come at 和 come up to 都有“达到”的意思，两者有什么区别？

come at
vt. 达到(袭击, 弄清)
例句与用法:
1. She desires you to come at once.
她希望你立即来.
2. All right, I'll come at six o'clock tomorrow.
行, 我明天六点钟来。
3. Put the food where dog can't come at it.
把食物放在狗够不到的地方。

******* stardict 给的例子通常连come at once 这样的例子也可能会出现，目的是为了让人比较两者的区别，不要误用。举个中文例子吧，用果然造句，“我在水果点拿了一个芒果然后飞快地跑了”，果然出现在句子里，可不是正确的用法。*****

come up to
符合，达到(标准), 比得上

例句与用法:
1. Why don't you come up to Scotland for a few days?
你怎麽不北上到苏格兰来住几天?
2. His film performance didn't come up to expectations.
他在影片中的演出有负众望.
3. His performance didn't really come up to his usual high standard.
他没有真正表现出平日的高水平来.