Using the Disassembler

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

Table 6-1 lists commands that disassemble memory into equivalent opcodes.

Table 6-1

Command	Stack Diagram	Description
+dis	( -- )	Continue disassembling where the last disassembly left off.
dis	( adr -- )	Begin disassembling at the given address.

dis begins to disassemble the data content of any desired location. The system pauses either if any key is pressed while disassembly is taking place or after every page of output. At that point, disassembly can be continued or stopped. Disassembly stops automatically when a call or jmp opcode is encountered. You can use the +dis command 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 Forth Monitor 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 page 89.) In all these cases, the Forth Monitor automatically saves all the CPU data register values in a buffer area. These values can then be inspected or altered for debugging purposes.

Table 6-2 lists the SPARC register commands.

Table 6-2

Command	Stack Diagram	Description
%f0 through %f31	( -- value )	Return the value in the given floating point register.
%fsr	( -- value )	Return the value in the given floating point register.
%g0 through %g7	( -- value )	Return the value in the given register.
%i0 through %i7	( -- value )	Return the value in the given register.
%L0 through %L7	( -- value )	Return the value in the given register.
%o0 through %o7	( -- value )	Return the value in the given register.
%pc %npc %psr	( -- value )	Return the value in the given register.
%y %wim %tbr	( -- value )	Return the value in the given register.
.fregisters	( -- )	Display values in %f0 through %f31.
.locals	( -- )	Display the values in the i, L and o registers.
.psr	( -- )	Formatted display of the %psr data.
.registers	( -- )	Display values in %g0 through %g7, plus %pc, %npc, %psr, %y, %wim, %tbr.
.window	( window# -- )	Same as w .locals; display the desired window.
ctrace	( -- )	Display the return stack showing C subroutines.
set-pc	( value -- )	Set %pc to the given value, and set %npc to (value+4).
to regname	( value -- )	Change the value stored in any of the above registers. Use in the form: value to regname.
w	( window# -- )	Set the current window for displaying %ix, %Lx, or %ox.

After you inspect the values, you can continue program execution by typing the go command. The saved register values are copied back into the CPU, and execution resumes at the location specified by the saved program counter.

These saved register values can be changed, if needed, with the to command. When execution is resumed, the changed values are copied back into the CPU and used.

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.

Breakpoints

The Forth Monitor provides a breakpoint capability to assist in the development and debugging of standalone 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.

Table 6-3 lists the breakpoint commands that control and monitor program execution.

Table 6-3

Command	Stack Diagram	Description
+bp	( adr -- )	Add a breakpoint at the given address.
-bp	( adr -- )	Remove the breakpoint at the given address.
--bp	( -- )	Remove the most-recently-set breakpoint.
.bp	( -- )	Display all currently set breakpoints.
.breakpoint	( -- )	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.
.instruction	( -- )	Display the address, opcode for the last-encountered breakpoint.

Table 6-3

Command	Stack Diagram	Description
.step	( -- )	Perform a specified action when a single step occurs (see .breakpoint).
bpoff	( -- )	Remove all breakpoints.
finish-loop	( -- )	Execute until the end of this loop.
go	( -- )	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.
gos	( n -- )	Execute go n times.
hop	( -- )	(Like the step command.) Treat a subroutine call as a single instruction.
hops	( n -- )	Execute hop n times.
return	( -- )	Execute until the end of this subroutine.
returnL	( -- )	Execute until the end of this leaf subroutine.
skip	( -- )	Skip (do not execute) the current instruction.
step	( -- )	Single-step one instruction.
steps	( n -- )	Execute step n times.
till	( adr -- )	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, "Loading and Executing Programs," 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.

Table 6-4

Command	Description
C	"Continue". Switch from stepping to tracing, thus tracing the remainder of the execution of the word being debugged.
D	"Down a level". Mark for debugging the word whose name was just displayed, then execute it.
F	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.
Q	"Quit". Abort the execution of the word being debugged and all its callers and return to the command interpreter.
U	"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 named Forth word for debugging. Enter the Forth Source-level Debugger with any subsequent attempts to execute that word. 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 ".".)
debug-off	Turn off the Forth Source-level Debugger so that no word is being debugged.
resume	Exit from a subordinate interpreter, and go back to the stepper (see the F command in this table).
stepping	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.
tracing	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.
Space	Execute the word just displayed and proceed to the next word.

Before the execution of each word called by the word that is being debugged, the contents of the stack are displayed, followed by the name of the word that is about to be executed. In trace mode, that word is then executed, and the process continues with the next word called by the debugged word. In step mode (the default), the user controls execution: before the execution of each word called by the debugged word, the user is prompted for one of the specified keystrokes.

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 ffefeff0 0 ffefebdc catch Called from (fload) at ffe8ced8 0 (fload) Called from interact at ffe8cf74 execute Called from catch at ffe8a8ba ffefefd4 0 ffefebdc 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.