the debugger displays information about the contents of the stack while executing each of the word's "component" words. Immediately before executing each component word, the debugger displays the contents of the stack and the name of the component word that is about to be executed.

In trace mode, that component word is then executed, and the process continues with the next component word.

In step mode (the default), the user controls the debugger's execution response. Before the execution of each component word, the user is prompted for one of the keystrokes specified in .

Using ftrace

The ftrace command shows the sequence of Forth words that were being executed at the time of the last exception. An example of ftrace follows.

ok : test1 1 ! ; 
ok : test2 1 test1 ; 
ok test2 
Memory address not aligned
ok ftrace 
! 		Called from test1 	at ffeacc5c
test1 		Called from test2 	at ffeacc6a
(ffe8b574) 	Called from (interpret 	at ffe8b6f8
execute 			Called from catch 	at ffe8a8ba
catch 			 Called from (fload) 	at ffe8ced8
(fload) 			Called from interact 	at ffe8cf74
execute 	Called from catch 	at ffe8a8ba
catch 		Called from (quit 	at ffe8cf98

In this example, test2 calls test1 , which tries to store a value to an unaligned address. This results in the exception: Memory address not aligned .

The first line of ftrace 's output shows the last command that caused the exception to occur. The next lines show locations from which the subsequent commands were being called.

The last thirteen lines are usually the same in any ftrace output, because that is the calling sequence in effect when the Forth interpreter interprets a word from the input stream.

Copyright © 2001, Sun Microsystems, Inc. All rights reserved.