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Sequential execution of loaded programs can be interrupted using the following debug commands:
This command causes the virtual machine to fetch and execute up to
ins_number instructions, beginning from the current program
counter position. Execution is interrupted either when the specified
number of instructions have been fetched or a breakpoint is found,
whatever happens first. If run without arguments, one instruction is
executed. If next is invoked again after program execution
completion (i.e., after the HLT instruction has been found in a
previous run), the program counter is repositioned and execution starts
again from the beginning (as a matter of fact, a load command
preserving the currently set breakpoints is issued before resuming
execution).
Sets a breakpoint at the specified source file line number. If the line
specified corresponds to a command or to a MIXAL pseudoinstruction which
does not produce a MIX instruction in the binary file (such as
ORIG or EQU) the breakpoint is set at the first source
code line giving rise to a MIX instruction after the specified
one. Thus, for our sample hello.mixal file:
* (1)
* hello.mixal: say 'hello world' in MIXAL (2)
* (3)
* label ins operand comment (4)
TERM EQU 19 the MIX console device number (5)
ORIG 1000 start address (6)
START OUT MSG(TERM) output data at address MSG (7)
...
trying to set a breakpoint at line 5, will produce the following result:
MIX > sbp 5 Breakpoint set at line 7 MIX >
since line 7 is the first one compiled into a MIX instruction (at address 3000).
The command cbp clears a (previously set) breakpoint at the given
source file line.
Sets a breakpoint at the given memory address. The argument must
be a valid MIX memory address, i.e., it must belong into the range
[0-3999]. Note that no check is performed to verify that the
specified address is reachable during program execution. No debug
information is needed to set a breakpoint by address with sbpa.
The command cbpa clears a (previously set) breakpoint at the
given memory address.
Sets a conditional breakpoint on the specified register change. For instance,
sbpr I1
will cause an interruption during program execution whenever the
contents of register I1 changes. A previously set breakpoint is
cleared using the cbpr command.
Sets a conditional breakpoint on the specified memory cell change. The argument must be a valid MIX memory address, i.e., it must belong into the range [0-3999]. For instance,
sbpm 1000
will cause an interruption during program execution whenever the
contents of the memory cell number 1000 changes. A previously set
breakpoint is cleared using the cbpm command.
Sets/clears a conditional breakpoint on overflow toggle change.
Sets/clears a conditional breakpoint on comparison flag change.
Clears all currently set breakpoints.
MIXAL programs can define symbolic constants, using either the
EQU pseudoinstruction or a label at the beginning of a
line. Thus, in the program fragment
VAR EQU 2168
ORIG 4000
START LDA VAR
the symbol VAR stands for the value 2168, while START is
assigned the value 4000. The symbol table can be consulted from
the mixvm command line using psym followed by the name of
the symbol whose contents you are interested in. When run without
arguments, psym will print all defined symbols and their values.
The virtual machine can also show you the instructions it is executing, using the following commands:
strace on enables instruction tracing. When tracing is enabled,
each time the virtual machine executes an instruction (due to your
issuing a run or next command), it is printed in its
canonical form (that is, with all expressions evaluated to their
numerical values) and, if the program was compiled with debug
information, as it was originally typed in the MIXAL source
file. Instruction tracing is disabled with strace off
command. A typical tracing session could be like this:
MIX > strace on MIX > next 3000: [OUT 3002,0(2:3)] START OUT MSG(TERM) MIXAL HELLO WORLD Elapsed time: 1 /Total program time: 1 (Total uptime: 1) MIX > next 3001: [HLT 0,0] HLT End of program reached at address 3002 Elapsed time: 10 /Total program time: 11 (Total uptime: 11) MIX > strace off MIX >
The executed instruction, as it was translated, is shown between square brackets after the memory address, and, following it, you can see the actual MIXAL code that was compiled into the executed instruction. The tracing behaviour is stored as a configuration parameter in ~/.mdk.
Prints the requested source line (or the current one if line_number is omitted:
MIX > load ../samples/hello Program loaded. Start address: 3000 MIX > pline Line 5: START OUT MSG(TERM) MIX > pline 6 Line 6: HLT MIX >
This command changes the limit for the backtrace of executed instructions. If the number is omitted, the command prints the current limit. If you use a 0, backtraces are turned off. This can improve performance. If you wish for all the instructions to be logged, a -1 will enable that. The amount of memory required for unlimited backtraces can be substantial for long-running programs.
This command prints a backtrace of executed instructions. Its optional argument ins_number is the number of instructions to print. If it is omitted or equals zero, all executed instructions are printed. For instance, if you compile and load the following program (bt.mixal):
ORIG 0
BEG JMP *+1
JMP *+1
FOO JMP BAR
BAR HLT
END BEG
you could get the following traces:
MIX > load bt Program loaded. Start address: 0 MIX > next MIX > pbt #0 BEG in bt.mixal:2 MIX > next MIX > pbt #0 1 in bt.mixal:3 #1 BEG in bt.mixal:2 MIX > run Running ... ... done MIX > pbt 3 #0 BAR in bt.mixal:5 #1 FOO in bt.mixal:4 #2 1 in bt.mixal:3 MIX > pbt #0 BAR in bt.mixal:5 #1 FOO in bt.mixal:4 #2 1 in bt.mixal:3 #3 BEG in bt.mixal:2 MIX >
Note that the executed instruction trace gives you the label of the executed line or, if it has no label, its address.
As you have probably observed, mixvm prints timing statistics
when running programs. This behaviour can be controlled using the
stime command (see Configuration commands).
mixvm is also able of evaluating w-expressions
(see W-expressions) using the following command:
Evaluates the given w-expression, WEXP. The w-expression can contain any currently defined symbol. For instance:
MIX > psym START + 00 00 00 46 56 (0000003000) MIX > weval START(0:1),START(3:4) + 56 00 46 56 00 (0939716096) MIX >
New symbols can be defined using the ssym command:
Defines the symbol named SYM with the value resulting from
evaluating WEXP, a w-expression. The newly defined symbol can be
used in subsequent weval commands, as part of the expression to
be evaluated. E.g.,
MIX > ssym S 2+23*START + 00 00 18 19 56 (0000075000) MIX > psym S + 00 00 18 19 56 (0000075000) MIX > weval S(3:4) + 00 00 19 56 00 (0000081408) MIX >
Finally, if you want to discover which is the decimal value of a MIX word expressed as five bytes plus sign, you can use
Computes the decimal value of the given word. WORD must be
expressed as a sign (+/-) followed by five space-delimited, two-digit
decimal values representing the five bytes composing the word. The
reverse operation (showing the word representation of a decimal value)
can be accomplished with weval. For instance:
MIX > w2d - 01 00 00 02 02 -16777346 MIX > weval -16777346 - 01 00 00 02 02 (0016777346) MIX >
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