pit.js

"use strict";

/**
 * @const
 * In kHz
 */
var OSCILLATOR_FREQ = 1193.1816666; // 1.193182 MHz

/**
 * @constructor
 *
 * Programmable Interval Timer
 */
function PIT(cpu, bus)
{
    /** @const @type {CPU} */
    this.cpu = cpu;

    this.bus = bus;

    this.counter_start_time = new Float64Array(3);
    this.counter_start_value = new Uint16Array(3);

    this.counter_next_low = new Uint8Array(4);
    this.counter_enabled = new Uint8Array(4);
    this.counter_mode = new Uint8Array(4);
    this.counter_read_mode = new Uint8Array(4);

    // 2 = latch low, 1 = latch high, 0 = no latch
    this.counter_latch = new Uint8Array(4);
    this.counter_latch_value = new Uint16Array(3);

    this.counter_reload = new Uint16Array(3);

    // TODO:
    // - counter2 can be controlled by an input

    cpu.io.register_read(0x61, this, function()
    {
        var now = v86.microtick();

        var ref_toggle = (now * (1000 * 1000 / 15000)) & 1;
        var counter2_out = this.did_rollover(2, now);

        return ref_toggle << 4 | counter2_out << 5;
    });
    cpu.io.register_write(0x61, this, function(data)
    {
        if(data & 1)
        {
            this.bus.send("pcspeaker-enable");
        }
        else
        {
            this.bus.send("pcspeaker-disable");
        }
    });

    cpu.io.register_read(0x40, this, function() { return this.counter_read(0); });
    cpu.io.register_read(0x41, this, function() { return this.counter_read(1); });
    cpu.io.register_read(0x42, this, function() { return this.counter_read(2); });

    cpu.io.register_write(0x40, this, function(data) { this.counter_write(0, data); });
    cpu.io.register_write(0x41, this, function(data) { this.counter_write(1, data); });
    cpu.io.register_write(0x42, this, function(data) {
        this.counter_write(2, data);
        this.bus.send("pcspeaker-update", [this.counter_mode[2], this.counter_reload[2]]);
    });

    cpu.io.register_write(0x43, this, this.port43_write);
}

PIT.prototype.get_state = function()
{
    var state = [];

    state[0] = this.counter_next_low;
    state[1] = this.counter_enabled;
    state[2] = this.counter_mode;
    state[3] = this.counter_read_mode;
    state[4] = this.counter_latch;
    state[5] = this.counter_latch_value;
    state[6] = this.counter_reload;
    state[7] = this.counter_start_time;
    state[8] = this.counter_start_value;

    return state;
};

PIT.prototype.set_state = function(state)
{
    this.counter_next_low = state[0];
    this.counter_enabled = state[1];
    this.counter_mode = state[2];
    this.counter_read_mode = state[3];
    this.counter_latch = state[4];
    this.counter_latch_value = state[5];
    this.counter_reload = state[6];
    this.counter_start_time = state[7];
    this.counter_start_value = state[8];
};

PIT.prototype.timer = function(now, no_irq)
{
    var time_to_next_interrupt = 100;

    // counter 0 produces interrupts
    if(!no_irq)
    {
        if(this.counter_enabled[0] && this.did_rollover(0, now))
        {
            this.counter_start_value[0] = this.get_counter_value(0, now);
            this.counter_start_time[0] = now;

            dbg_log("pit interrupt. new value: " + this.counter_start_value[0], LOG_PIT);

            // This isn't strictly correct, but it's necessary since browsers
            // may sleep longer than necessary to trigger the else branch below
            // and clear the irq
            this.cpu.device_lower_irq(0);

            this.cpu.device_raise_irq(0);
            var mode = this.counter_mode[0];

            if(mode === 0)
            {
                this.counter_enabled[0] = 0;
            }
        }
        else
        {
            this.cpu.device_lower_irq(0);
        }

        if(this.counter_enabled[0])
        {
            const diff = now - this.counter_start_time[0];
            const diff_in_ticks = Math.floor(diff * OSCILLATOR_FREQ);
            const ticks_missing = this.counter_start_value[0] - diff_in_ticks; // XXX: to simplify
            time_to_next_interrupt = ticks_missing / OSCILLATOR_FREQ;
        }
    }

    return time_to_next_interrupt;
};

PIT.prototype.get_counter_value = function(i, now)
{
    if(!this.counter_enabled[i])
    {
        return 0;
    }

    var diff = now - this.counter_start_time[i];
    var diff_in_ticks = Math.floor(diff * OSCILLATOR_FREQ);

    var value = this.counter_start_value[i] - diff_in_ticks;

    dbg_log("diff=" + diff + " dticks=" + diff_in_ticks + " value=" + value + " reload=" + this.counter_reload[i], LOG_PIT);

    var reload = this.counter_reload[i];

    if(value >= reload)
    {
        dbg_log("Warning: Counter" + i + " value " + value  + " is larger than reload " + reload, LOG_PIT);
        value %= reload;
    }
    else if(value < 0)
    {
        value = value % reload + reload;
    }

    return value;
};

PIT.prototype.did_rollover = function(i, now)
{
    var diff = now - this.counter_start_time[i];

    if(diff < 0)
    {
        // should only happen after restore_state
        dbg_log("Warning: PIT timer difference is negative, resetting (timer " + i + ")");
        return true;
    }
    var diff_in_ticks = Math.floor(diff * OSCILLATOR_FREQ);
    //dbg_log(i + ": diff=" + diff + " start_time=" + this.counter_start_time[i] + " diff_in_ticks=" + diff_in_ticks + " (" + diff * OSCILLATOR_FREQ + ") start_value=" + this.counter_start_value[i] + " did_rollover=" + (this.counter_start_value[i] < diff_in_ticks), LOG_PIT);

    return this.counter_start_value[i] < diff_in_ticks;
};

PIT.prototype.counter_read = function(i)
{
    var latch = this.counter_latch[i];

    if(latch)
    {
        this.counter_latch[i]--;

        if(latch === 2)
        {
            return this.counter_latch_value[i] & 0xFF;
        }
        else
        {
            return this.counter_latch_value[i] >> 8;
        }
    }
    else
    {
        var next_low = this.counter_next_low[i];

        if(this.counter_mode[i] === 3)
        {
            this.counter_next_low[i] ^= 1;
        }

        var value = this.get_counter_value(i, v86.microtick());

        if(next_low)
        {
            return value & 0xFF;
        }
        else
        {
            return value >> 8;
        }
    }
};

PIT.prototype.counter_write = function(i, value)
{
    if(this.counter_next_low[i])
    {
        this.counter_reload[i] = this.counter_reload[i] & ~0xFF | value;
    }
    else
    {
        this.counter_reload[i] = this.counter_reload[i] & 0xFF | value << 8;
    }

    if(this.counter_read_mode[i] !== 3 || !this.counter_next_low[i])
    {
        if(!this.counter_reload[i])
        {
            this.counter_reload[i] = 0xFFFF;
        }

        // depends on the mode, should actually
        // happen on the first tick
        this.counter_start_value[i] = this.counter_reload[i];

        this.counter_enabled[i] = true;

        this.counter_start_time[i] = v86.microtick();

        dbg_log("counter" + i + " reload=" + h(this.counter_reload[i]) +
                " tick=" + (this.counter_reload[i] || 0x10000) / OSCILLATOR_FREQ + "ms", LOG_PIT);
    }

    if(this.counter_read_mode[i] === 3)
    {
        this.counter_next_low[i] ^= 1;
    }
};

PIT.prototype.port43_write = function(reg_byte)
{
    var mode = reg_byte >> 1 & 7,
        binary_mode = reg_byte & 1,
        i = reg_byte >> 6 & 3,
        read_mode = reg_byte >> 4 & 3;

    if(i === 1)
    {
        dbg_log("Unimplemented timer1", LOG_PIT);
    }

    if(i === 3)
    {
        dbg_log("Unimplemented read back", LOG_PIT);
        return;
    }

    if(read_mode === 0)
    {
        // latch
        this.counter_latch[i] = 2;
        var value = this.get_counter_value(i, v86.microtick());
        dbg_log("latch: " + value, LOG_PIT);
        this.counter_latch_value[i] = value ? value - 1 : 0;

        return;
    }

    if(mode >= 6)
    {
        // 6 and 7 are aliased to 2 and 3
        mode &= ~4;
    }

    dbg_log("Control: mode=" + mode + " ctr=" + i +
            " read_mode=" + read_mode + " bcd=" + binary_mode, LOG_PIT);

    if(read_mode === 1)
    {
        // msb
        this.counter_next_low[i] = 0;
    }
    else if(read_mode === 2)
    {
        // lsb
        this.counter_next_low[i] = 1;
    }
    else
    {
        // first lsb then msb
        this.counter_next_low[i] = 1;
    }

    if(i === 0)
    {
        this.cpu.device_lower_irq(0);
    }

    if(mode === 0)
    {
    }
    else if(mode === 3 || mode === 2)
    {
        // what is the difference
    }
    else
    {
        dbg_log("Unimplemented counter mode: " + h(mode), LOG_PIT);
    }

    this.counter_mode[i] = mode;
    this.counter_read_mode[i] = read_mode;

    if(i === 2)
    {
        this.bus.send("pcspeaker-update", [this.counter_mode[2], this.counter_reload[2]]);
    }
};

PIT.prototype.dump = function()
{
    const reload = this.counter_reload[0];
    const time = (reload || 0x10000) / OSCILLATOR_FREQ;
    dbg_log("counter0 ticks every " + time + "ms (reload=" + reload + ")");
};