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
nextis invoked again after program execution completion (i.e., after the
HLTinstruction 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
loadcommand 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
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).
cbpclears 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
cbpaclears 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 or register
I1changes. A previously set breakpoint is cleared using the
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
Sets/clears a conditional breakpoint on overflow toggle change.
Sets/clears a conditional breakpoint on comparison flag change.
MIXAL programs can define symbolic constants, using either the
EQUpseudoinstruction or a label at the beginning of a line. Thus, in the program fragmentVAR EQU 2168 ORIG 4000 START LDA VAR
VARstands for the value 2168, while
STARTis assigned the value 4000. The symbol table can be consulted from the
mixvmcommand line using
psymfollowed by the name of the symbol whose contents you are interested in. When run without arguments,
psymwill print all defined symbols and their values.
The virtual machine can also show you the instructions it is executing, using the following commands:
strace onenables instruction tracing. When tracing is enabled, each time the virtual machine executes an instruction (due to your issuing a
nextcommand), 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 offcommand. 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 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
Defines the symbol named SYM with the value resulting from evaluating WEXP, an w-expression. The newly defined symbol can be used in subsequent
wevalcommands, 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 >