-- CRTC mode table: 256x256, 256 colours, 31.5kHz, graphics double-scanned.
--
-- DERIVED from MAME 0.277 src/mame/sharp/x68k_crtc.cpp, not recalled. The
-- derivation is self-checking, which is why it is trustworthy:
--
--   refresh_mode() picks the dot clock as (reg20 bit4 ? 69.55199MHz : 38.86363MHz)/div
--   with div from the (reg20 & 0x1f) ladder:  0x16 -> 2   (IPL's 768x512)
--                                             0x11 -> 3   (512 wide)
--                                             0x10 -> 6   (256 wide, double-scan)
--
--   IPL 768 mode: div 2 -> 34.776 MHz, m_htotal = (137+1)*8 = 1104 dots
--                 34.776e6 / 1104 = 31500.0 Hz  exactly.
--   256 mode:     div 6 -> 11.592 MHz.  Same 31.5kHz line rate requires
--                 11.592e6 / 31500 = 368 dots = 46 chars -> R00 = 45.
--
--   368 = 1104/3 exactly, so EVERY horizontal register is the 768-mode value
--   divided by 3 -- no rounding for the active window:
--       visible chars   (124-28) / 3 = 32  ->  32*8 = 256 dots  exact
--   Only the blanking split needs rounding. 768 mode is sync/back/front =
--   14/14/14 chars; /3 = 4.67 each; the closest integer triple summing to
--   46-32 = 14 is 5/5/4.  ->  R01=5, R02=10, R03=42.
--
--   Total blanking time is identical to the 768 mode (112 dots @ 11.592MHz =
--   336 dots @ 34.776MHz = 9.66us), which is what a real monitor needs.
--
-- VERTICAL registers are NOT halved. The CRTC still generates a 568-line
-- 31.5kHz raster (31500/568 = 55.46 Hz); "256 lines" is a graphics-layer
-- double-scan (draw_gfx() halves gfxrect, x68k_v.cpp:401). Halving them would
-- ask the monitor for 110 Hz. So R04-R07 keep the 31kHz text-mode values.
-- MAME logerrors "visarea larger then reg[20]" for this; it is cosmetic.

local M = {}

M.regs = {
  [0] = 45,   -- H total      (46 chars = 368 dots @ 11.592MHz = 31500.0 Hz)
  [1] = 5,    -- H sync end   (5 chars = 3.45us)
  [2] = 10,   -- H disp begin (hbegin = 10*8+1 = 81)
  [3] = 42,   -- H disp end   (hend = 336; inclusive width = 336-81+1 = 256)
  [4] = 567,  -- V total      (568 scanlines -> 55.46 Hz)
  [5] = 5,    -- V sync end
  [6] = 40,   -- V disp begin (vbegin = 41)
  [7] = 552,  -- V disp end   (512 scanlines -> 256 gfx rows, double-scanned)
  [8] = 27,   -- H sync adjust (MAME stores but does not use it; IPL value)
}

-- R20: bit11=0 display (not buffer), bits9-8=01 256-colour,
--      bit4=1 31.5kHz, bits3-2=00 256 lines, bits1-0=00 256 dots
M.r20 = 0x0110

M.width, M.height = 256, 256

function M.apply(SP)
  for r, v in pairs(M.regs) do SP:write_u16(0xE80000 + r*2, v) end
  SP:write_u16(0xE80000 + 20*2, M.r20)
  SP:write_u16(0xE82400, 0x0001)   -- video ctrl reg 0: 256 colours
  SP:write_u16(0xE82600, 0x001F)   -- reg 2: graphics on, all 4 pages, text/PCG off
  SP:write_u8 (0xE8E001, 15)       -- monitor contrast (IPL leaves 14 = 7% dark)
end

return M