-- Real frame, 256-colour, with the ACTUAL display gate from MAME's source:
--   CRTC R20 bit 11 = "G-VRAM set to buffer" -> graphics layer invisible.
--   CRTC R20 bits 9-8 = colour setup: 0x0100 = 256-colour, low byte per word.
-- The IPL leaves R20 = 0x0B16 (buffered + 65536c), which is why every earlier
-- attempt rendered black no matter what the video controller said.
M=manager.machine; SP=M.devices[":maincpu"].spaces["program"]; SUB=nil

local CRTC20 = 0xE80000 + 20*2
local GVRAM, GPAL = 0xC00000, 0xE82000

local f=io.open("frame.bin","rb"); local d=f:read("a"); f:close()
local function B(i) return string.byte(d,i) end
local W,H = B(5)*256+B(6), B(7)*256+B(8)
local PAL0, PIX0 = 9, 9+256*3

-- GGGGGRRRRRBBBBBI, confirmed from x68k_v.cpp
local function pack(r,g,b) return ((g>>3)<<11)|((r>>3)<<6)|((b>>3)<<1)|1 end

local function T() local t=M.time; return t.seconds+t.attoseconds/1e18 end
local st,tp="wait",nil

SUB = emu.add_machine_frame_notifier(function()
  local t=T()
  if st=="wait" then
    if t<3.0 then return end
    local old = SP:read_u16(CRTC20)
    local new = (old & ~0x0B00) | 0x0100      -- clear buffer bit, select 256c
    SP:write_u16(CRTC20, new)
    SP:write_u16(0xE82400, 0x0001)            -- video ctrl: 256 colours
    SP:write_u16(0xE82600, 0x001F)            -- graphics + pages on, text off
    SP:write_u8(0xE8E001, 15)                 -- monitor contrast: IPL leaves it at 14
    for c=0,255 do
      local o=PAL0+c*3
      SP:write_u16(GPAL+c*2, pack(B(o),B(o+1),B(o+2)))
    end
    for y=0,H-1 do
      local row,base = PIX0+y*W, GVRAM+y*1024
      for x=0,W-1 do SP:write_u16(base+x*2, B(row+x)) end
    end
    print(string.format("[SHOW3] R20 %04X->%04X  E82400=%04X E82600=%04X gv=%04X t=%.3f",
      old, SP:read_u16(CRTC20), SP:read_u16(0xE82400), SP:read_u16(0xE82600),
      SP:read_u16(GVRAM), t))
    st,tp="painted",t
  elseif st=="painted" and t>tp+0.30 then
    M.video:snapshot(); print("[SHOW3] snapshot"); st="done"; M:exit()
  end
end)