mod.callback = function () {
    var exports = []
    var uris = mod.resolve()

    for (var i = 0, len = uris.length; i < len; i++) {
        exports[i] = cachedMods[uris[i]].exec()
    }

    if (callback) {
        callback.apply(global, exports)
    }

    delete mod.callback
}

// Execute a module
Module.prototype.exec = function () {
    var mod = this

    // When module is executed, DO NOT execute it again. When module
    // is being executed, just return `module.exports` too, for avoiding
    // circularly calling
    if (mod.status >= STATUS.EXECUTING) {
        return mod.exports
    }

    mod.status = STATUS.EXECUTING

    // Create require
    var uri = mod.uri

    function require(id) {
        return Module.get(require.resolve(id)).exec()
    }

    require.resolve = function (id) {
        return Module.resolve(id, uri)
    }

    require.async = function (ids, callback) {
        Module.use(ids, callback, uri + "_async_" + cid())
        return require
    }

    // Exec factory
    var factory = mod.factory

    var exports = isFunction(factory) ? factory(require, mod.exports = {},
    mod) : factory

    if (exports === undefined) {
        exports = mod.exports
    }

    // Emit `error` event
    if (exports === null && ! IS_CSS_RE.test(uri)) {
        emit("error", mod)
    }

    // Reduce memory leak
    delete mod.factory

    mod.exports = exports
    mod.status = STATUS.EXECUTED

    // Emit `exec` event
    emit("exec", mod)

    return exports
}