Chip-8 (#88)

As some of you may know, the annual ICFP contest was just two weekends
ago and
this year’s problem involved the creation of a small virtual machine,
much as
this quiz does. We didn’t plan that, but it turned out to be a pleasant
synergy.

The ICFP machine was really a non-ideal environment for Ruby. The
performance
issues there really called for C speed. In contrast, Chip-8 programs
seem to
fit Ruby emulation nicely.

I’m not going to cover it below, but do spend a little time playing with
Sander
Land’s complete emulator if you have a Windows box handy. It runs games
from
the quiz-linked site and isn’t much more code than some of the other
solutions.

I do want to take a look at Boris P.'s solution though, so let’s get
to it.
This first chunk of code isn’t the actual quiz solution, but it’s a
wonderful
tool for seeing how solutions process Chip-8 files. Here’s Boris’s
disassembler:

class Chip8Disassembler
  CODES = {
    /0000/     => 'exit',
    /1(...)/   => 'goto \1',
    /3(.)(..)/ => 'skip next if V\1 == 0x\2',
    /6(.)(..)/ => 'V\1 = 0x\2',
    /7(.)(..)/ => 'V\1 = V\1 + 0x\2',
    /8(.)(.)0/ => 'V\1 = V\2',
    /8(.)(.)1/ => 'V\1 = V\1 | V\2',
    /8(.)(.)2/ => 'V\1 = V\1 & V\2',
    /8(.)(.)3/ => 'V\1 = V\1 ^ V\2',
    /8(.)(.)4/ => 'V\1 = V\1 + V\2',
    /8(.)(.)5/ => 'V\1 = V\1 - V\2',
    /8(.)06/   => 'V\1 = V\1 >> 1',
    /8(.)(.)7/ => 'V\1 = V\2 - V\1',
    /8(.)0E/   => 'V\1 = V\1 << 1',
    /C(.)(..)/ => 'V\1 = rand() & 0x\2',
  }

  def self.code2text hexcode
    CODES.each do |re, subs|
      if hexcode =~ re
        return hexcode.sub(re, subs)
      end
    end
    '???'
  end

  def self.dump binary
    opcodes = binary.unpack "n*"
    opcodes.each_with_index do |code, waddr|
      code_hex = "%04X" % code
      printf("%03X: [%s] %s\n", waddr*2, code_hex, 

code2text(code_hex));
end
end
end

binary = File.read(ARGV[0])
Chip8Disassembler.dump(binary)

If you skip to the end, you can see that this code just slurps the
Chip-8
program passed as an argument and feeds the whole thing to dump().

The dump() method separates the program into opcodes with a simple call
to
unpack(). The “n*” pattern for unpack() has it read two characters at a
time as
an unsigned integer. It also dictates that the number is in “network
byte-order” which avoids endian issues on different architectures. From
there
each opcode is traversed, turned into a hex code, and handed off to the
code2text() method for translation.

Inside code2text(), the passed hexcode is compared to each pattern of
the CODES
Hash in turn. When a match is found, the key and value Hash pair are
used to
perform a regular expression conversion from code to source statement
and that
statement is returned.

Each of those statements is decorated with the code_hex and the address
of the
instruction in the program file, then printed. The end result looks
like this,
for the quiz test program:

000: [6177] V1 = 0x77
002: [6245] V2 = 0x45
004: [7101] V1 = V1 + 0x01
006: [8320] V3 = V2
008: [8121] V1 = V1 | V2
00A: [8122] V1 = V1 & V2
00C: [8233] V2 = V2 ^ V3
00E: [8134] V1 = V1 + V3
010: [8235] V2 = V2 - V3
012: [8106] V1 = V1 >> 1
014: [8327] V3 = V2 - V3
016: [830E] V3 = V3 << 1
018: [64FF] V4 = 0xFF
01A: [C411] V4 = rand() & 0x11
01C: [32BB] skip next if V2 == 0xBB
01E: [1000] goto 000
020: [0000] exit

The hex codes from the quiz description are in brackets in the middle
and you
can see the operations just to the right of that. Hopefully that makes
it clear
how you break down instructions and what they are suppose to do.

Now we should be well equipped to read Boris’s actual solution. Here’s
how it
begins:

class Chip8Emulator
  attr_accessor :register

  VF = 15 # index of the carry/borrow register

  def initialize
    @register = [0] * 16 # V0..VF
  end

  # ...

I doubt there is anything tricky there. The registers are just an Array
of
Integers which are made available through the accessor. We also see a
convenient VF constant defined for accessing that register.

Here’s the main event loop of the emulator:

	# ...

	def exec(program)
	  opcodes = program.unpack('n*')
	  current = 0 # current opcode index
	  loop do
	    opcode = opcodes[current]

	    # these are needed often:
	    x   = opcode >> 8 & 0x0F
	    y   = opcode >> 4 & 0x0F
	    kk  = opcode & 0xFF
	    nnn = opcode & 0x0FFF

	    case opcode >> 12 # first nibble
	    when 0 then return
	    when 1 then current = (nnn) / 2 and next
	    when 3 then current += 1 if @register[x] == kk
	    when 6 then @register[x] = kk
	    when 7 then add(x, kk)
	    when 8
	      case opcode & 0x0F # last nibble
	      when 0 then @register[x]  = @register[y]
	      when 1 then @register[x] |= @register[y]
	      when 2 then @register[x] &= @register[y]
	      when 3 then @register[x] ^= @register[y]
	      when 4 then add(x, @register[y])
	      when 5 then subtract(x, x, y)
	      when 6 then shift_right(x)
	      when 7 then subtract(x, y, x)
	      when 0xE then shift_left(x)
	      else raise "Unknown opcode: " + opcode.to_s(16)
	      end
	    when 0xC then random(x, kk)
	    else raise "Unknown opcode: " + opcode.to_s(16)
	    end
	    current += 1 # next opcode
	  end
	end

	# ...

This entire method basically comes right out of the quiz. The program
is first
unpack()ed into opcodes, as we saw with the disassembler and a current
opcode
pointer is initialized.

Next the code enters into an infinite loop() of opcode processing.
Inside that
loop(), the current opcode is pulled and then split into the x, y, kk,
and nnn
variables the quiz discusses.

The big case statement after that is the opcode instruction processor.
Most of
those are trivial implementations of the quiz descriptions, but you can
see that
the more complex operations are delegated to helper methods, which we
will
examine in just a moment.

The last thing of note here is the jump implementation with the and next code
on the end of it. That forces the next iteration of the loop() to start
immediately, bypassing the current opcode pointer increment after the
case
statement.

Let’s see those helper methods:

  # ...

  def add(reg, value)
    result = @register[reg] + value
    @register[reg] = result & 0xFF
    @register[VF]  = result >> 8 # carry
  end

  def subtract(reg, a, b)
    result = @register[a] - @register[b]
    @register[reg] = result & 0xFF
    @register[VF]  = - (result >> 8) # borrow
  end

  def shift_right(reg)
    @register[VF] = @register[reg] & 0x01
    @register[reg] >>= 1
  end

  def shift_left(reg)
    @register[VF] = @register[reg] >> 7
    @register[reg] = (@register[reg] << 1) & 0xFF
  end

  def random(reg, kk)
    @register[reg] = rand(256) & kk
  end

  # ...

These are mainly just the operations that affect the VF register.
Because they
require multiple steps, it was easier to move these into separate
methods and
leave the event loop case statement clear. These definitions still come
right
out of the quiz.

Finally, we have the last bit of code needed to dump registers and
kick-off the
program run in the first place. Here’s the code:

  # ...

  # show all registers
  def dump
    0.upto(VF) do |reg|
      printf("V%1X:%08b\n", reg, @register[reg])
    end
  end
end

if $0 == __FILE__
  ARGV.each do |program|
    emu = Chip8Emulator.new
    emu.exec(File.read(program))
    emu.dump
  end
end

The dump() method is a simple print routine for register names and
values,
iterating over all the registers. Below that we have the code the
executes and
dumps each program provided as an argument, using the methods we have
been
examining.

Boris’s code did include unit tests. Each opcode was tested against a
hand
crafted mini-program to check functionality. Here’s the subtraction
test, for
example:

  # ... other test code not shown ...

  def test_subtract
    @emu.exec("\x60\x00" + "\x61\x01" + "\x80\x15" + "\0\0")
    assert_equal [255, 1] + [0]*13 + [1], @emu.register
  end

  # ... other test code not shown ...

Don’t miss the other solutions folks. Take some time to look them over.
All
are quite clever. You will even find elements not covered in this
summary, like
Alexandru E. Ungur’s assembler!

My thanks to all who took tried their hand at building their own
emulator. I
hope you found it a lot easier than I did building a good ICFP emulator.

In tomorrow’s quiz, we will see just how far a simple method like
upcase() can
really go in terms of useful application…

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