OC-ZX : A 48K ZX Spectrum Emulator for OpenComputers

local component = require("component")
local computer = require("computer")
local shell = require("shell")
local fs = require("filesystem")

local PUMP_INTERVAL = 0.15
local RENDER_EVERY = 2
local MASTER_VOLUME = 0.6
local FRAME_TS = 69888
local CPU_HZ = 3500000
local CONTENTION = true
local KEY_HOLD = 3
local AUTOLOAD_FRAME = 150

local args = {...}
local gpu = component.gpu
local snd = component.isAvailable("sound") and component.sound or nil

local function readFile(path)
  local p = shell.resolve(path)
  local f = io.open(p, "rb")
  if not f then return nil end
  local d = f:read("*a")
  f:close()
  return d
end

local function rd16(d, i)
  return d:byte(i) | (d:byte(i + 1) << 8)
end

local function rd24(d, i)
  return d:byte(i) | (d:byte(i + 1) << 8) | (d:byte(i + 2) << 16)
end

local function rd32(d, i)
  return d:byte(i) | (d:byte(i + 1) << 8) |
         (d:byte(i + 2) << 16) | (d:byte(i + 3) << 24)
end

local romPath, snaPath, tapPath, z80Path, tzxPath, scrPath, binPath

for _, a in ipairs(args) do
  local ext = a:lower():match("%.(%w+)$")
  if ext == "rom" then
    romPath = a
  elseif ext == "bin" then
    binPath = a
  elseif ext == "sna" then
    snaPath = a
  elseif ext == "tap" then
    tapPath = a
  elseif ext == "z80" then
    z80Path = a
  elseif ext == "tzx" then
    tzxPath = a
  elseif ext == "scr" then
    scrPath = a
  else
    io.stderr:write("zx: unknown file type: " .. a .. "\n")
    return
  end
end

local romData
do
  local candidates = {
    romPath,
    "48.rom",
    "/home/48.rom",
    "/usr/share/zx/48.rom"
  }

  for _, p in ipairs(candidates) do
    if p and fs.exists(shell.resolve(p)) then
      romData = readFile(p)
      if romData then break end
    end
  end
end

if not romData or #romData ~= 16384 then
  io.stderr:write("zx: need the 16384-byte 48K ROM image (48.rom).\n")
  io.stderr:write("usage: zx [48.rom] [game.sna|.z80|.tap|.tzx|.scr|.bin]\n")
  return
end

local snaData, tapBlocks, z80Data, tzxData, scrData, binData

if snaPath then
  snaData = readFile(snaPath)
  if not snaData then
    io.stderr:write("zx: cannot read " .. snaPath .. "\n")
    return
  end
  if #snaData ~= 49179 then
    io.stderr:write("zx: only 48K .sna snapshots (49179 bytes) are supported\n")
    return
  end
end

if z80Path then
  z80Data = readFile(z80Path)
  if not z80Data then
    io.stderr:write("zx: cannot read " .. z80Path .. "\n")
    return
  end
  if #z80Data < 30 then
    io.stderr:write("zx: .z80 file too short\n")
    return
  end
end

if tzxPath then
  tzxData = readFile(tzxPath)
  if not tzxData then
    io.stderr:write("zx: cannot read " .. tzxPath .. "\n")
    return
  end
end

if scrPath then
  scrData = readFile(scrPath)
  if not scrData or #scrData ~= 6912 then
    io.stderr:write("zx: .scr must be exactly 6912 bytes\n")
    return
  end
end

if binPath then
  binData = readFile(binPath)
  if not binData then
    io.stderr:write("zx: cannot read " .. binPath .. "\n")
    return
  end
  if #binData > 49152 then
    io.stderr:write("zx: .bin larger than 48K\n")
    return
  end
end

if tapPath then
  local d = readFile(tapPath)
  if not d then
    io.stderr:write("zx: cannot read " .. tapPath .. "\n")
    return
  end

  tapBlocks = {}
  local p = 1

  while p + 1 <= #d do
    local len = d:byte(p) | (d:byte(p + 1) << 8)
    if p + 1 + len > #d then break end
    tapBlocks[#tapBlocks + 1] = d:sub(p + 2, p + 1 + len)
    p = p + 2 + len
  end
end

local function loadTZX(d)
  local blocks = {}
  local p = 1
  local n = #d

  if d:sub(1, 7) == "ZXTape!" then
    p = 11
  end

  while p <= n do
    local id = d:byte(p)
    p = p + 1

    if id == 0x10 then
      local len = rd16(d, p + 2)
      blocks[#blocks + 1] = d:sub(p + 4, p + 3 + len)
      p = p + 4 + len

    elseif id == 0x11 then
      local len = rd24(d, p + 15)
      blocks[#blocks + 1] = d:sub(p + 18, p + 17 + len)
      p = p + 18 + len

    elseif id == 0x12 then
      p = p + 4

    elseif id == 0x13 then
      p = p + 1 + d:byte(p) * 2

    elseif id == 0x14 then
      p = p + 10 + rd24(d, p + 7)

    elseif id == 0x15 then
      p = p + 8 + rd24(d, p + 5)

    elseif id == 0x18 or id == 0x19 then
      p = p + 4 + rd32(d, p)

    elseif id == 0x20 or id == 0x23 or id == 0x24 then
      p = p + 2

    elseif id == 0x21 or id == 0x30 then
      p = p + 1 + d:byte(p)

    elseif id == 0x22 or id == 0x25 or id == 0x27 then

    elseif id == 0x26 then
      p = p + 2 + rd16(d, p) * 2

    elseif id == 0x28 or id == 0x32 then
      p = p + 2 + rd16(d, p)

    elseif id == 0x2A then
      p = p + 4

    elseif id == 0x2B then
      p = p + 5

    elseif id == 0x31 then
      p = p + 2 + d:byte(p + 1)

    elseif id == 0x33 then
      p = p + 1 + d:byte(p) * 3

    elseif id == 0x34 then
      p = p + 8

    elseif id == 0x35 then
      p = p + 20 + rd32(d, p + 16)

    elseif id == 0x40 then
      p = p + 4 + rd24(d, p + 1)

    elseif id == 0x5A then
      p = p + 9

    else
      break
    end
  end

  return blocks
end

if tzxData then
  tapBlocks = loadTZX(tzxData)
end

local tapePos = 1

local M = {}
for i = 0, 65535 do M[i] = 0 end
for i = 0, 16383 do M[i] = romData:byte(i + 1) end

local dirty = {}
local anyDirty = true

local ox, oy = 0, 0
local border = 7
local borderDirty = true
local flashOn = false

local ts = 0
local frameStart = 0

local function markScreenByte(a)
  if a < 0x4000 or a > 0x5AFF then return end

  if a < 0x5800 then
    local y = ((a >> 5) & 0xC0) | ((a >> 2) & 0x38) | ((a >> 8) & 7)
    dirty[(y >> 2) * 32 + (a & 31)] = true
  else
    local n = a - 0x5800
    local k = (n >> 5) * 64 + (n & 31)
    dirty[k] = true
    dirty[k + 32] = true
  end

  anyDirty = true
end

local CT = {[0]=6, 5, 4, 3, 2, 1, 0, 0}

local function wr(a, v)
  a = a & 0xFFFF
  v = v & 255

  if a >= 0x4000 then
    if CONTENTION and a < 0x8000 then
      local t = ts - frameStart - 14335
      if t >= 0 and t < 43008 then
        local c = t % 224
        if c < 128 then
          ts = ts + CT[c & 7]
        end
      end
    end

    if M[a] ~= v then
      M[a] = v
      markScreenByte(a)
    end
  end
end

local R = {[0]=0,0,0,0,0,0,0,0}
local S = {[0]=0,0,0,0,0,0,0,0}

local F, F2 = 0, 0
local PC, SP, IX, IY = 0, 0, 0, 0
local I = 0
local RR = 0
local IFF1, IFF2 = false, false
local IM = 0
local halted = false
local eiDelay = 0

local prefixed = false
local sh, sl = 0, 0

local function incR()
  RR = (RR & 0x80) | ((RR + 1) & 0x7F)
end

local function fetch()
  local v = M[PC]
  PC = (PC + 1) & 0xFFFF
  return v
end

local function fetchOp()
  local v = M[PC]
  PC = (PC + 1) & 0xFFFF
  RR = (RR & 0x80) | ((RR + 1) & 0x7F)
  return v
end

local function fetch16()
  local lo = fetch()
  local hi = fetch()
  return lo | (hi << 8)
end

local function push(v)
  SP = (SP - 2) & 0xFFFF
  wr(SP, v & 255)
  wr((SP + 1) & 0xFFFF, (v >> 8) & 255)
end

local function pop()
  local lo = M[SP]
  local hi = M[(SP + 1) & 0xFFFF]
  SP = (SP + 2) & 0xFFFF
  return lo | (hi << 8)
end

local function getrp(p)
  if p == 0 then
    return (R[0] << 8) | R[1]
  elseif p == 1 then
    return (R[2] << 8) | R[3]
  elseif p == 2 then
    return (R[4] << 8) | R[5]
  else
    return SP
  end
end

local function setrp(p, v)
  v = v & 0xFFFF
  if p == 0 then
    R[0], R[1] = v >> 8, v & 255
  elseif p == 1 then
    R[2], R[3] = v >> 8, v & 255
  elseif p == 2 then
    R[4], R[5] = v >> 8, v & 255
  else
    SP = v
  end
end

local SZ53, SZ53P = {}, {}

for i = 0, 255 do
  local v = i
  v = v ~ (v >> 4)
  v = v ~ (v >> 2)
  v = v ~ (v >> 1)

  local b = (i & 0xA8) | ((i == 0) and 0x40 or 0)
  SZ53[i] = b
  SZ53P[i] = b | (((v & 1) == 0) and 4 or 0)
end

local CM = {[0]=0x40,0x01,0x04,0x80}

local function cc(y)
  local set = (F & CM[y >> 1]) ~= 0
  if (y & 1) == 0 then
    return not set
  end
  return set
end

local function alu(op, v)
  local a = R[7]

  if op == 0 or op == 1 then
    local r = a + v + ((op == 1) and (F & 1) or 0)
    local res = r & 255

    F = SZ53[res]
      | ((r >> 8) & 1)
      | ((a ~ v ~ res) & 0x10)
      | (((~(a ~ v)) & (a ~ res) & 0x80) >> 5)

    R[7] = res

  elseif op == 2 or op == 3 or op == 7 then
    local r = a - v - ((op == 3) and (F & 1) or 0)
    local res = r & 255

    local fl = ((r >> 8) & 1)
      | 2
      | ((a ~ v ~ res) & 0x10)
      | (((a ~ v) & (a ~ res) & 0x80) >> 5)

    if op == 7 then
      F = (SZ53[res] & 0xC0)
        | (v & 0x28)
        | (fl & 0x17)
    else
      F = SZ53[res] | fl
      R[7] = res
    end

  elseif op == 4 then
    local res = a & v
    F = SZ53P[res] | 0x10
    R[7] = res

  elseif op == 5 then
    local res = a ~ v
    F = SZ53P[res]
    R[7] = res

  else
    local res = a | v
    F = SZ53P[res]
    R[7] = res
  end
end

local function inc8(v)
  local res = (v + 1) & 255
  F = (F & 1)
    | SZ53[res]
    | (((v & 15) == 15) and 0x10 or 0)
    | ((v == 0x7F) and 4 or 0)
  return res
end

local function dec8(v)
  local res = (v - 1) & 255
  F = (F & 1)
    | SZ53[res]
    | 2
    | (((v & 15) == 0) and 0x10 or 0)
    | ((v == 0x80) and 4 or 0)
  return res
end

local function rot(op, v)
  local c, r

  if op == 0 then
    c = v >> 7
    r = ((v << 1) | c) & 255
  elseif op == 1 then
    c = v & 1
    r = (v >> 1) | (c << 7)
  elseif op == 2 then
    c = v >> 7
    r = ((v << 1) | (F & 1)) & 255
  elseif op == 3 then
    c = v & 1
    r = (v >> 1) | ((F & 1) << 7)
  elseif op == 4 then
    c = v >> 7
    r = (v << 1) & 255
  elseif op == 5 then
    c = v & 1
    r = (v >> 1) | (v & 0x80)
  elseif op == 6 then
    c = v >> 7
    r = ((v << 1) | 1) & 255
  else
    c = v & 1
    r = v >> 1
  end

  F = SZ53P[r] | c
  return r
end

local function ea()
  if prefixed then
    local d = fetch()
    if d >= 128 then d = d - 256 end

    local base = (R[4] << 8) | R[5]
    return (base + d) & 0xFFFF
  end

  return (R[4] << 8) | R[5]
end

local spk = 0
local tgN = 0
local tgT = {}
local tgL = {}

local kb = {}
local kcount = {}

for r = 0, 7 do
  kb[r] = 0x1F
end

local function portIn(p)
  if (p & 1) == 0 then
    local k = 0x1F
    local hi = (p >> 8) & 255

    for r = 0, 7 do
      if (hi & (1 << r)) == 0 then
        k = k & kb[r]
      end
    end

    return 0xA0 | k
  end

  if (p & 0xFF) == 0x1F then
    return 0
  end

  return 0xFF
end

local function portOut(p, v)
  if (p & 1) == 0 then
    local b = v & 7

    if b ~= border then
      border = b
      borderDirty = true
    end

    local s = (v >> 4) & 1
    if s ~= spk then
      spk = s
      if snd and tgN < 4096 then
        tgN = tgN + 1
        tgT[tgN] = ts
        tgL[tgN] = s
      end
    end
  end
end

local function exec(op)
  local x = op >> 6

  if x == 1 then
    local y = (op >> 3) & 7
    local z = op & 7

    if z == 6 then
      if y == 6 then
        halted = true
        return 4
      end

      local a = ea()
      local v = M[a]

      if prefixed and (y == 4 or y == 5) then
        if y == 4 then sh = v else sl = v end
      else
        R[y] = v
      end

      return prefixed and 19 or 7

    elseif y == 6 then
      local a = ea()
      local v

      if prefixed and (z == 4 or z == 5) then
        v = (z == 4) and sh or sl
      else
        v = R[z]
      end

      wr(a, v)
      return prefixed and 19 or 7

    else
      R[y] = R[z]
      return 4
    end

  elseif x == 2 then
    local y = (op >> 3) & 7
    local z = op & 7

    if z == 6 then
      alu(y, M[ea()])
      return prefixed and 19 or 7
    end

    alu(y, R[z])
    return 4

  elseif x == 0 then
    local y = (op >> 3) & 7
    local z = op & 7

    if z == 0 then
      if y == 0 then
        return 4

      elseif y == 1 then
        R[7], S[7] = S[7], R[7]
        F, F2 = F2, F
        return 4

      elseif y == 2 then
        local d = fetch()
        local b = (R[0] - 1) & 255
        R[0] = b

        if b ~= 0 then
          PC = (PC + (d >= 128 and d - 256 or d)) & 0xFFFF
          return 13
        end

        return 8

      elseif y == 3 then
        local d = fetch()
        PC = (PC + (d >= 128 and d - 256 or d)) & 0xFFFF
        return 12

      else
        local d = fetch()
        if cc(y - 4) then
          PC = (PC + (d >= 128 and d - 256 or d)) & 0xFFFF
          return 12
        end
        return 7
      end

    elseif z == 1 then
      local p = y >> 1

      if (y & 1) == 0 then
        setrp(p, fetch16())
        return prefixed and 14 or 10
      end

      local hl = (R[4] << 8) | R[5]
      local rr = getrp(p)
      local r = hl + rr

      F = (F & 0xC4)
        | ((r >> 8) & 0x28)
        | (((hl ~ rr ~ r) >> 8) & 0x10)
        | ((r >> 16) & 1)

      r = r & 0xFFFF
      R[4], R[5] = r >> 8, r & 255
      return prefixed and 15 or 11

    elseif z == 2 then
      local p = y >> 1

      if (y & 1) == 0 then
        if p == 0 then
          wr((R[0] << 8) | R[1], R[7])
          return 7

        elseif p == 1 then
          wr((R[2] << 8) | R[3], R[7])
          return 7

        elseif p == 2 then
          local a = fetch16()
          wr(a, R[5])
          wr((a + 1) & 0xFFFF, R[4])
          return 16

        else
          wr(fetch16(), R[7])
          return 13
        end

      else
        if p == 0 then
          R[7] = M[(R[0] << 8) | R[1]]
          return 7

        elseif p == 1 then
          R[7] = M[(R[2] << 8) | R[3]]
          return 7

        elseif p == 2 then
          local a = fetch16()
          R[5] = M[a]
          R[4] = M[(a + 1) & 0xFFFF]
          return 16

        else
          R[7] = M[fetch16()]
          return 13
        end
      end

    elseif z == 3 then
      local p = y >> 1
      local v = getrp(p)

      if (y & 1) == 0 then
        setrp(p, (v + 1) & 0xFFFF)
      else
        setrp(p, (v - 1) & 0xFFFF)
      end

      return prefixed and 10 or 6

    elseif z == 4 then
      if y == 6 then
        local a = ea()
        wr(a, inc8(M[a]))
        return prefixed and 23 or 11
      end

      R[y] = inc8(R[y])
      return 4

    elseif z == 5 then
      if y == 6 then
        local a = ea()
        wr(a, dec8(M[a]))
        return prefixed and 23 or 11
      end

      R[y] = dec8(R[y])
      return 4

    elseif z == 6 then
      if y == 6 then
        local a = ea()
        wr(a, fetch())
        return prefixed and 19 or 10
      end

      R[y] = fetch()
      return 7

    else
      local a = R[7]

      if y == 0 then
        local c = a >> 7
        a = ((a << 1) | c) & 255
        R[7] = a
        F = (F & 0xC4) | (a & 0x28) | c

      elseif y == 1 then
        local c = a & 1
        a = (a >> 1) | (c << 7)
        R[7] = a
        F = (F & 0xC4) | (a & 0x28) | c

      elseif y == 2 then
        local c = a >> 7
        a = ((a << 1) | (F & 1)) & 255
        R[7] = a
        F = (F & 0xC4) | (a & 0x28) | c

      elseif y == 3 then
        local c = a & 1
        a = (a >> 1) | ((F & 1) << 7)
        R[7] = a
        F = (F & 0xC4) | (a & 0x28) | c

      elseif y == 4 then
        local f = F
        local corr = 0
        local c = f & 1

        if (f & 0x10) ~= 0 or (a & 15) > 9 then
          corr = 6
        end

        if c ~= 0 or a > 0x99 then
          corr = corr | 0x60
          c = 1
        else
          c = 0
        end

        local na
        local h

        if (f & 2) ~= 0 then
          na = (a - corr) & 255
          h = (((f & 0x10) ~= 0) and (a & 15) < 6) and 0x10 or 0
        else
          na = (a + corr) & 255
          h = ((a & 15) > 9) and 0x10 or 0
        end

        R[7] = na
        F = SZ53P[na] | (f & 2) | c | h

      elseif y == 5 then
        a = a ~ 255
        R[7] = a
        F = (F & 0xC5) | 0x12 | (a & 0x28)

      elseif y == 6 then
        F = (F & 0xC4) | 1 | (a & 0x28)

      else
        local c = F & 1
        F = (F & 0xC4) | (c << 4) | (a & 0x28) | (1 - c)
      end

      return 4
    end

  else
    local y = (op >> 3) & 7
    local z = op & 7

    if z == 0 then
      if cc(y) then
        PC = pop()
        return 11
      end
      return 5

    elseif z == 1 then
      local p = y >> 1

      if (y & 1) == 0 then
        local v = pop()

        if p == 3 then
          R[7] = v >> 8
          F = v & 255
        else
          setrp(p, v)
        end

        return 10
      end

      if p == 0 then
        PC = pop()
        return 10

      elseif p == 1 then
        for i = 0, 5 do
          R[i], S[i] = S[i], R[i]
        end
        return 4

      elseif p == 2 then
        PC = (R[4] << 8) | R[5]
        return prefixed and 8 or 4
      end

      SP = (R[4] << 8) | R[5]
      return prefixed and 10 or 6

    elseif z == 2 then
      local nn = fetch16()
      if cc(y) then PC = nn end
      return 10

    elseif z == 3 then
      if y == 0 then
        PC = fetch16()
        return 10

      elseif y == 2 then
        local n = fetch()
        portOut((R[7] << 8) | n, R[7])
        return 11

      elseif y == 3 then
        local n = fetch()
        R[7] = portIn((R[7] << 8) | n)
        return 11

      elseif y == 4 then
        local lo = M[SP]
        local hi = M[(SP + 1) & 0xFFFF]

        wr(SP, R[5])
        wr((SP + 1) & 0xFFFF, R[4])

        R[5], R[4] = lo, hi
        return prefixed and 23 or 19

      elseif y == 5 then
        R[2], R[4] = R[4], R[2]
        R[3], R[5] = R[5], R[3]
        return 4

      elseif y == 6 then
        IFF1 = false
        IFF2 = false
        return 4

      else
        IFF1 = true
        IFF2 = true
        eiDelay = 2
        return 4
      end

    elseif z == 4 then
      local nn = fetch16()
      if cc(y) then
        push(PC)
        PC = nn
        return 17
      end
      return 10

    elseif z == 5 then
      if (y & 1) == 0 then
        local p = y >> 1
        local v

        if p == 3 then
          v = (R[7] << 8) | F
        else
          v = getrp(p)
        end

        push(v)
        return 11
      end

      local nn = fetch16()
      push(PC)
      PC = nn
      return 17

    elseif z == 6 then
      alu(y, fetch())
      return 7

    else
      push(PC)
      PC = y << 3
      return 11
    end
  end
end

local function cbop()
  local op = fetchOp()

  local x = op >> 6
  local y = (op >> 3) & 7
  local z = op & 7

  local a, v

  if z == 6 then
    a = (R[4] << 8) | R[5]
    v = M[a]
  else
    v = R[z]
  end

  if x == 0 then
    v = rot(y, v)

  elseif x == 1 then
    local m = v & (1 << y)
    F = (F & 1)
      | 0x10
      | (v & 0x28)
      | ((m == 0) and 0x44 or (m & 0x80))

    return (z == 6) and 12 or 8

  elseif x == 2 then
    v = v & ~(1 << y)

  else
    v = v | (1 << y)
  end

  if z == 6 then
    wr(a, v)
    return 15
  end

  R[z] = v
  return 8
end

local function edop()
  local op = fetchOp()

  local x = op >> 6
  local y = (op >> 3) & 7
  local z = op & 7

  if x == 1 then
    local p = y >> 1

    if z == 0 then
      local v = portIn((R[0] << 8) | R[1])

      F = (F & 1) | SZ53P[v]

      if y ~= 6 then
        R[y] = v
      end

      return 12

    elseif z == 1 then
      portOut(
        (R[0] << 8) | R[1],
        (y == 6) and 0 or R[y]
      )
      return 12

    elseif z == 2 then
      local hl = (R[4] << 8) | R[5]
      local rr = getrp(p)
      local c = F & 1

      local r, res

      if (y & 1) == 0 then
        r = hl - rr - c
        res = r & 0xFFFF

        F = ((res >> 8) & 0xA8)
          | ((res == 0) and 0x40 or 0)
          | (((hl ~ rr ~ res) >> 8) & 0x10)
          | (((hl ~ rr) & (hl ~ res) & 0x8000) >> 13)
          | 2
          | ((r >> 16) & 1)
      else
        r = hl + rr + c
        res = r & 0xFFFF

        F = ((res >> 8) & 0xA8)
          | ((res == 0) and 0x40 or 0)
          | (((hl ~ rr ~ res) >> 8) & 0x10)
          | (((~(hl ~ rr)) & (hl ~ res) & 0x8000) >> 13)
          | ((r >> 16) & 1)
      end

      R[4], R[5] = res >> 8, res & 255
      return 15

    elseif z == 3 then
      local nn = fetch16()

      if (y & 1) == 0 then
        local v = getrp(p)
        wr(nn, v & 255)
        wr((nn + 1) & 0xFFFF, v >> 8)
      else
        setrp(
          p,
          M[nn] | (M[(nn + 1) & 0xFFFF] << 8)
        )
      end

      return 20

    elseif z == 4 then
      local v = R[7]
      local res = (-v) & 255

      F = SZ53[res]
        | 2
        | ((v ~ res) & 0x10)
        | ((v == 0x80) and 4 or 0)
        | ((v ~= 0) and 1 or 0)

      R[7] = res
      return 8

    elseif z == 5 then
      PC = pop()
      IFF1 = IFF2
      eiDelay = 0
      return 14

    elseif z == 6 then
      if y == 0 or y == 1 or y == 4 or y == 5 then
        IM = 0
      elseif y == 2 or y == 6 then
        IM = 1
      else
        IM = 2
      end
      return 8

    else
      if y == 0 then
        I = R[7]
        return 9

      elseif y == 1 then
        RR = R[7]
        return 9

      elseif y == 2 then
        R[7] = I
        F = (F & 1) | SZ53[I] | (IFF2 and 4 or 0)
        return 9

      elseif y == 3 then
        R[7] = RR
        F = (F & 1) | SZ53[RR] | (IFF2 and 4 or 0)
        return 9

      elseif y == 4 then
        local hl = (R[4] << 8) | R[5]
        local m = M[hl]
        local a = R[7]

        wr(hl, ((a & 15) << 4) | (m >> 4))
        a = (a & 0xF0) | (m & 15)

        R[7] = a
        F = (F & 1) | SZ53P[a]
        return 18

      elseif y == 5 then
        local hl = (R[4] << 8) | R[5]
        local m = M[hl]
        local a = R[7]

        wr(hl, ((m << 4) & 0xF0) | (a & 15))
        a = (a & 0xF0) | (m >> 4)

        R[7] = a
        F = (F & 1) | SZ53P[a]
        return 18
      end

      return 8
    end

  elseif x == 2 and y >= 4 and z <= 3 then
    local dec = (y & 1) == 1
    local rep = y >= 6
    local d = dec and -1 or 1

    local hl = (R[4] << 8) | R[5]

    if z == 0 then
      local de = (R[2] << 8) | R[3]
      local bc = (((R[0] << 8) | R[1]) - 1) & 0xFFFF
      local v = M[hl]

      wr(de, v)

      hl = (hl + d) & 0xFFFF
      de = (de + d) & 0xFFFF

      R[4], R[5] = hl >> 8, hl & 255
      R[2], R[3] = de >> 8, de & 255
      R[0], R[1] = bc >> 8, bc & 255

      local n = v + R[7]

      F = (F & 0xC1)
        | ((bc ~= 0) and 4 or 0)
        | (n & 8)
        | ((n & 2) << 4)

      if rep and bc ~= 0 then
        PC = (PC - 2) & 0xFFFF
        return 21
      end

      return 16

    elseif z == 1 then
      local bc = (((R[0] << 8) | R[1]) - 1) & 0xFFFF
      local v = M[hl]

      local res = (R[7] - v) & 255
      local hf = (R[7] ~ v ~ res) & 0x10
      local n = res - (hf >> 4)

      hl = (hl + d) & 0xFFFF

      R[4], R[5] = hl >> 8, hl & 255
      R[0], R[1] = bc >> 8, bc & 255

      F = (F & 1)
        | 2
        | (res & 0x80)
        | ((res == 0) and 0x40 or 0)
        | hf
        | ((bc ~= 0) and 4 or 0)
        | (n & 8)
        | ((n & 2) << 4)

      if rep and bc ~= 0 and res ~= 0 then
        PC = (PC - 2) & 0xFFFF
        return 21
      end

      return 16

    elseif z == 2 then
      local v = portIn((R[0] << 8) | R[1])

      wr(hl, v)

      R[0] = (R[0] - 1) & 255
      hl = (hl + d) & 0xFFFF

      R[4], R[5] = hl >> 8, hl & 255
      F = SZ53[R[0]] | 2

      if rep and R[0] ~= 0 then
        PC = (PC - 2) & 0xFFFF
        return 21
      end

      return 16

    else
      local v = M[hl]

      R[0] = (R[0] - 1) & 255
      portOut((R[0] << 8) | R[1], v)

      hl = (hl + d) & 0xFFFF
      R[4], R[5] = hl >> 8, hl & 255

      F = SZ53[R[0]] | 2

      if rep and R[0] ~= 0 then
        PC = (PC - 2) & 0xFFFF
        return 21
      end

      return 16
    end
  end

  return 8
end

local function indexedCB(ireg)
  local d = fetch()
  if d >= 128 then d = d - 256 end

  local op = fetch()
  local a = (ireg + d) & 0xFFFF

  local x = op >> 6
  local y = (op >> 3) & 7
  local z = op & 7

  local v = M[a]

  if x == 0 then
    v = rot(y, v)

  elseif x == 1 then
    local m = v & (1 << y)

    F = (F & 1)
      | 0x10
      | ((a >> 8) & 0x28)
      | ((m == 0) and 0x44 or (m & 0x80))

    return 20

  elseif x == 2 then
    v = v & ~(1 << y)

  else
    v = v | (1 << y)
  end

  wr(a, v)

  if z ~= 6 then
    R[z] = v
  end

  return 23
end

local function prefTotal(ins)
  local x = ins >> 6
  local y = (ins >> 3) & 7
  local z = ins & 7

  if x == 1 then
    return z == 6 or y == 6
  elseif x == 2 then
    return z == 6
  elseif x == 0 then
    if z == 1 or z == 3 then return true end
    return y == 6 and (z == 4 or z == 5 or z == 6)
  else
    if z == 1 then return y == 5 or y == 7 end
    if z == 3 then return y == 4 end
  end

  return false
end

local function prefix(op)
  local isIX = (op == 0xDD)
  local extra = 0

  while true do
    local n = M[PC]

    if n == 0xDD then
      fetchOp()
      isIX = true
      extra = extra + 4
    elseif n == 0xFD then
      fetchOp()
      isIX = false
      extra = extra + 4
    else
      break
    end
  end

  local ireg = isIX and IX or IY
  local nxt = M[PC]

  if nxt == 0xCB then
    fetchOp()
    return indexedCB(ireg) + extra
  end

  if nxt == 0xED then
    fetchOp()
    return edop() + 4 + extra
  end

  if nxt == 0xEB then
    fetchOp()
    R[2], R[4] = R[4], R[2]
    R[3], R[5] = R[5], R[3]
    return 8 + extra
  end

  if nxt == 0xD9 then
    fetchOp()
    for i = 0, 5 do
      R[i], S[i] = S[i], R[i]
    end
    return 8 + extra
  end

  local ignored =
      nxt == 0x00 or nxt == 0x08 or nxt == 0x10 or nxt == 0x18
   or nxt == 0x20 or nxt == 0x28 or nxt == 0x30 or nxt == 0x38
   or nxt == 0x76
   or nxt == 0xC3 or nxt == 0xC9
   or nxt == 0xCD
   or nxt == 0xC0 or nxt == 0xC8 or nxt == 0xD0 or nxt == 0xD8
   or nxt == 0xE0 or nxt == 0xE8 or nxt == 0xF0 or nxt == 0xF8
   or nxt == 0xC7 or nxt == 0xCF or nxt == 0xD7 or nxt == 0xDF
   or nxt == 0xE7 or nxt == 0xEF or nxt == 0xF7 or nxt == 0xFF
   or nxt == 0xF3 or nxt == 0xFB
   or nxt == 0x27 or nxt == 0x2F or nxt == 0x37 or nxt == 0x3F

  if ignored then
    return exec(fetchOp()) + 4 + extra
  end

  sh, sl = R[4], R[5]

  R[4] = ireg >> 8
  R[5] = ireg & 255

  prefixed = true

  local instruction = fetchOp()
  local t = exec(instruction)

  prefixed = false

  local nv = (R[4] << 8) | R[5]

  if isIX then
    IX = nv
  else
    IY = nv
  end

  R[4], R[5] = sh, sl

  if not prefTotal(instruction) then
    t = t + 4
  end

  return t + extra
end

local function tapTrap()
  local want = R[7]
  local isLoad = (F & 1) ~= 0

  local de = (R[2] << 8) | R[3]
  local ok = false

  while tapePos <= #tapBlocks do
    local blk = tapBlocks[tapePos]
    tapePos = tapePos + 1

    if #blk > 0 and blk:byte(1) == want then
      if isLoad then
        local n = #blk - 2
        local cnt = math.min(n, de)

        for i = 0, cnt - 1 do
          wr((IX + i) & 0xFFFF, blk:byte(2 + i))
        end

        ok = (n == de)

        IX = (IX + cnt) & 0xFFFF
        de = de - cnt
      else
        ok = true
      end

      break
    end
  end

  R[2], R[3] = de >> 8, de & 255

  if ok then
    F = F | 1
  else
    F = F & 0xFE
  end

  PC = pop()

  IFF1 = true
  IFF2 = true
  eiDelay = 0

  return 10
end

local function step()
  if PC == 0x0556 and tapBlocks then
    return tapTrap()
  end

  local op = fetchOp()

  if op == 0xCB then
    return cbop()

  elseif op == 0xED then
    return edop()

  elseif op == 0xDD or op == 0xFD then
    return prefix(op)
  end

  return exec(op)
end

local function resetMachine()
  for i = 0x4000, 0xFFFF do
    M[i] = 0
  end

  for i = 0, 7 do
    R[i] = 0
    S[i] = 0
  end

  R[7] = 0xFF
  S[7] = 0xFF
  F = 0xFF
  F2 = 0xFF

  PC = 0
  SP = 0xFFFF
  IX = 0xFFFF
  IY = 0xFFFF
  I = 0
  RR = 0

  IFF1 = false
  IFF2 = false
  IM = 0
  halted = false
  eiDelay = 0

  border = 7
  borderDirty = true

  spk = 0
  tgN = 0

  tapePos = 1

  for i = 0, 1535 do
    dirty[i] = true
  end

  anyDirty = true
end

local function loadSNA(d)
  local function b(i)
    return d:byte(i + 1)
  end

  I = b(0)

  S[5], S[4] = b(1), b(2)
  S[3], S[2] = b(3), b(4)
  S[1], S[0] = b(5), b(6)
  F2, S[7] = b(7), b(8)

  R[5], R[4] = b(9), b(10)
  R[3], R[2] = b(11), b(12)
  R[1], R[0] = b(13), b(14)

  IY = b(15) | (b(16) << 8)
  IX = b(17) | (b(18) << 8)

  IFF2 = (b(19) & 4) ~= 0
  IFF1 = IFF2

  RR = b(20)

  F, R[7] = b(21), b(22)

  SP = b(23) | (b(24) << 8)
  IM = b(25) & 3
  border = b(26) & 7

  for i = 0, 49151 do
    M[0x4000 + i] = d:byte(28 + i)
  end

  PC = pop()

  halted = false
  eiDelay = 0
  borderDirty = true

  for i = 0, 1535 do
    dirty[i] = true
  end

  anyDirty = true
end

local function z80Unpack(d, pos, last, want)
  local out, n, o = {}, 0, 0

  while o < want and pos <= last do
    local b = d:byte(pos)

    if b == 0xED and pos + 3 <= last and d:byte(pos + 1) == 0xED then
      local cnt = d:byte(pos + 2)
      local v = d:byte(pos + 3)
      n = n + 1
      out[n] = string.rep(string.char(v), cnt)
      o = o + cnt
      pos = pos + 4
    else
      n = n + 1
      out[n] = string.char(b)
      o = o + 1
      pos = pos + 1
    end
  end

  local s = table.concat(out)

  if #s < want then
    s = s .. string.rep("\0", want - #s)
  end

  return s:sub(1, want)
end

local function loadZ80(d)
  local function b(i)
    return d:byte(i + 1) or 0
  end

  R[7] = b(0)
  F = b(1)
  R[1], R[0] = b(2), b(3)
  R[5], R[4] = b(4), b(5)

  local pc = b(6) | (b(7) << 8)

  SP = b(8) | (b(9) << 8)
  I = b(10)

  local f12 = b(12)
  if f12 == 255 then f12 = 1 end

  RR = (b(11) & 0x7F) | ((f12 & 1) << 7)
  border = (f12 >> 1) & 7

  local compressed = (f12 & 0x20) ~= 0

  R[3], R[2] = b(13), b(14)
  S[1], S[0] = b(15), b(16)
  S[3], S[2] = b(17), b(18)
  S[5], S[4] = b(19), b(20)
  S[7], F2 = b(21), b(22)

  IY = b(23) | (b(24) << 8)
  IX = b(25) | (b(26) << 8)

  IFF1 = b(27) ~= 0
  IFF2 = b(28) ~= 0
  IM = b(29) & 3

  if pc ~= 0 then
    local mem

    if compressed then
      mem = z80Unpack(d, 31, #d, 49152)
    else
      mem = d:sub(31, 30 + 49152)
    end

    for i = 0, 49151 do
      M[0x4000 + i] = mem:byte(i + 1) or 0
    end

    PC = pc

  else
    local hlen = b(30) | (b(31) << 8)

    PC = b(32) | (b(33) << 8)

    local mode = b(34)

    if (hlen == 23 and mode >= 3) or (hlen ~= 23 and mode >= 4) then
      error("z80: only 48K snapshots are supported")
    end

    local pos = 33 + hlen

    while pos + 2 <= #d do
      local len = d:byte(pos) | (d:byte(pos + 1) << 8)
      local page = d:byte(pos + 2)

      pos = pos + 3

      local mem

      if len == 0xFFFF then
        mem = d:sub(pos, pos + 16384)
        pos = pos + 16384
      else
        mem = z80Unpack(d, pos, pos + len - 1, 16384)
        pos = pos + len
      end

      local base

      if page == 8 then
        base = 0x4000
      elseif page == 4 then
        base = 0x8000
      elseif page == 5 then
        base = 0xC000
      end

      if base then
        for i = 0, 16383 do
          M[base + i] = mem:byte(i + 1) or 0
        end
      end
    end
  end

  halted = false
  eiDelay = 0
  borderDirty = true

  for i = 0, 1535 do dirty[i] = true end
  anyDirty = true
end

local function loadSCR(d)
  for i = 0, 6911 do
    M[0x4000 + i] = d:byte(i + 1)
  end
  for i = 0, 1535 do
    dirty[i] = true
  end
  anyDirty = true
end

local function loadBIN(d)
  local addr = tonumber(os.getenv("BIN_ADDR") or "0x8000") or 0x8000
  for i = 1, #d do
    M[(addr + i - 1) & 0xFFFF] = d:byte(i)
  end
  for i = 0, 1535 do dirty[i] = true end
  anyDirty = true
end

local LINE = {}

for y = 0, 191 do
  LINE[y] =
      0x4000
    | ((y & 0xC0) << 5)
    | ((y & 7) << 8)
    | ((y & 0x38) << 2)
end

local BL = {1, 2, 4, 0x40}
local BR = {8, 0x10, 0x20, 0x80}

local PX = {}

for k = 1, 4 do
  PX[k] = {}

  for pair = 0, 3 do
    PX[k][pair] =
      (((pair & 2) ~= 0) and BL[k] or 0)
      | (((pair & 1) ~= 0) and BR[k] or 0)
  end
end

local PX1, PX2, PX3, PX4 =
  PX[1], PX[2], PX[3], PX[4]

local uchar
do
  local ok, unicode = pcall(require, "unicode")
  if ok and unicode and unicode.char then
    uchar = unicode.char
  else
    uchar = utf8.char
  end
end

local curFG, curBG = -1, -1

local function cell(by, bx)
  local y0 = by << 2

  local attr =
    M[0x5800 + ((y0 >> 3) << 5) + bx]

  local ink = attr & 7
  local paper = (attr >> 3) & 7

  if (attr & 0x80) ~= 0 and flashOn then
    ink, paper = paper, ink
  end

  if (attr & 0x40) ~= 0 then
    ink = ink + 8
    paper = paper + 8
  end

  local b0 = M[LINE[y0] + bx]
  local b1 = M[LINE[y0 + 1] + bx]
  local b2 = M[LINE[y0 + 2] + bx]
  local b3 = M[LINE[y0 + 3] + bx]

  local s = uchar(
      0x2800
      | PX1[(b0 >> 6) & 3]
      | PX2[(b1 >> 6) & 3]
      | PX3[(b2 >> 6) & 3]
      | PX4[(b3 >> 6) & 3],

      0x2800
      | PX1[(b0 >> 4) & 3]
      | PX2[(b1 >> 4) & 3]
      | PX3[(b2 >> 4) & 3]
      | PX4[(b3 >> 4) & 3],

      0x2800
      | PX1[(b0 >> 2) & 3]
      | PX2[(b1 >> 2) & 3]
      | PX3[(b2 >> 2) & 3]
      | PX4[(b3 >> 2) & 3],

      0x2800
      | PX1[b0 & 3]
      | PX2[b1 & 3]
      | PX3[b2 & 3]
      | PX4[b3 & 3]
  )

  return ink, paper, s
end

local function flush(by, x0, fg, bg, str)
  if fg ~= curFG then
    gpu.setForeground(fg, true)
    curFG = fg
  end

  if bg ~= curBG then
    gpu.setBackground(bg, true)
    curBG = bg
  end

  gpu.set(
    ox + x0 * 4 + 1,
    oy + by + 1,
    str
  )
end

local function drawBorder()
  borderDirty = false

  if ox == 0 then return end

  if curBG ~= border then
    gpu.setBackground(border, true)
    curBG = border
  end

  gpu.fill(1, 1, 160, oy, " ")
  gpu.fill(1, 51 - oy, 160, oy, " ")
  gpu.fill(1, oy + 1, ox, 48, " ")
  gpu.fill(161 - ox, oy + 1, ox, 48, " ")
end

local function render()
  if borderDirty then
    drawBorder()
  end

  if not anyDirty then
    return
  end

  anyDirty = false

  for by = 0, 47 do
    local base = by * 32
    local bx = 0

    while bx < 32 do
      if dirty[base + bx] then
        local x0 = bx
        local fg, bg, str = cell(by, bx)

        dirty[base + bx] = nil
        bx = bx + 1

        while bx < 32 and dirty[base + bx] do
          local f2, b2, s2 = cell(by, bx)

          if f2 ~= fg or b2 ~= bg then
            break
          end

          dirty[base + bx] = nil
          str = str .. s2
          bx = bx + 1
        end

        flush(by, x0, fg, bg, str)
      else
        bx = bx + 1
      end
    end
  end
end

local function markFlash()
  for n = 0, 767 do
    if (M[0x5800 + n] & 0x80) ~= 0 then
      local k = (n >> 5) * 64 + (n & 31)

      dirty[k] = true
      dirty[k + 32] = true

      anyDirty = true
    end
  end
end

local CAPS = {{0, 0}}
local SYM = {{7, 1}}

local KEYS = {
  [42] = CAPS,
  [54] = SYM,
  [29] = SYM,
  [157] = SYM,

  [44] = {{0,1}},
  [45] = {{0,2}},
  [46] = {{0,3}},
  [47] = {{0,4}},

  [30] = {{1,0}},
  [31] = {{1,1}},
  [32] = {{1,2}},
  [33] = {{1,3}},
  [34] = {{1,4}},

  [16] = {{2,0}},
  [17] = {{2,1}},
  [18] = {{2,2}},
  [19] = {{2,3}},
  [20] = {{2,4}},

  [2] = {{3,0}},
  [3] = {{3,1}},
  [4] = {{3,2}},
  [5] = {{3,3}},
  [6] = {{3,4}},

  [11] = {{4,0}},
  [10] = {{4,1}},
  [9]  = {{4,2}},
  [8]  = {{4,3}},
  [7]  = {{4,4}},

  [25] = {{5,0}},
  [24] = {{5,1}},
  [23] = {{5,2}},
  [22] = {{5,3}},
  [21] = {{5,4}},

  [28] = {{6,0}},
  [38] = {{6,1}},
  [37] = {{6,2}},
  [36] = {{6,3}},
  [35] = {{6,4}},

  [57] = {{7,0}},
  [50] = {{7,2}},
  [49] = {{7,3}},
  [48] = {{7,4}},

  [14] = {CAPS[1], {4,0}},

  [203] = {CAPS[1], {3,4}},
  [208] = {CAPS[1], {4,4}},
  [200] = {CAPS[1], {4,3}},
  [205] = {CAPS[1], {4,2}},

  [51] = {SYM[1], {7,3}},
  [52] = {SYM[1], {7,2}},
  [53] = {SYM[1], {0,4}},
  [39] = {SYM[1], {5,1}},
  [12] = {SYM[1], {6,3}},
  [13] = {SYM[1], {6,1}},
  [40] = {SYM[1], {4,3}},
}

local function pressList(list)
  for _, k in ipairs(list) do
    local idx = k[1] * 8 + k[2]
    local n = (kcount[idx] or 0) + 1

    kcount[idx] = n

    if n == 1 then
      kb[k[1]] = kb[k[1]] & ~(1 << k[2])
    end
  end
end

local function releaseList(list)
  for _, k in ipairs(list) do
    local idx = k[1] * 8 + k[2]
    local n = (kcount[idx] or 0) - 1

    if n <= 0 then
      kcount[idx] = 0
      kb[k[1]] = kb[k[1]] | (1 << k[2])
    else
      kcount[idx] = n
    end
  end
end

local frames = 0
local running = true
local muted = false
local resetAudio
local adjustVolume

local downAt = {}
local pendingUp = {}

local typeQ
local typeI = 0
local typeT = 0

local LOAD_SEQ = {
  {{6,3}},
  {SYM[1], {5,0}},
  {SYM[1], {5,0}},
  {{6,0}},
}

local function typeTick()
  if not typeQ then return end

  typeT = typeT + 1

  local e = typeQ[typeI]

  if typeT == 1 then
    pressList(e)

  elseif typeT == 5 then
    releaseList(e)

  elseif typeT >= 8 then
    typeT = 0
    typeI = typeI + 1

    if typeI > #typeQ then
      typeQ = nil
    end
  end
end

local function handle(name, _, _, code)
  if name == "key_down" then

    if code == 88 then
      running = false
      return
    end

    if code == 63 then
      resetMachine()
      return
    end

    if code == 61 then
      typeQ = LOAD_SEQ
      typeI = 1
      typeT = 0
      return
    end

    if code == 62 then
      tapePos = 1
      return
    end

    if code == 64 then
      if adjustVolume then adjustVolume(-0.1) end
      return
    end

    if code == 65 then
      if adjustVolume then adjustVolume(0.1) end
      return
    end

    if code == 60 then
      muted = not muted
      if resetAudio then resetAudio() end
      return
    end

    local list = KEYS[code]

    if list then
      if not downAt[code] then
        downAt[code] = frames
        pressList(list)
      end

      pendingUp[code] = nil
    end

  elseif name == "key_up" then
    local t = downAt[code]

    if t then
      if frames - t >= KEY_HOLD then
        releaseList(KEYS[code])
        downAt[code] = nil
      else
        pendingUp[code] = t + KEY_HOLD
      end
    end

  elseif name == "interrupted" then
    running = false
  end
end

local function pump(timeout)
  local s, a, b, c = computer.pullSignal(timeout)

  while s and running do
    handle(s, a, b, c)
    s, a, b, c = computer.pullSignal(0)
  end
end

local SNDCFG = {spFs = 16384, spN = 1024, polyMin = 0.02, polyRel = 0.06, polyMax = 8, gain = 0.85, tilt = 0.2, maxHz = 15000}

local SP_FS = SNDCFG.spFs or 16384
local SP_N = SNDCFG.spN or 1024
local SP_M = SP_N // 2
local SP_TS = CPU_HZ / SP_FS
local SP_DF = SP_FS / SP_N
local SP_NYQ = SP_FS / 2
local SP_KMIN = math.max(2, math.ceil(45 / SP_DF))

local spRing, spW, spS, spLast, spAcc, spLvl = {}, 0, 0, 0, 0, 0
local spHann, spBR, spTwr, spTwi, spPwr, spPwi = {}, {}, {}, {}, {}, {}
local spX, spRe, spIm, spAmp, spMark = {}, {}, {}, {}, {}
local spPrev = {}
local spStamp = 0
local spCount = 0
local spCache = nil

do
  for i = 1, SP_N do
    spHann[i] = 0.5 - 0.5 * math.cos(2 * math.pi * (i - 1) / SP_N)
    spX[i] = 0
    spRing[i] = 0
  end

  local bits = 0
  while (1 << bits) < SP_M do bits = bits + 1 end

  for m = 0, SP_M - 1 do
    local r, x = 0, m
    for _ = 1, bits do
      r = (r << 1) | (x & 1)
      x = x >> 1
    end
    spBR[m + 1] = r + 1
    spRe[m + 1] = 0
    spIm[m + 1] = 0
  end

  for t = 0, SP_M // 2 - 1 do
    spTwr[t + 1] = math.cos(2 * math.pi * t / SP_M)
    spTwi[t + 1] = -math.sin(2 * math.pi * t / SP_M)
  end

  for k = 0, SP_M do
    spPwr[k + 1] = math.cos(2 * math.pi * k / SP_N)
    spPwi[k + 1] = -math.sin(2 * math.pi * k / SP_N)
    spAmp[k + 1] = 0
    spMark[k + 1] = 0
  end
end

local function spInit(t)
  spW = 0
  spS = t
  spLast = t
  spAcc = 0
  spPrev = {}
  spCache = nil
  spCount = 0

  for i = 1, SP_N do spRing[i] = 0 end
end

local function spAdvance(t)
  if t <= spLast then return end

  local lvl = spLvl

  while spS + SP_TS <= t do
    local e = spS + SP_TS
    local v = (spAcc + lvl * (e - spLast)) / SP_TS

    spW = spW + 1
    spRing[(spW - 1) % SP_N + 1] = v

    spS = e
    spLast = e
    spAcc = 0
  end

  spAcc = spAcc + lvl * (t - spLast)
  spLast = t
end

local function spFeed(fe, tT, tL, tN)
  for i = 1, tN do
    spAdvance(tT[i])
    spLvl = tL[i]
  end

  spAdvance(fe)
end

local function spSpectrum()
  local N, M = SP_N, SP_M
  local base = spW - N
  local sum = 0

  for i = 1, N do
    local idx = base + i
    local v = 0

    if idx >= 1 then v = spRing[(idx - 1) % N + 1] end

    spX[i] = v
    sum = sum + v
  end

  local mean = sum / N

  for m = 0, M - 1 do
    local j = spBR[m + 1]
    local i0 = 2 * m + 1
    spRe[j] = (spX[i0] - mean) * spHann[i0]
    spIm[j] = (spX[i0 + 1] - mean) * spHann[i0 + 1]
  end

  local re, im = spRe, spIm
  local size = 2

  while size <= M do
    local half = size >> 1
    local step = M // size

    for st = 0, M - 1, size do
      local tw = 1

      for k = st + 1, st + half do
        local wr, wi = spTwr[tw], spTwi[tw]
        local b = k + half
        local br, bi = re[b], im[b]
        local xr = br * wr - bi * wi
        local xi = br * wi + bi * wr
        local ar, ai = re[k], im[k]

        re[b] = ar - xr
        im[b] = ai - xi
        re[k] = ar + xr
        im[k] = ai + xi

        tw = tw + step
      end
    end

    size = size << 1
  end

  local scale = 4 / N

  for k = 0, M do
    local ka = k % M
    local kb = (M - k) % M
    local zr, zi = re[ka + 1], im[ka + 1]
    local cr, ci = re[kb + 1], -im[kb + 1]
    local er, ei = (zr + cr) * 0.5, (zi + ci) * 0.5
    local orr, oi = (zi - ci) * 0.5, -(zr - cr) * 0.5
    local wr, wi = spPwr[k + 1], spPwi[k + 1]
    local xr = er + orr * wr - oi * wi
    local xi = ei + orr * wi + oi * wr

    spAmp[k + 1] = math.sqrt(xr * xr + xi * xi) * scale
  end
end

local function spBand(f, tol)
  local k0 = math.floor(f * (1 - tol) / SP_DF)
  local k1 = math.ceil(f * (1 + tol) / SP_DF)

  if k0 < 1 then k0 = 1 end
  if k1 > SP_M then k1 = SP_M end

  local best = 0

  for k = k0, k1 do
    local a = spAmp[k + 1]
    if a > best then best = a end
  end

  return best
end

local SP_FUND = {sq = 4 / math.pi, tri = 8 / (math.pi * math.pi), sin = 1}
local SP_HARM = {
  sq = function(h) return (h % 2 == 1) and 1 / h or 0 end,
  tri = function(h) return (h % 2 == 1) and 1 / (h * h) or 0 end,
  sin = function(h) return 0 end,
}

local function spVoices()
  spSpectrum()

  local amp = spAmp
  local kmax = SP_M - 1
  local top = 0
  local tot = 0

  for k = SP_KMIN, kmax do
    local a = amp[k + 1]
    tot = tot + a * a
    if a > top then top = a end
  end

  if top < (SNDCFG.polyMin or 0.02) or tot <= 0 then return nil, 0 end

  local thr = math.max(SNDCFG.polyMin or 0.02, (SNDCFG.polyRel or 0.06) * top)
  local cands = {}

  for k = SP_KMIN, kmax do
    local a = amp[k + 1]

    if a >= thr * 0.6 and a >= amp[k] and a > amp[k + 2] then
      local al, ac, ar = amp[k], a, amp[k + 2]
      local la, lb, lc = math.log(math.max(al, 1e-9)), math.log(ac),
                         math.log(math.max(ar, 1e-9))
      local den = la - 2 * lb + lc
      local d = 0

      if den < -1e-9 then d = 0.5 * (la - lc) / den end
      if d > 0.5 then d = 0.5 elseif d < -0.5 then d = -0.5 end

      local f = (k + d) * SP_DF
      local pa = math.exp(lb - 0.25 * (la - lc) * d)
      local keep = pa >= thr

      if not keep then
        for i = 1, #spPrev do
          if math.abs(f / spPrev[i] - 1) < 0.04 then keep = true break end
        end
      end

      if keep then cands[#cands + 1] = {f = f, a = pa, k = k} end
    end
  end

  if #cands == 0 then return nil, 0 end

  table.sort(cands, function(x, y) return x.a > y.a end)

  spStamp = spStamp + 1

  local pe = 0

  for i = 1, math.min(#cands, 10) do
    local k = cands[i].k

    for j = math.max(SP_KMIN, k - 2), math.min(kmax, k + 2) do
      if spMark[j + 1] ~= spStamp then
        spMark[j + 1] = spStamp
        pe = pe + amp[j + 1] * amp[j + 1]
      end
    end
  end

  local conc = pe / tot

  local maxV = math.min(8, SNDCFG.polyMax or 8)
  local voices = {}

  for _, p in ipairs(cands) do
    local harmonic = false

    for _, v in ipairs(voices) do
      local h = math.floor(p.f / v.f + 0.5)

      if h >= 2 and h <= 8 then
        local near = math.abs(p.f / (h * v.f) - 1) < 0.03
                     or math.abs(p.f - h * v.f) < 1.3 * SP_DF

        if near and p.a <= 1.6 * v.a / h then
          v.harm[#v.harm + 1] = {h = h, f = p.f, a = p.a}
          harmonic = true
          break
        end
      end
    end

    if not harmonic and #voices < maxV then
      voices[#voices + 1] = {f = p.f, a = p.a, harm = {}}
    end
  end

  local g = SNDCFG.gain or 0.85
  local tilt = SNDCFG.tilt or 0.2
  local out, extras = {}, {}
  local sumv = 0

  for _, v in ipairs(voices) do
    local r2 = (2 * v.f < SP_NYQ) and spBand(2 * v.f, 0.03) / v.a or 0
    local r3 = (3 * v.f < SP_NYQ) and spBand(3 * v.f, 0.03) / v.a or 1 / 3
    local wave

    if r3 < 0.05 and r2 < 0.1 then
      wave = "sin"
    elseif r3 < 0.2 and r2 < 0.2 then
      wave = "tri"
    else
      wave = "sq"
    end

    local fm = SP_HARM[wave]
    local lift = math.min(1.6, math.max(0.85, (v.f / 250) ^ tilt))
    local vol = v.a * 2 * g * lift / SP_FUND[wave]

    out[#out + 1] = {v.f, vol, wave, v.a}
    sumv = sumv + vol

    for _, hm in ipairs(v.harm) do
      local res = hm.a - fm(hm.h) * v.a

      if res >= math.max(SNDCFG.polyMin or 0.02, 0.12 * v.a)
         and hm.f < SNDCFG.maxHz then
        extras[#extras + 1] = {hm.f, res * 2 * g, "sin", res}
      end
    end
  end

  table.sort(extras, function(x, y) return x[4] > y[4] end)

  for i = 1, #extras do
    if #out >= 8 then break end

    out[#out + 1] = extras[i]
    sumv = sumv + extras[i][2]
  end

  local sc = 1

  if sumv > 1.15 then sc = 1.15 / sumv end

  for _, o in ipairs(out) do
    o[2] = math.min(1, o[2] * sc)
  end

  spPrev = {}

  for i = 1, #out do spPrev[i] = out[i][1] end

  return out, conc
end

local masterVol = MASTER_VOLUME
local chs = { held = false, cw = {}, cv = {}, cf = {}, co = {} }

local function resetChannels()
  chs.cw, chs.cv, chs.cf, chs.co = {}, {}, {}, {}
  chs.init = false
  chs.held = false
end

resetChannels()

local function cardClear()
  if snd and snd.clear then
    pcall(function() snd.clear() end)
  end

  if chs.held and snd then
    pcall(function()
      for c = 1, 8 do snd.close(c) end
      snd.process()
    end)
  end

  resetChannels()
end

resetAudio = function()
  tgN = 0
  spInit(ts)
  cardClear()
end

adjustVolume = function(delta)
  if not snd then return end
  masterVol = math.max(0, math.min(1, masterVol + delta))
  pcall(function() snd.setTotalVolume(masterVol) end)
end

local function initSound()
  if not snd then return end
  pcall(function() snd.setTotalVolume(masterVol) end)
  spInit(0)
end

local function closeSound()
  if not snd then return end
  pcall(function()
    for c = 1, 8 do snd.close(c) end
    snd.process()
  end)
end

local WAVE_MAP = {
  sq = 1,
  sin = 2,
  tri = 3
}

local function analyseBeeper(fs, fe)
  if not snd or muted then
    tgN = 0
    return
  end

  spFeed(fe, tgT, tgL, tgN)
  tgN = 0
end

local function submitAudio()
  if not snd or muted then return end

  local voices, conc = spVoices()

  if not voices or #voices == 0 then
    if chs.held then
      pcall(function()
        for c = 1, 8 do snd.close(c) end
        snd.process()
      end)
      resetChannels()
    end
    return
  end

  pcall(function()
    local modes = snd.modes or {}
    local waveTypes = {
      sq = modes.square or 1,
      sin = modes.sine or modes.square or 1,
      tri = modes.triangle or modes.square or 1
    }

    if not chs.init then
      for c = 1, 8 do
        snd.close(c)
        pcall(snd.setADSR, c, 1, 1, 1, 1)
      end
      chs.init = true
    end

    local usedChannels = #voices

    for c = 1, usedChannels do
      local v = voices[c]
      local freq = v[1]
      local vol = v[2]
      local waveStr = v[3]
      local waveType = waveTypes[waveStr] or 1

      if chs.cw[c] ~= waveType then
        snd.setWave(c, waveType)
        chs.cw[c] = waveType
      end

      if not chs.cv[c] or math.abs(chs.cv[c] - vol) > 0.02 then
        snd.setVolume(c, vol)
        chs.cv[c] = vol
      end

      local fi = math.max(1, math.floor(freq + 0.5))
      if chs.cf[c] ~= fi then
        snd.setFrequency(c, fi)
        chs.cf[c] = fi
      end

      if not chs.co[c] then
        snd.open(c)
        chs.co[c] = true
      end
    end

    for c = usedChannels + 1, 8 do
      if chs.co[c] then
        snd.close(c)
        chs.co[c] = false
      end
    end

    snd.delay(20)
    snd.process()
    chs.held = true
  end)
end

local saved = {}

local function setupGPU()
  local mw, mh = gpu.maxResolution()

  saved.w, saved.h = gpu.getResolution()
  saved.depth = gpu.getDepth()
  saved.fg, saved.fgp = gpu.getForeground()
  saved.bg, saved.bgp = gpu.getBackground()

  if mw >= 160 and mh >= 50 then
    gpu.setResolution(160, 50)
    ox, oy = 16, 1

  elseif mw >= 128 and mh >= 48 then
    gpu.setResolution(128, 48)
    ox, oy = 0, 0

  else
    error(
      "a Tier 3 GPU and screen (at least 128x48 characters) are required"
    )
  end

  gpu.setDepth(gpu.maxDepth())

  saved.pal = {}

  for c = 0, 15 do
    local v = (c >= 8) and 0xFF or 0xD7
    local cc = c & 7

    local r = ((cc & 2) ~= 0) and v or 0
    local g = ((cc & 4) ~= 0) and v or 0
    local b = ((cc & 1) ~= 0) and v or 0

    saved.pal[c] =
      gpu.setPaletteColor(
        c,
        (r << 16) | (g << 8) | b
      )
  end

  local w, h = gpu.getResolution()

  gpu.setBackground(7, true)
  curBG = 7

  gpu.fill(1, 1, w, h, " ")

  curFG = -1
end

local function restoreGPU()
  pcall(function()
    if saved.pal then
      for c = 0, 15 do
        if saved.pal[c] then
          gpu.setPaletteColor(c, saved.pal[c])
        end
      end
    end

    if saved.depth then
      gpu.setDepth(saved.depth)
    end

    if saved.w then
      gpu.setResolution(saved.w, saved.h)
    end

    if saved.fg then
      gpu.setForeground(saved.fg, saved.fgp)
    end

    if saved.bg then
      gpu.setBackground(saved.bg, saved.bgp)
    end

    local w, h = gpu.getResolution()
    gpu.fill(1, 1, w, h, " ")
  end)
end

local function serviceKeyboard()
  for code, f in pairs(pendingUp) do
    if frames >= f then
      local list = KEYS[code]

      if list then
        releaseList(list)
      end

      downAt[code] = nil
      pendingUp[code] = nil
    end
  end
end

local function acceptInterrupt()
  halted = false
  IFF1 = false
  IFF2 = false
  incR()

  if IM == 2 then
    push(PC)

    local v = (I << 8) | 0xFF
    PC =
      M[v]
      | (M[(v + 1) & 0xFFFF] << 8)

    return 19
  end

  push(PC)
  PC = 0x38

  return 13
end

local function main()
  setupGPU()
  initSound()

  resetMachine()

  local haveSnapshot = false

  if snaData then
    loadSNA(snaData)
    haveSnapshot = true
  elseif z80Data then
    loadZ80(z80Data)
    haveSnapshot = true
  elseif scrData then
    loadSCR(scrData)
  elseif binData then
    loadBIN(binData)
    haveSnapshot = true
  end

  local autoLoad = tapBlocks ~= nil and not haveSnapshot

  frameStart = ts

  local nextT = computer.uptime()
  local lastPump = nextT

  while running do
    frames = frames + 1

    local frameEnd = frameStart + FRAME_TS

    local intPending = true
    local intEnd = frameStart + 32

    while ts < frameEnd do
      if intPending then
        if ts >= intEnd then
          intPending = false
        elseif IFF1 and eiDelay == 0 then
          intPending = false
          ts = ts + acceptInterrupt()
        end
      end

      if halted then
        ts = frameEnd
      else
        ts = ts + step()

        if eiDelay > 0 then
          eiDelay = eiDelay - 1
        end
      end
    end

    analyseBeeper(frameStart, frameEnd)

    frameStart = frameEnd

    if (frames & 15) == 0 then
      flashOn = not flashOn
      markFlash()
    end

    if autoLoad and frames == AUTOLOAD_FRAME then
      typeQ = LOAD_SEQ
      typeI = 1
      typeT = 0
    end

    typeTick()
    serviceKeyboard()

    if frames % RENDER_EVERY == 0 then
      render()
    end

    submitAudio()

    local now = computer.uptime()

    if now < nextT then
      repeat
        pump(nextT - now)
        now = computer.uptime()
      until now >= nextT or not running

      lastPump = now

    elseif now - lastPump >= PUMP_INTERVAL then
      pump(0)
      lastPump = computer.uptime()
    end

    nextT = nextT + FRAME_TS / CPU_HZ

    if nextT < now - 0.1 then
      nextT = now
    end
  end
end

local ok, err = xpcall(main, debug.traceback)

closeSound()
restoreGPU()

if not ok then
  io.stderr:write(tostring(err) .. "\n")
end