local MAXADDR = 255
local t = {}
-- internal state
--- program counter (pc)
--- current instruction (ci) -- only passed from the environment to the emulator
--- stack (stack)
local function pop(s)
 return table.remove(s,#s) or 0
end
local function push(s,v)
 v=v or 0
 s[#s+1] = v%256
 if #s > 16 then
  table.remove(s,1)
 end
end

function t.run(stab,ov)
 -- variables
 local pc = stab.pc or 1
 local ci = stab.cins or 0
 if ov then
  ci = tonumber(ov) or 0
 end
 local stack = stab.stack or {}
 local rstack = stab.rstack or {}
 local pci = 1
 local rw = nil
 local addr = 0
 local dat = 0
 -- instruction processing
 if ci == 255 then -- ppc
  push(stack,pc)
 elseif ci == 254 then -- swp
  a=pop(stack)
  b=pop(stack)
  push(stack,a)
  push(stack,b)
 elseif ci == 253 then -- read
  rw = "read" -- terrible IPC
  addr = pop(stack)
 elseif ci == 252 then -- write
  rw = "write"
  addr = pop(stack)
  dat = pop(stack)
  --print("writing "..tostring(dat).." to "..tostring(addr))
 elseif ci == 251 then -- add
  push(stack,pop(stack)+pop(stack))
 elseif ci == 250 then -- sub
  push(stack,pop(stack)+pop(stack))
 elseif ci == 249 then -- jmp
  pc = pop(stack)
  pci = 0
 elseif ci == 248 then -- sez
  if pop(stack) == 0 then
   pci = pci + 1
  end
 elseif ci == 247 then -- jez
  local a = pop(stack)
  if pop(stack) == 0 then
   pc = a
   pci = 0
  end
 elseif ci == 246 then -- jsr
  push(rstack,pc)
  pc = pop(stack)
  pci = 0
 elseif ci == 245 then -- ret
  pc = pop(rstack)
 elseif ci == 244 then -- dup
  local v = pop(stack)
  push(stack,v)
  push(stack,v)
 elseif ci == 243 then -- drop
  pop(stack)
 elseif ci == 242 then -- hlt
  rw = "halt"
 elseif ci == 241 then -- prd
  rw = "preread"
  addr = pop(stack)
 else
  push(stack,ci)
 end
 -- return stuff
 stab = {}
 stab.pc = pc + pci -- increment pc
 if stab.pc > MAXADDR then
  stab.pc = 0
 end
 stab.cins = 0
 stab.stack = stack
 stab.rstack = rstack
 if t.debug then print("t408: pc: "..tostring(stab.pc),"cins: "..tostring(ci),"tos: "..tostring(stab.stack[1]).." #s: "..tostring(#stab.stack),stab.rw,stab.addr,stab.dat) end
 return stab, rw, addr, dat
end

return t