-- Diagnostic for FINDINGS 46.6: separate the WRITE path from the DISPLAY path.
M=manager.machine; SP=M.devices[":maincpu"].spaces["program"]; SUB=nil
local function load_mode()
  for _,p in ipairs{"../tools/bench/crtc_mode.lua","tools/bench/crtc_mode.lua"} do
    local f=loadfile(p); if f then return f() end end
  error("no crtc_mode.lua") end
local MODE = load_mode()
local CRTC, GV = 0xE80000, 0xC00000
local function T() local t=M.time; return t.seconds+t.attoseconds/1e18 end
local st="wait"
SUB = emu.add_machine_frame_notifier(function()
  if st~="wait" then return end
  if T()<3.0 then return end
  MODE.apply(SP)
  local function trial(name, r20)
    SP:write_u16(CRTC+20*2, r20)
    -- clear both bytes of word 0 via the two aliases, masked mode
    SP:write_u16(CRTC+20*2, MODE.r20)
    SP:write_u16(GV, 0); SP:write_u16(GV+0x80000, 0)
    SP:write_u16(CRTC+20*2, r20)
    SP:write_u16(GV, 0xAB5C)                 -- the write under test
    local raw   = SP:read_u16(GV)
    SP:write_u16(CRTC+20*2, MODE.r20)        -- back to masked to read pages
    local p0    = SP:read_u16(GV)            -- page 0 alias -> low byte
    local p1    = SP:read_u16(GV+0x80000)    -- page 1 alias -> high byte
    SP:write_u16(CRTC+20*2, r20)
    print(string.format("[PROBE] %-22s R20=%04X  wrote AB5C  raw=%04X  page0=%02X  page1=%02X",
                        name, r20, raw, p0 & 0xff, p1 & 0xff))
  end
  trial("masked (bit11=0)",  MODE.r20)
  trial("buffer (bit11=1)",  MODE.r20 | 0x0800)
  -- what does the video controller look like after MODE.apply?
  print(string.format("[PROBE] VC R0=%04X R1=%04X R2=%04X",
        SP:read_u16(0xE82400), SP:read_u16(0xE82500), SP:read_u16(0xE82600)))
  st="done"; M:exit()
end)