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6.2.2 Debug commands

Sequential execution of loaded programs can be interrupted using the following debug commands:

debug command: next [ins_number]

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).

debug command: sbp line_number
debug command: cbp line_no

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

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.

debug command: spba address
debug command: cbpa address

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.

debug command: sbpr A | X | J | Ii
debug command: cbpr A | X | J | Ii

Sets a conditional breakpoint on the specified register change. For instance,

sbpr I1

will cause an interruption during program execution whenever the contents or register I1 changes. A previously set breakpoint is cleared using the cbpr command.

debug command: sbpm address
debug command: cbpm address

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 or of the memory cell number 1000 changes. A previously set breakpoint is cleared using the cbpm command.

debug command: sbpo
debug command: cbpo

Sets/clears a conditional breakpoint on overflow toggle change.

debug command: sbpc
debug command: cbpc

Sets/clears a conditional breakpoint on comparison flag change.

debug command: cabp

Clears all currently set breakpoints.

debug command: psym [symbol_name]

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

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:

debug command: strace [on|off]

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)
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

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.

debug command: pline [LINE_NUMBER]

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
debug command: pbt [INS_NUMBER]

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
    JMP *+1

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

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:

debug command: weval WEXP

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)

New symbols can be defined using the ssym command:

debug command: ssym SYM WEXP

Defines the symbol named SYM with the value resulting from evaluating WEXP, an 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)

Finally, if you want to discover which is the decimal value of a MIX word expressed as five bytes plus sign, you can use

debug command: w2d WORD

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
MIX > weval -16777346
- 01 00 00 02 02 (0016777346)

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