FORTH CPU

term.setTextColor(colors.green)

local pc = 0

local rstack = {}
for i = 0, 31 do
  rstack[i] = i
end
local rstack_pointer = 0

local dstack = {}
for i = 0, 31 do
  dstack[i] = i
end
local dstack_T = 0
local dstack_N = 0
local dstack_depth = 0

local video_ram = {}
for i = 0, 10239 do
  video_ram[i] = 0x20
end
video_ram[0x2600] = 0
video_ram[0x2602] = 0
video_ram[0x2604] = 0

local kbd_buf = {}
local kbd_buf_head = 0
local kbd_buf_tail = 0
for i = 0, 15 do
  kbd_buf[i] = 0
end

if not fs.exists("forth_img") then
  shell.run("pastebin", "get", "0QuVj5Zy", "forth_img")
end
local ram = {}
local forth_file = assert(io.open("forth_img", "r"))
for i = 0, 0x1FFF do
  ram[i] = tonumber(forth_file:read())
end
forth_file:close()

-- Default: 51x19
function display_all()
  local i = video_ram[0x2600]
  local cursor_x = video_ram[0x2602]
  local cursor_y = video_ram[0x2604]
  for y = 0, 18 do
    local str = ""
    for x = 0, 50 do
      str = str..string.char(video_ram[i * 128 + x])
    end
    term.setCursorPos(1, y + 1)
    term.write(str)
    i = (i + 1) % 19
  end
  term.setCursorPos(cursor_x + 1, cursor_y + 1)
end

function save_ram(filename)
  local forth_file = assert(io.open(filename, "w"))
  for i = 0, 0x1FFF do
    forth_file:write(tostring(ram[i]))
    forth_file:write("\n")
  end
  forth_file:close()
end

function read_mem(addr)
  local a = bit.band(addr, 0xFFFE)
  if addr < 0x4000 then
    return ram[bit.blogic_rshift(addr, 1)]
  elseif a == 0x7000 then
    return kbd_buf_head
  elseif a == 0x7002 then
    return kbd_buf_tail
  elseif a == 0x7004 then
    return kbd_buf[kbd_buf_head]
  else
    return 0
  end
end

function write_mem(addr, data)
  local a = bit.band(addr, 0xFFFE)
  if addr < 0x4000 then
    ram[bit.blogic_rshift(addr, 1)] = data
  elseif (addr >= 0x4000) and (addr < (0x4000 + 10240)) then
    video_ram[addr - 0x4000] = data
    --print("addr = "..(addr - 0x4000).." data = "..data)
  elseif addr == 0x7000 then
    kbd_buf_head = bit.band(data, 0x0F)
  end
end

function ALU(op, read_mem)
  if op == 0 then return dstack_T
  elseif op == 1 then return dstack_N
  elseif op == 2 then return bit.band(dstack_T + dstack_N, 0xFFFF)
  elseif op == 3 then return bit.band(dstack_T, dstack_N)
  elseif op == 4 then return bit.bor(dstack_T, dstack_N)
  elseif op == 5 then return bit.bor(bit.band(bit.blshift(dstack_T, 8), 0xFF00), bit.band(bit.blogic_rshift(dstack_T, 8), 0xFF))
  elseif op == 6 then return bit.band(bit.bnot(dstack_T), 0xFFFF)
  elseif op == 7 then
    if dstack_N == dstack_T then
      return 0xFFFF
    else
      return 0
    end
  elseif op == 8 then
    -- N < T
    --print("T = "..dstack_T.." N = "..dstack_N)
    if (bit.band(dstack_N, 0x8000) ~= 0) or (bit.band(dstack_T, 0x8000) ~= 0) then
      if dstack_N >= dstack_T then
        return 0
      else
        return 0xFFFF
      end
    elseif bit.band(dstack_N, 0x8000) ~= 0 then
      return 0xFFFF
    elseif bit.band(dstack_T, 0x8000) ~= 0 then
      return 0
    else
      if dstack_N < dstack_T then
        return 0xFFFF
      else
        return 0
      end
    end
  elseif op == 9 then return bit.blogic_rshift(dstack_T, 1)
  elseif op == 10 then return bit.band(dstack_T - 1, 0xFFFF)
  elseif op == 11 then return rstack[rstack_pointer]
  elseif op == 12 then return read_mem(dstack_T)
  elseif op == 13 then return bit.band(bit.blshift(dstack_T, 1), 0xFFFF)
  elseif op == 14 then return dstack_depth
  elseif op == 15 then
    if dstack_N < dstack_T then
      return 0xFFFF
    else
      return 0
    end
  else
    print("Invalid ALU operation. op = "..op)
  end
end

function execute_insn(read_mem, write_mem)
  local dstack_T_buf = dstack_T
  local istr = read_mem(bit.blshift(pc, 1))

  if bit.band(istr, 0x8000) == 0x8000 then
    -- literal.
    --print("literal pc = "..pc.." T = "..bit.band(istr, 0x7FFF))
    dstack_depth = bit.band(dstack_depth + 1, 0x1F)
    dstack[dstack_depth] = dstack_N
    dstack_N = dstack_T
    dstack_T = bit.band(istr, 0x7FFF)
    pc = bit.band(pc + 1, 0x1FFF)
  elseif bit.band(istr, 0xE000) == 0 then
    -- jump.
    --print("jump pc = "..pc)
    pc = bit.band(istr, 0x1FFF)
  elseif bit.band(istr, 0xE000) == 0x2000 then
    -- conditional jump.
    --print("cond_jump pc = "..pc)
    if dstack_T ~= 0 then
      pc = bit.band(pc + 1, 0x1FFF)
    else
      pc = bit.band(istr, 0x1FFF)
    end
    dstack_T = dstack_N
    dstack_N = dstack[dstack_depth]
    dstack_depth = bit.band(dstack_depth - 1, 0x1F)
  elseif bit.band(istr, 0xE000) == 0x4000 then
    -- call.
    --print("call pc = "..pc)
    rstack_pointer = bit.band(rstack_pointer + 1, 0x1F)
    rstack[rstack_pointer] = bit.blshift(bit.band(pc + 1, 0x1FFF), 1)
    pc = bit.band(istr, 0x1FFF)
  elseif bit.band(istr, 0xE000) == 0x6000 then
    -- ALU.
    --print("ALU pc = "..pc)
    -- N -> [T]
    if bit.band(istr, 0x20) ~= 0 then
      write_mem(dstack_T, dstack_N)
    end
    -- dstack+-
    if bit.band(istr, 3) == 0 then
      dstack_T = ALU(bit.band(bit.blogic_rshift(istr, 8), 0x0F), read_mem)
      if bit.band(istr, 0x80) ~= 0 then dstack_N = dstack_T_buf end -- T -> N
    elseif bit.band(istr, 3) == 1 then
      dstack_T = ALU(bit.band(bit.blogic_rshift(istr, 8), 0x0F), read_mem)
      dstack_depth = bit.band(dstack_depth + 1, 0x1F)
      dstack[dstack_depth] = dstack_N
      if bit.band(istr, 0x80) ~= 0 then dstack_N = dstack_T_buf end -- T -> N
    elseif bit.band(istr, 3) == 3 then
      dstack_T = ALU(bit.band(bit.blogic_rshift(istr, 8), 0x0F), read_mem)
      dstack_N = dstack[dstack_depth]
      dstack_depth = bit.band(dstack_depth - 1, 0x1F)
    else
      -- Error.
    end
    -- R -> PC
    if bit.band(istr, 0x1000) ~= 0 then
      pc = bit.blogic_rshift(rstack[rstack_pointer], 1)
    else
      pc = bit.band(pc + 1, 0x1FFF)
    end
    -- rstack+-
    if bit.band(istr, 0xC) == 4 then
      rstack_pointer = bit.band(rstack_pointer + 1, 0x1F)
      rstack[rstack_pointer] = dstack_T_buf
    elseif bit.band(istr, 0xC) == 0xC then
      rstack_pointer = bit.band(rstack_pointer - 1, 0x1F)
    end
  end
end

while true do
  for i = 0, 1000 do
    execute_insn(read_mem, write_mem)
  end
  display_all()

  os.startTimer(0.05)
  local key = 0xFF
  local event, p = os.pullEvent()
  if event == "key" then
    if p == 28 then
      key = 0x0D
    elseif p == 14 then
      key = 0x08
    end
  elseif event == "char" then
    key = string.byte(p)
  end
  if (key ~= 0xFF) and (((kbd_buf_tail + 1) % 16) ~= kbd_buf_head) then
    kbd_buf_tail = (kbd_buf_tail + 1) % 16
    kbd_buf[kbd_buf_tail] = key
  end
end