path: root/doc/mdk_mixvm.texi

@c -*-texinfo-*-
@c This is part of the GNU MDK Reference Manual.
@c Copyright (C) 2000, 2001, 2002, 2003, 2004
@c   Free Software Foundation, Inc.
@c See the file mdk.texi for copying conditions.

@c $Id: mdk_mixvm.texi,v 1.20 2005/09/19 21:29:42 jao Exp $

@node mixvm, gmixvm, mixasm, Top
@comment  node-name,  next,  previous,  up
@chapter @code{mixvm}, the MIX computer simulator

@cindex mixvm

This chapter describes @code{mixvm}, the MIX computer
simulator. @code{mixvm} is a command line interface programme which
simulates the MIX computer (@pxref{The MIX computer}). It is able
to run MIXAL programs (@pxref{MIXAL}) previously compiled with the MIX
assembler (@pxref{mixasm}). The simulator allows inspection of the MIX
computer components (registers, memory cells, comparison flag and overflow
toggle), step by step execution of MIX programmes, and breakpoint
setting to aid you in debugging your code. For a tutorial description of
@code{mixvm} usage, @xref{Running the program}.

@menu
* Invocation::
* Commands::                    Commands available in interactive mode.
* Devices::                     MIX block devices implementation.
@end menu

@node Invocation, Commands, mixvm, mixvm
@comment  node-name,  next,  previous,  up
@section Invoking @code{mixvm}

@code{mixvm} can be invoked with the following command line options
(note that, following GNU's conventions, we provide a long option name
for each available single letter switch):

@example
mixvm [-vhurdtq] [--version] [--help] [--usage] [--run] [--dump]
      [--time] [--noinit]  [FILE[.mix]]
@end example

@noindent
The meaning of these options is as follows:

@defopt -v
@defoptx --version
Prints version and copyleft information and exits.
@end defopt

@defopt -h
@defoptx --help
@defoptx -u
@defoptx --usage
Prints a summary of available options and exits.
@end defopt

@defopt -r
@defoptx --run
Loads the specified @var{FILE} and executes it. After the program
execution, @code{mixvm} exits. @var{FILE} must be the name of a binary
@file{.mix} program compiled with @code{mixasm}. If your program does
not produce any output, use the @code{-d} flag (see below) to peek at
the virtual machine's state after execution.
@end defopt

@defopt -d
@defoptx --dump
This option must be used in conjuction with @code{-r}, and tells
@code{mixvm} to print the value of the virtual machine's registers,
comparison flag and overflow toggle after executing the program named
@var{FILE}. See @xref{Non-interactive mode}, for sample usage.
@end defopt

@defopt -t
@defoptx --time
This option must be used in conjuction with @code{-r}, and tells
@code{mixvm} to print virtual time statistics for the program's
execution.
@end defopt

When run without the @code{-r} flag, @code{mixvm} enters its interactive
mode, showing you a prompt like this one:

@example
MIX >
@end example

@noindent
and waiting for your commands (@pxref{Commands}). If the
optional @var{FILE} argument is given, the file @file{FILE.mix} will be
loaded into the virtual machine memory before entering the interactive
mode.

The first time @code{mixvm} is invoked, a directory named @file{.mdk} is
created in your home directory. It contains the @code{mixvm}
configuration file, the command history file and (by default) the block
devices files (@pxref{Devices}). Before showing you the command prompt,
@code{mixvm} looks in the @file{~/.mdk} directory for a file named
@code{mixguile.scm}; if it exists, it is read and evaluated by the
embedded Guile interpreter (@pxref{Defining new functions}). You can use
the @code{-q} command line option to skip this file loading:

@defopt -q
@defoptx --noinit
Do not load the Guile initialisation file @code{~/.mdk/mixguile.scm} at
startup.
@end defopt

@node Commands, Devices, Invocation, mixvm
@comment  node-name,  next,  previous,  up
@section Interactive commands

You can enter the interactive mode of the MIX virtual machine by simply
invoking @code{mixvm} without arguments. You will then be greeted by a shell
prompt@footnote{The default command prompt, @samp{MIX > }, can be
changed using the @code{prompt} command (@pxref{Configuration commands})}

@example
MIX >
@end example

@noindent
which indicates that a new virtual machine has been initialised and is
ready to execute your commands. As we have already mentioned, this
command prompt offers you command line editing facilities which are
described in the Readline user's manual (chances are that you are
already familiar with these command line editing capabilities, as they
are present in many GNU utilities, e.g. the @code{bash}
shell)@footnote{The readline functionality will be available if you have
compiled @sc{mdk} with readline support, i.e., if GNU readline is
installed in your system. This is ofte the case in GNU/Linux and BSD
systems}. In a nutshell, readline provides command completion using the
@kbd{TAB} key and command history using the cursor keys. A history file
containing the last commands typed in previous sessions is stored in the
@sc{mdk} configuration directory (@file{~/.mdk}).

As a beginner, your best friend will be the @code{help} command, which
shows you a summary of all available MIX commands and their usage; its
syntax is as follows:

@deffn {@code{mixvm} command} help [command]
Prints a short description of the given @var{command} and its usage. If
@var{command} is omitted, @code{help} prints the short description for
all available commands.
@end deffn

@menu
* File commands::               Loading and executing programs.
* Debug commands::              Debugging programs.
* State commands::              Inspecting the virtual machine state.
* Configuration commands::      Changing and storing mixvm settings.
* Scheme commands::
@end menu

@node File commands, Debug commands, Commands, Commands
@subsection File commands

You have at your disposal a series of commands that let you load and
execute MIX executable files, as well as manipulate MIXAL source files:

@deffn {file command} load file[.mix]
This command loads a binary file, @var{file.mix} into the virtual
machine memory, and positions the program counter at the beginning of
the loaded program. This address is indicated in the MIXAL source file
as the operand of the @code{END} pseudoinstruction. Thus, if your
@file{sample.mixal} source file contains the line:

@example
     END 3000
@end example

@noindent
and you compile it with @code{mixasm} to produce the binary file
@file{sample.mix}, you will load it into the virtual machine as follows:

@example
MIX > load sample
Program loaded. Start address: 3000
MIX >
@end example

@end deffn

@deffn {file command} run [file[.mix]]
When executed without argument, this command initiates or resumes
execution of instructions from the current program counter
address. Therefore, issuing this command after a successful @code{load},
will run the loaded program until either a @code{HLT} instruction or a
breakpoint is found. If you provide a MIX filename as argument, the
given file will be loaded (as with @code{load} @var{file}) and
executed. If @code{run} is invoked again after program execution
completion (i.e., after the @code{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 @code{load} command
preserving the currently set breakpoints is issued before resuming
execution).
@end deffn

@deffn {file command} edit [file[.mixal]]
The source file @var{file.mixal} is edited using the editor defined in
the environment variable @var{MDK_EDITOR}. If this variable is not set,
the following ones are tried out in order: @var{X_EDITOR}, @var{EDITOR}
and @var{VISUAL}. If invoked without argument, the source file for the
currently loaded MIX file is edited. The command used to edit source
files can also be configured using the @code{sedit} command
(@pxref{Configuration commands}).
@end deffn

@deffn {file command} compile file[.mixal]
The source file @var{file.mixal} is compiled (with debug information
enabled) using @code{mixasm}. If invoked without argument, the source
file for the currently loaded MIX file is recompiled. The compilation
command can be set using the @code{sasm} command (@pxref{Configuration
commands}).
@end deffn

@deffn {file command} pprog
@deffnx {file command} psrc
Print the path of the currently loaded MIX program and its source file:

@example
MIX > load ../samples/primes
Program loaded. Start address: 3000
MIX > pprog
../samples/primes.mix
MIX > psrc
/home/jao/projects/mdk/gnu/samples/primes.mixal
MIx>
@end example
@end deffn

Finally, you can use the @code{quit} command to exit @code{mixvm}:

@deffn {file command} quit
Exit @code{mixvm}, saving the current configuration parameters in
@file{~/.mdk/mixvm.config}.
@end deffn


@node Debug commands, State commands, File commands, Commands
@subsection Debug commands

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

@deffn {debug command} next [ins_number]
This command causes the virtual machine to fetch and execute  up to
@var{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 @code{next} is invoked again after program execution
completion (i.e., after the @code{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 @code{load} command
preserving the currently set breakpoints is issued before resuming
execution).
@end deffn

@deffn {debug command} sbp line_number
@deffnx {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
@code{ORIG} or @code{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 @file{hello.mixal} file:

@example
*                                                        (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)
...
@end example

@noindent
trying to set a breakpoint at line 5, will produce the following result:

@example
MIX > sbp 5
Breakpoint set at line 7
MIX >
@end example

@noindent
since line 7 is the first one compiled into a MIX instruction (at
address 3000).

The command @code{cbp} clears a (previously set) breakpoint at the given
source file line.
@end deffn

@deffn {debug command} spba address
@deffnx {debug command} cbpa address
Sets a breakpoint at the given memory @var{address}. The argument must
be a valid MIX memory address, i.e., it must belong into the range
@w{[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 @code{sbpa}.
The command @code{cbpa} clears a (previously set) breakpoint at the
given memory address.
@end deffn

@deffn {debug command} sbpr A | X | J | Ii
@deffnx {debug command} cbpr A | X | J | Ii
Sets a conditional breakpoint on the specified register change. For
instance,

@example
sbpr I1
@end example

@noindent
will cause an interruption during program execution whenever the
contents or register @code{I1} changes. A previously set breakpoint is
cleared using the @code{cbpr} command.
@end deffn

@deffn {debug command} sbpm address
@deffnx {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 @w{[0-3999]}. For instance,

@example
sbpm 1000
@end example

@noindent
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 @code{cbpm} command.
@end deffn

@deffn {debug command} sbpo
@deffnx {debug command} cbpo
Sets/clears a conditional breakpoint on overflow toggle change.
@end deffn

@deffn {debug command} sbpc
@deffnx {debug command} cbpc
Sets/clears a conditional breakpoint on comparison flag change.
@end deffn

@deffn {debug command} cabp
Clears all currently set breakpoints.
@end deffn

@deffn {debug command} psym [symbol_name]
MIXAL programs can define symbolic constants, using either the
@code{EQU} pseudoinstruction or a label at the beginning of a
line. Thus, in the program fragment

@example
VAR     EQU  2168
        ORIG 4000
START   LDA  VAR
@end example

@noindent
the symbol @code{VAR} stands for the value 2168, while @code{START} is
assigned the value 4000. The symbol table can be consulted from
the @code{mixvm} command line using @code{psym} followed by the name of
the symbol whose contents you are interested in. When run without
arguments, @code{psym} will print all defined symbols and their values.
@end deffn

The virtual machine can also show you the instructions it is executing,
using the following commands:

@deffn {debug command} strace [on|off]
@code{strace on} enables instruction tracing. When tracing is enabled,
each time the virtual machine executes an instruction (due to your
issuing a @code{run} or @code{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 @code{strace off}
command. A typical tracing session could be like this:

@example
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 >
@end example
@noindent
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 @file{~/.mdk}.
@end deffn

@deffn {debug command} pline [LINE_NUMBER]
Prints the requested source line (or the current one if
@var{line_number} is omitted:

@example
MIX > load ../samples/hello
Program loaded. Start address: 3000
MIX > pline
Line 5: START       OUT   MSG(TERM)
MIX > pline 6
Line 6:             HLT
MIX >
@end example
@end deffn

@deffn {debug command} pbt [INS_NUMBER]
This command prints a backtrace of executed instructions. Its optional
argument @var{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 (@file{bt.mixal}):

@example
    ORIG 0
BEG JMP *+1
    JMP *+1
FOO JMP BAR
BAR HLT
    END BEG
@end example

@noindent
you could get the following traces:

@example
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 >
@end example

Note that the executed instruction trace gives you the label of the
executed line or, if it has no label, its address.
@end deffn

As you have probably observed, @code{mixvm} prints timing statistics
when running programs. This behaviour can be controlled using the
@code{stime} command (@pxref{Configuration commands}).

@code{mixvm} is also able of evaluating w-expressions
(@pxref{W-expressions}) using the following command:

@deffn {debug command} weval WEXP
Evaluates the given w-expression, @var{WEXP}. The w-expression can
contain any currently defined symbol. For instance:

@example
MIX > psym START
+ 00 00 00 46 56 (0000003000)
MIX > weval START(0:1),START(3:4)
+ 56 00 46 56 00 (0939716096)
MIX >
@end example
@end deffn

New symbols can be defined using the @code{ssym} command:

@deffn {debug command} ssym SYM WEXP
Defines the symbol named @var{SYM} with the value resulting from
evaluating @var{WEXP}, an w-expression. The newly defined symbol can be
used in subsequent @code{weval} commands, as part of the expression to
be evaluated. E.g.,

@example
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 >
@end example
@end deffn

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

@deffn {debug command} w2d WORD
Computes the decimal value of the given word. @var{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 @code{weval}. For instance:

@example
MIX > w2d - 01 00 00 02 02
-16777346
MIX > weval -16777346
- 01 00 00 02 02 (0016777346)
MIX >
@end example
@end deffn

@node State commands, Configuration commands, Debug commands, Commands
@subsection State commands

Inspection and modification of the virtual machine state (memory,
registers, overflow toggle and comparison flag contents) is accomplished
using the following commands:

@deffn {state command} pstat
This commands prints the current virtual machine state, which can be one
of the following:
@itemize @minus
@item
No program loaded
@item
Program successfully loaded
@item
Execution stopped (@code{next} executed)
@item
Execution stopped: breakpoint encountered
@item
Execution stopped: conditional breakpoint encountered
@item
Program successfully terminated
@end itemize
@end deffn

@deffn {state command} pc
Prints the current value of the program counter, which stores the
address of the next instruction to be executed in a non-halted program.
@end deffn

@deffn {state command} sreg A | X | J | I[1-6] value
@deffnx {state command} preg [A | X | J | I[1-6]]
@deffnx {state command} pall
@code{preg} prints the contents of a given MIX register. For instance,
@w{@code{preg} @var{A}} will print the contents of the A-register. When
invoked without arguments, all registers shall be printed:

@example
MIX > preg
rA: - 00 00 00 00 35 (0000000035)
rX: + 00 00 00 15 40 (0000001000)
rJ: + 00 00 (0000)
rI1: + 00 00 (0000)	rI2: + 00 00 (0000)
rI3: + 00 00 (0000)	rI4: + 00 00 (0000)
rI5: + 00 00 (0000)	rI6: + 00 00 (0000)
MIX >
@end example

As you can see in the above sample, the contents is printed as the sign
plus the values of the MIX bytes stored in the register and, between
parenthesis, the decimal representation of its module.

@code{pall} prints the contents of all registers plus the comparison
flag and overflow toggle.

Finally, @code{sreg} Sets the contents of the given register to
@var{value}, expressed as a decimal constant. If @var{value} exceeds the
maximum value storable in the given register, @code{VALUE mod
MAXIMU_VALUE} is stored, e.g.

@example
MIX > sreg I1 1000
MIX > preg I1
rI1: + 15 40 (1000)
MIX > sreg I1 1000000
MIX > preg I1
rI1: + 09 00 (0576)
MIX >
@end example

@end deffn


@deffn {state command} pflags
@deffnx {state command} scmp E | G | L
@deffnx {state command} sover F | T
@code{pflags} prints the value of the comparison flag and overflow
toggle of the virtual machine, e.g.

@example
MIX > pflags
Overflow: F
Cmp: E
MIX >
@end example

@noindent
The values of the overflow toggle are either @var{F} (false) or @var{T}
(true), and, for the comparison flag, @var{E}, @var{G}, @var{L} (equal,
greater, lesser). @code{scmp} and @code{sover} are setters of the
comparison flag and overflow toggle values.
@end deffn

@deffn {state command} pmem from[-to]
@deffnx {state command} smem address value
@code{pmem} prints the contents of memory cells in the address range
@w{[@var{FROM}-@var{TO}]}. If the upper limit @var{to} is omitted, only
the contents of the memory cell with address @var{FROM} is printed, as
in

@example
MIX > pmem 3000
3000: + 46 58 00 19 37 (0786957541)
MIX >
@end example

The memory contents is displayed both as the set of five MIX bytes plus
sign composing the stored MIX word and, between parenthesis, the decimal
representation of the module of the stored value.

@code{smem} sets the content of the memory cell with address
@var{address} to @var{value}, expressed as a decimal constant.

@end deffn

@node Configuration commands, Scheme commands, State commands, Commands
@subsection Configuration commands

This section describes commands that allow you to configure the virtual
machine behaviour. This configuration is stored in the @sc{mdk}
directory @file{~/.mdk}.

As you can see in their description, some commands print, as a side
effect, informational messages to the standard output (e.g. @code{load}
prints a message telling you the loaded program's start address): these
messages can be enabled/disabled using @code{slog}:

@deffn {config command} slog on|off
Turns on/off the logging of informational messages. Note that error
messages are always displayed, as well as state messages required using
commands prefixed with @code{p} (@code{preg}, @code{pmem} and the like).
@end deffn

@deffn {config command} stime  on|off
@deffnx {config command} ptime
The @code{stime} command (un)sets the printing of timing statistics, and
@code{ptime} prints their current value:

@example
MIX > ptime
Elapsed time: 10 /Total program time: 11 (Total uptime: 11)
MIX >
@end example
@end deffn

@deffn {config command} sedit TEMPLATE
@deffnx {config command} pedit
@code{sedit} sets the command to be used to edit MIXAL source files with
the @code{edit} command. @var{TEMPLATE} must contain the control
characters @code{%s} to mark the place where the source's file name will
be inserted. For instance, if you type

@example
MIX > sedit emacsclient %s
MIX >
@end example

issuing the @code{mixvm} command @w{@code{edit foo.mixal}} will invoke
the operating system command @w{@code{emacsclient foo.mixal}}.

@code{pedit} prints the current value of the edit command template.

@end deffn

@deffn {config command} sasm TEMPLATE
@deffnx {config command} pasm
@code{sasm} sets the command to be used to compile MIXAL source files with
the @code{compile} command. @var{template} must contain the control
characters @code{%s} to mark the place where the source's file name will
be inserted. For instance, if you type

@example
MIX > sasm mixasm -l %s
MIX >
@end example

issuing the @code{mixvm} command @w{@code{compile foo.mixal}} will invoke
the operating system command @w{@code{mixasm -l foo.mixal}}.

@code{pasm} prints the current value of the compile command template.

@end deffn

@deffn {config command} sddir DIRNAME
@deffnx {config command} pddir
MIX devices (@pxref{Devices}) are implemented as regular files stored,
by default, inside @file{~/.mdk}. The @code{sddir} command lets you
specify an alternative location for storing these device files, while
@code{pddir} prints the current device directory.
@end deffn

Finally, you can change the default command prompt, @samp{MIX > },
using the @code{prompt} command:

@deffn {config command} prompt PROMPT
Changes the command prompt to @var{prompt}. If you want to include
white space(s) at the end of the new prompt, bracket @var{prompt} using
double quotes (e.g., @code{prompt ">> "}).
@end deffn

@node Scheme commands,  , Configuration commands, Commands
@subsection Scheme commands

If you have compiled @sc{mdk} with @code{libguile} support
(@pxref{Special configure flags}), @code{mixvm} will start and
initialise an embedded Guile Scheme interpret when it is invoked. That
means that you have at your disposal, at @code{mixvm}'s command prompt,
all the Scheme primitives described in @ref{Using mixguile} and
@ref{mixguile}, as well as any other function or hook that you have
defined in the initialisation file @file{~/.mdk/mixguile.scm}. To
evaluate a Scheme function, simply type it at the @code{mixvm} command
prompt (see @ref{Using Scheme in mixvm and gmixvm} for a
sample). Compared to the @code{mixguile} program, this has only one
limitation: the expressions used in @code{mixvm} cannot span more than
one line. You can get over this inconvenience writing your multiline
Scheme expressions in a file and loading it using the @code{scmf}
command:

@deffn {scheme command} scmf FILE_NAME
Loads the given Scheme file and evaluates it using the embedded Guile
interpreter.
@end deffn


@node Devices,  , Commands, mixvm
@section MIX block devices

The MIX computer comes equipped with a set of block devices for
input-output operations (@pxref{Input-output operators}). @code{mixvm}
implements these block devices as disk files, with the exception of
block device no. 19 (typewriter terminal) which is redirected to
standard input/output. When you request an output operation on any other
(output) device, a file named according to the following table will be
created, and the specified MIX words will be
written to the file in binary form (for binary devices) or in ASCII (for
char devices). Files corresponding to input block devices should be
created and filled beforehand to be used by the MIX virtual machine (for
input-output devices this creation can be accomplished by a MIXAL
program writing to the device the required data, or, if you prefer, with
your favourite editor). The device files are stored, by default, in the
directory @file{~/.mdk}; this location can be changed using the
@code{mixvm} command @code{devdir} (@pxref{Configuration commands}).

@multitable {the device name} { xx-xx } {filename[x-x].dev} {bin  i/o/char }
@item @emph{Device}  @tab @emph{No.} @tab @emph{filename} @tab @emph{type and block size}
@item Tape @tab 0-7 @tab @file{tape[0-7].dev} @tab bin i/o - 100 words
@item Disks @tab 8-15 @tab @file{disk[0-7].dev} @tab bin i/o - 100 words
@item Card reader @tab 16 @tab @file{cardrd.dev} @tab char in - 16 words
@item Card writer @tab 17 @tab @file{cardwr.dev} @tab char out - 16 words
@item Line printer @tab 18 @tab @file{printer.dev} @tab char out - 24 words
@item Terminal @tab 19 @tab @code{stdin/stdout} @tab char i/o - 14 words
@item Paper tape @tab 20 @tab @file{paper.dev} @tab char in - 14 words
@end multitable

Devices of type @i{char} are stored as ASCII files, using one line per
block. For instance, since the card reader has blocks of size 16, that
is, 80 characters, it will be emulated by an ASCII file consisting of
lines with length 80. If the reader finds a line with less than the
required number of characters, it pads the memory with zeroes (MIX
character 'space') to complete the block size.

Note that the virtual machine automatically converts between the MIX and
ASCII character encodings, so that you can manipulate char device files
with any ASCII editor. In addition, the reader is not case-sensitive,
i.e., it automatically converts lowercase letters to their uppercase
counterparts (since the MIX character set does not include the former).

The typewriter (device no. 19) lets you use the standard input and
output in your MIXAL programs. For instance, here is a simple 'echo'
program:

@example
* simple echo program
TERM    EQU     19          the typewriter device
BUF     EQU     500         input buffer
        ORIG    1000
START   IN      BUF(TERM)   read a block (70 chars)
        OUT     BUF(TERM)   write the read chars
        HLT
        END     START
@end example

@noindent Input lines longer than 70 characters (14 words) are trimmed.
On the other hand, if you type less than a block of characters,
whitespace (MIX character zero) is used as padding.