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%4096
 if #s > 16 then
  table.remove(s,1)
 end
end

function t.run(stab)
 -- variables
 local pc = stab.pc or 1
 local ci = stab.cins or 0
 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 == 4095 then -- ppc
  push(stack,pc)
 elseif ci == 4094 then -- swp
  a=pop(stack)
  b=pop(stack)
  push(stack,a)
  pusk(stack,b)
 elseif ci == 4093 then -- read
  rw = "read" -- terrible IPC
  addr = pop(stack)
 elseif ci == 4092 then -- write
  rw = "write"
  addr = pop(stack)
  dat = pop(stack)
 elseif ci == 4091 then -- add
  push(stack,pop(stack)+pop(stack))
 elseif ci == 4090 then -- sub
  push(stack,pop(stack)+pop(stack))
 elseif ci == 4089 then -- jmp
  pc = pop(stack)
  pci = 0
 elseif ci == 4088 then -- sez
  if pop(stack) == 0 then
   pci = pci + 1
  end
 elseif ci == 4087 then -- jez
  local a = pop(stack)
  if pop(stack) == 0 then
   pc = a
   pci = 0
  end
 elseif ci == 4086 then -- jsr
  push(rstack,pc)
  pc = pop(stack)
  pci = 0
 elseif ci == 4085 then -- ret
  pc = pop(rstack)
 elseif ci == 4084 then -- dup
  local v = pop(stack)
  push(stack,v)
  push(stack,v)
 elseif ci == 4083 then -- drop
  pop(stack)
 elseif ci == 4082 then -- hlt
  rw = "halt"
 else
  push(stack,ci)
 end
 -- return stuff
 stab = {}
 stab.pc = pc + pci -- increment pc
 stab.cins = 0
 stab.stack = stack
 stab.rstack = rstack
 return stab, rw, addr, dat
end

return t