pit.lua

-- 8253 PIT
-- TODO: Actually emulate it. Tough, as we only get 20 "ticks" a second on OpenComputers.
-- Maybe it's not worth to accurately emulate it?

pit_osc_freq = 1.193182

local channels = {nil, nil, nil}

function pit_channel(c)
	return channels[c]
end

local function pit_init(c,first)
	channels[c] = {
		mode=3,
		addr_mode=3,
		reload=0x10000,
		reload_set_lo=false,
		reload_set_hi=false,
		bcd=false,
		paused=false,
		count=0
	}
end

-- THIS IS REALLY INACCURATE OKAY
-- but that is fine as we're not using it right now anyway
local pit_last_tick = -1

function pit_tick(curr_t)
	if pit_last_tick == -1 then
		pit_last_tick = curr_t
		return
	end	
	local osc_count = math.floor((curr_t - pit_last_tick) * 1000000 * pit_osc_freq)
	pit_last_tick = curr_t
	for c=1,3 do
		local trig=0
		local ch=channels[c]
		local ch_ready=(not ch.paused) and ch.reload_set_lo and ch.reload_set_hi
		if ch_ready then
			emu_debug(2, "PIT tick " .. c .. " mode " .. ch.mode)
		end
		if ch.reload == 0 then
			ch.reload = 0x10000
		end
		if (ch.mode == 0 or ch.mode == 4) and ch_ready then
			if ch.count == 0 then ch.count = ch.reload end
			ch.count = ch.count - osc_count
			if ch.count < 1 then
				trig = 1
				ch.count = 0
				ch.paused = true
			end
		elseif (ch.mode == 2 or ch.mode == 3 or ch.mode == 6 or ch.mode == 7) and ch_ready then
			emu_debug(2, "PIT " .. ch.count .. " -> " .. (ch.count - osc_count))
			ch.count = ch.count - osc_count
			while ch.count < 0 do
				ch.count = ch.count + ch.reload
				trig = trig + 1
			end
		end
		for i=1,trig do
			if c == 1 and pic_interrupt_enabled(PIC_INTERRUPT_TIMER) then
				cpu_emit_interrupt(0x08, false)
			end
		end
	end
end

local access_lohi = false

local function pit_data(n) return function(cond, val)
	local ch = channels[n]
	if not val then
		emu_debug(2, "PIT read data " .. n .. " mode " .. ch.addr_mode)
		if (ch.addr_mode == 3) or (ch.addr_mode == 0) then
			access_lohi = not access_lohi
			if access_lohi then
				return ch.count -band- 0xFF
			else
				return (ch.count -brshift- 8) -band- 0xFF
			end
		elseif ch.addr_mode == 1 then
			return ch.count -band- 0xFF
		elseif ch.addr_mode == 2 then
			return (ch.count -brshift- 8) -band- 0xFF
		elseif ch.addr_mode == 0 then
			return 0x00 -- TODO
		end
	else
		emu_debug(2, "PIT write data " .. n .. " mode " .. ch.addr_mode)
		if ch.addr_mode == 3 then
			access_lohi = not access_lohi
			if access_lohi then
				ch.reload = (ch.reload -band- 0xFF00) -bor- val
				ch.reload_set_lo = true
				ch.reload_set_hi = false
			else
				ch.reload = (ch.reload -band- 0xFF) -bor- (val -blshift- 8)
				ch.reload_set_hi = true
			end
		elseif ch.addr_mode == 1 then
			ch.reload = (ch.reload -band- 0xFF00) -bor- val
			ch.reload_set_lo = true
		elseif ch.addr_mode == 2 then
			ch.reload = (ch.reload -band- 0xFF) -bor- (val -blshift- 8)
			ch.reload_set_hi = true
		end
	end
end end

cpu_port_set(0x40, pit_data(1))
cpu_port_set(0x41, pit_data(2))
cpu_port_set(0x42, pit_data(3))
cpu_port_set(0x43, function(cond, val)
	if val then
		emu_debug(2, "PIT write control " .. val)
		pit_tick(os.epoch("utc") / 1000)
		local c = (val -brshift- 6) + 1
		if c < 4 then
			pit_init(c,false)
			channels[c].addr_mode = (val -brshift- 4) -band- 0x03
			if channels[c].addr_mode ~= 0 then
				channels[c].mode = (val -brshift- 1) -band- 0x07
				channels[c].bcd = (val -band- 1) and true or false
				channels[c].paused = false
			end
			access_lohi = false
		end
	else
		emu_debug(2, "PIT read control")
		return 0xFF
	end
end)

pit_init(1,true)
pit_init(2,true)
pit_init(3,true)

channels[1].reload_set_lo = true
channels[1].reload_set_hi = true