gistfile1.nim

#The MIX memory architecture consists of 4000 words, where each word is 5 6-bit values and a sign bit

type TWord* = int32

const 
  masks: array[0..5, int32] = [0b01_000000_000000_000000_000000_000000'i32,
    0b00_111111_000000_000000_000000_000000,
    0b00_000000_111111_000000_000000_000000,
    0b00_000000_000000_111111_000000_000000,
    0b00_000000_000000_000000_111111_000000,
    0b00_000000_000000_000000_000000_111111]
  data_mask = 0b00_111111_111111_111111_111111_111111


var memory*: array[0..3999, TWord]

converter toInt32*(x: TWord): int32 = 
  var sign = (x and masks[0])
  if sign == 0:
    return x and data_mask
  return -(x and data_mask)

converter toTWord(x: int32): TWord =
  if abs(x) > data_mask:
    raise newexception(EInvalidValue,"Value will not fit in a 30 bit word")
  var t: TWord = data_mask and x
  if x < 0:
    t = t or masks[0]
  return t

proc writeword *(addr, value: TWord): = 
  if addr < 4000:
    memory[addr] = value
  else:
    raise newexception(EAccessViolation, "Address our of range")

proc readword *(addr: TWord, start, stop: int): int32
  var mask = 0 
  for x in countup(start, stop):
    mask or= masks[x]
  var data = (memory[addr] and mask) shr (6 * (6 - stop))
  if memory[addr] and sign != 0:
    return -data
  return data