#!/usr/bin/perl
#
# a7out: user-mode simulator for PDP-7 Unix applications
#
# (c) 2016 Warren Toomey, GPL3
#
use strict;
use warnings;
use Data::Dumper;

### Global variables ###
my $debug = 0;    # Debug flag
my @Mem;          # 8K 18-bit words of main memory
my @FD;           # Array of open filehandles

# Registers
my $PC = 0;       # Program counter
my $AC;           # Accumulator
my $LINK;         # Link register
my $MQ;           # MQ register

### Main program ###

# Optional debug argument
if ( ( @ARGV > 0 ) && ( $ARGV[0] eq "-d" ) ) {
    $debug = 1; shift(@ARGV);
}

# Check the arguments
die("Usage: $0 [-d] a.outfile [arg1 arg2 ...]\n") if ( @ARGV < 1 );

# Load the a.out file into memory
# and simulate it
load_code( shift(@ARGV) );
set_arguments(@ARGV);
#dump_memory();
#exit(0);
simulate();
exit(0);

### Load the a.out file into memory
sub load_code {
    my $filename = shift;

    # Fill all the 8K words in memory with zeroes
    foreach my $i ( 0 .. 017777 ) {
        $Mem[$i] = 0;
    }

    # Set up two file open filehandles
    $FD[0] = \*STDIN;
    $FD[1] = \*STDOUT;
    $FD[8] = \*STDERR;		# According to cat.s (uses d8 == 8)

    # Open up the file
    open( my $IN, "<", $filename ) || die("Unable to open $filename: $!\n");
    while (<$IN>) {
        chomp;

        # Lose any textual stuff after a tab character
        $_ =~ s{\t.*}{};

        # Split into location and value, both in octal
        my ( $loc, $val ) = split( /:\s+/, $_ );

        # Convert from octal and save
        $loc       = oct($loc);
        $val       = oct($val);
        $Mem[$loc] = $val;
    }
    close($IN);
}

### Copy the arguments into the PDP-7 memory space, and build
### an array of pointers to these arguments. Build a pointer
### at 017777 that points at the array.
###
### At the moment, this is NOT what PDP-7 Unix uses, but it's
### a start and it will help us to grok the real thing.
###
### For now, assume abc, def and ghi are stored in memory.
### The layout of the pointers and strings would be:
###
###    +-------+
###    |       |
###    | +-----|---------+
###    | |     |         |
###    | | +---|---------|---------+
###    | | |   |         |         |
###    | | |   V         V         V
###   |o|o|o|0|<ab>|<c 0|<de>|<f 0|<gh>|<i 0|o|
###    ^                                     |
###    |                                     |
###    +-------------------------------------+
###    0 0 0 0  0    0    0    0    0    0   0
###    1 1 1 1  1    1    1    1    1    1   1
###    7 7 7 7  7    7    7    7    7    7   7
###    7 7 7 7  7    7    7    7    7    7   7
###    6 6 6 7  7    7    7    7    7    7   7
###    5 6 7 0  1    2    3    4    5    6   7
###
sub set_arguments {
  # No arguments, set the 017777 pointer to 017776 which is NULL
  if (@ARGV==0) {
    $Mem[ 017777 ] = 017776;
    print( STDERR "No arguments, so NULL 017777 pointer\n") if ($debug);
    return;
  }

  # Count the number of words to store each string and its pointer
  my $wordcount=2;  # 2 as we count the NULL pointer at end of array
  my $argcount= @ARGV;
  foreach my $arg (@ARGV) {
    # +1 before /2 to allow for the NUL character at end of string
    # += +1 to include the pointer to the string
    $wordcount += 1 + ( (length($arg) +1)/2);
  }

  my $arraybase= 017777 - $wordcount;
  my $stringbase= $arraybase + $argcount + 1;	# include NULL pointer
  $Mem[ 017777 ] = $arraybase;

  # Now copy each string into memory and initialise the pointer
  foreach my $arg (@ARGV) {
    $Mem[$arraybase++]= $stringbase;
    $stringbase= string2mem($arg, $stringbase);
  }
}

### Simulate the machine code loaded into memory
sub simulate {

    # List of opcodes that we can simulate
    my %Oplist = (
        oct("004") => \&dac,
        oct("020") => \&lac,
        oct("034") => \&tad,
        oct("060") => \&jmp,
        oct("070") => \&iot,
        oct("074") => \&special,
    );

    # Loop indefinitely
    while (1) {

        # Get the instruction pointed to by PC and decode it
        my $instruction = $Mem[$PC];
        my $opcode      = ( $instruction >> 12 ) & 074;
        my $indirect    = ( $instruction >> 13 ) & 1;
        my $addr        = $instruction & 017777;

	# Work out what any indirect address would be
	my $indaddr= ($indirect) ? $Mem[$addr] & 017777 : $addr;
        printf( STDERR
              "PC %06o: instr %06o, op %03o, ind %o, addr %06o ind %06o\n",
            $PC, $instruction, $opcode, $indirect, $addr, $indaddr )
          if ($debug);

        # Simulate the instruction. Each subroutine updates the $PC
        if ( defined( $Oplist{$opcode} ) ) {
            $Oplist{$opcode}->( $instruction, $addr, $indaddr );
        } else {
            printf( STDERR "Unknown instruction 0%o at location 0%o\n",
                $instruction, $PC );
            die("\n");
        }
    }
}

# Debug code: dump memory contents
sub dump_memory {
    foreach my $i ( 0 .. 017777 ) {
        printf( STDERR "%06o: %06o\n", $i, $Mem[$i] ) if ( $Mem[$i] != 0 );
    }
}

# Load AC
sub lac {
    my ( $instruction, $addr, $indaddr ) = @_;
    printf( STDERR "PC %06o: lac %05o (value %06o) into AC\n",
        $PC, $indaddr, $Mem[$indaddr] )
      if ($debug);
    $AC = $Mem[$indaddr];
    $PC++;
}

# Deposit AC
sub dac {
    my ( $instruction, $addr, $indaddr ) = @_;
    printf( STDERR "PC %06o: dac AC (value %06o) into %05o\n",
        $PC, $AC, $indaddr )
      if ($debug);
        $Mem[$indaddr] = $AC;
    $PC++;
}

# Add to AC
sub tad {
    my ( $instruction, $addr, $indaddr ) = @_;
    printf( STDERR "PC %06o: tac AC (value %06o) from addr %05o\n",
        $PC, $AC, $indaddr )
      if ($debug);
      $AC+= $Mem[$indaddr];
    $PC++;
}

# Jump
sub jmp {
    my ( $instruction, $addr, $indaddr ) = @_;
    printf( STDERR "PC %06o: jmp %06o\n", $PC, $indaddr ) if ($debug);
    $PC = $indaddr;
}

# Special instructions
sub special {
    my $instruction = shift;

    # Deal with each one in turn
    # hlt
    if ( $instruction == 0740040 ) {
        printf( STDERR "PC %06o: program halted\n", $PC );
        dump_memory() if ($debug);
        exit(1);
    }
    if ( $instruction == 0741100 ) {    # spa: skip on positive AC
        printf( STDERR "PC %06o: spa AC %06o\n", $PC, $AC ) if ($debug);
        # Because we are dealing with 18 bits, compare the range
        $PC += ( ($AC >= 0) && ($AC < 0400000) ) ? 2 : 1;
        return;
    }
    if ( $instruction == 0741200 ) {    # sna: skip on non-zero AC
        printf( STDERR "PC %06o: sna AC %06o\n", $PC, $AC ) if ($debug);
        $PC += ( $AC != 0 ) ? 2 : 1;
        return;
    }
    if ( $instruction == 0740200 ) {    # sza: skip on zero AC
        printf( STDERR "PC %06o: sza AC %06o\n", $PC, $AC ) if ($debug);
        $PC += ( $AC == 0 ) ? 2 : 1;
        return;
    }
    printf( STDERR "PC %06o: unknown instruction %06o\n", $PC, $instruction );
    exit(1);
}

# I/O transfer: used for system calls
sub iot {
    my ( $instruction, $addr, $indaddr ) = @_;

    # Syscalls that we can simulate
    my %Syscallist = (
        3  => \&sys_open,
        4  => \&sys_read,
        5  => \&sys_write,
        9  => \&sys_close,
        14 => \&sys_exit,
    );

    # Simulate the syscall. Each syscall updates the $PC
    if ( defined( $Syscallist{$addr} ) ) {
        $Syscallist{$addr}->();
    } else {
        printf( STDERR "PC %06o: Unknown syscall %d\n", $PC, $addr );
        die("\n");
    }
}

# Exit system call
sub sys_exit {
    printf( STDERR "PC %06o: exit system call\n", $PC ) if ($debug);
    exit(0);
}

# Close system call
sub sys_close {

    # AC is the file descriptor
    my $fd = $AC;
    printf( STDERR "PC %06o: close: closing fd %d\n", $PC, $fd ) if ($debug);

    # Bump up the PC
    $PC += 1;

    # That filehandle is not open, set an error -1 in octal
    if ( !defined( $FD[$fd] ) ) {
        print( STDERR "close: fd $fd is not open\n") if ($debug);
        $AC = 0777777;
        return;
    }
    close( $FD[$fd] );
    $FD[$fd] = undef;
    $AC = 0;
    return;
}

# Open system call
sub sys_open {

    # Open seems to have arguments: PC+1 has a pointer to the filename,
    # PC+2 and PC+3 I don't know yet, probably read/write and mask?
    # AC is the opened fd on success, or -1 on error

    # Get the start address of the string
    my $start = $Mem[ $PC + 1 ];

    # Bump up the PC
    $PC += 4;

    # Convert this to a sensible ASCII filename
    my $filename = mem2string($start);
    printf( STDERR "PC %06o: open: file %s\n", $PC, $filename ) if ($debug);

    # Open the file
    if ( open( my $FH, "<", $filename ) ) {

       # Find a place in the @FD array to store this filehandle. 99 is arbitrary
       foreach my $fd ( 0 .. 99 ) {
            if ( !defined( $FD[$fd] ) ) {
                $FD[$fd] = $FH;
        	$AC = $fd;
                last;
            }
        }
        return;
    } else {
        # No filehandle, so it's an error
        print( STDERR "open failed: $!\n") if ($debug);
        $AC = 0777777;
        return;
    }
}

# Read system call
sub sys_read {

    # Read seems to have arguments: AC is the file descriptor, PC+1 is
    # the pointer to the buffer and PC+2 is the number of words to read.
    # Return the number of words read in AC on success, or -1 on error.

    # Get the file descriptor, start address and end address
    my $fd    = $AC;
    my $start = $Mem[ $PC + 1 ];
    my $count = $Mem[ $PC + 2 ];
    my $end   = $start + $count - 1;
    printf( STDERR "PC %06o: read: %d words into %o from fd %d\n",
        $PC, $count, $start, $fd )
      if ($debug);

    # Bump up the PC
    $PC += 3;

    # That filehandle is not open, set an error -1 in octal
    if ( !defined( $FD[$fd] ) ) {
        print( STDERR "read: fd $fd is not open\n") if ($debug);
        $AC = 0777777;
        return;
    }

    # Read each word in
    my $FH = $FD[$fd];
    $count = 0;
    foreach my $addr ( $start .. $end ) {

        # It's a terminal, so convert from ASCII
        if ( -t $FH ) {
            my $c1 = getc($FH);
            last if ( !defined($c1) );    # No character, leave the loop
            my $c2 = getc($FH) || "";     # No character, make it a NUL
            $Mem[$addr] =
              ( ord($c1) << 9 ) | ord($c2);    # Pack both into one word
            $count++;
        } else {
            # otherwise (for now) read in one line and convert to octal
            my $line = <$FH>;
            last if ( !defined($line) );       # No line, leave the loop
            chomp($line);
            $Mem[$addr] = oct($line) & 0777777;
            $count++;
        }
    }

    # No error
    $AC = $count;
    return;
}

# Write system call
sub sys_write {

    # Write seems to have arguments: AC is the file descriptor, PC+1 is
    # the pointer to the buffer and PC+2 is the number of words to write

    # Get the file descriptor, start address and end address
    my $fd    = $AC;
    my $start = $Mem[ $PC + 1 ];
    my $count = $Mem[ $PC + 2 ];
    my $end   = $start + $count - 1;
    printf( STDERR "PC %06o: write: %d words from %o to fd %d\n",
        $PC, $count, $start, $fd )
      if ($debug);

    # Bump up the PC
    $PC += 3;

    # That filehandle is not open, set an error -1 in octal
    if ( !defined( $FD[$fd] ) ) {
        print( STDERR "write: fd $fd is not open\n") if ($debug);
        $AC = 0777777;
        return;
    }

    # Write each word out
    my $FH = $FD[$fd];
    foreach my $addr ( $start .. $end ) {

        # It's a terminal, so convert to ASCII
        # otherwise (for now) print in octal
        if ( -t $FH ) {
            print( $FH word2ascii( $Mem[$addr] ) );
        } else {
            printf( $FH "%06o\n", $Mem[$addr] );
        }
    }

    # No error
    $AC = 0;
    return;
}

# Convert an 18-bit word into two ASCII characters and return them.
# Don't return NUL characters
sub word2ascii {
    my $word   = shift;
    my $c1     = ( $word >> 9 ) & 0177;
    my $c2     = $word & 0177;
    my $result = "";
    $result .= chr($c1) if ($c1);
    $result .= chr($c2) if ($c2);
    return ($result);
}

# Given the address of a word in memory, interpret that location
# and those following as a NUL-terminated ASCII string and return
# a copy of this string
sub mem2string {
    my $addr   = shift;
    my $result = "";

    while (1) {

        # Stop when the address leave the 8K word address space
        return ($result) if ( $addr > 017777 );

        # Stop when the value there is zero
        my $word = $Mem[$addr];
        return ($result) if ( $word == 0 );

        # Get the top ASCII character, return if NUL
        my $c1 = ( $word >> 9 ) & 0177;
        return ($result) if ( $c1 == 0 );
        $result .= chr($c1);

        # Get the bottom ASCII character, return if NUL
        my $c2 = $word & 0177;
        return ($result) if ( $c2 == 0 );
        $result .= chr($c2);

        # Move up to the next address
        $addr++;
    }
}

# Given a string and the address of a word in memory, copy
# the string into memory starting at that address and NUL
# terminate the string. Return the first address after the string.
#
# We will go off the end of the string: suppress warnings
no warnings ('substr');
sub string2mem {
    my ($str, $base)= @_;

    # <= length so we go off the end and insert a NUL
    for (my $i=0; $i <= length($str); $i += 2) {
      my $c1= substr($str, $i, 1) || "";
      my $c2= substr($str, $i+1, 1) || "";
      #printf("Saving %06o to %05o\n", (ord($c1) << 9 ) | ord($c2), $base);
      $Mem[$base++]= (ord($c1) << 9 ) | ord($c2);
    }
    return($base);
}