DDR2（Double Data Rate 2） SDRAM是由JEDEC（电子设备工程联合委员会）进行开发的新生代内存技术标准，它与上一代DDR内存技术标准最大的不同就是，虽然同是采用了在时钟的上升/下降延同时进行数据传输的基本方式，但DDR2内存却拥有两倍于上一代DDR内存预读取能力（即：4bit数据读预取）。换句话说，DDR2内存每个时钟能够以4倍外部总线的速度读/写数据，并且能够以内部控制总线4倍的速度运行。

    此外，由于DDR2标准规定所有DDR2内存均采用FBGA封装形式，而不同于目前广泛应用的TSOP/TSOP-II封装形式，FBGA封装可以提供了更为良好的电气性能与散热性，为DDR2内存的稳定工作与未来频率的发展提供了坚实的基础。回想起DDR的发展历程，从第一代应用到个人电脑的DDR200经过DDR266、DDR333到今天的双通道DDR400技术，第一代DDR的发展也走到了技术的极限，已经很难通过常规办法提高内存的工作速度；随着Intel最新处理器技术的发展，前端总线对内存带宽的要求是越来越高，拥有更高更稳定运行频率的DDR2内存将是大势所趋。

DDR2与DDR的区别：

    在了解DDR2内存诸多新技术前，先让我们看一组DDR和DDR2技术对比的数据。

1、延迟问题：

    从上表可以看出，在同等核心频率下，DDR2的实际工作频率是DDR的两倍。这得益于DDR2内存拥有两倍于标准DDR内存的4BIT预读取能力。换句话说，虽然DDR2和DDR一样，都采用了在时钟的上升延和下降延同时进行数据传输的基本方式，但DDR2拥有两倍于DDR的预读取系统命令数据的能力。也就是说，在同样100MHz的工作频率下，DDR的实际频率为200MHz，而DDR2则可以达到400MHz。

    这样也就出现了另一个问题：在同等工作频率的DDR和DDR2内存中，后者的内存延时要慢于前者。举例来说，DDR 200和DDR2-400具有相同的延迟，而后者具有高一倍的带宽。实际上，DDR2-400和DDR 400具有相同的带宽，它们都是3.2GB/s，但是DDR400的核心工作频率是200MHz，而DDR2-400的核心工作频率是100MHz，也就是说DDR2-400的延迟要高于DDR400。

2、封装和发热量：

    DDR2内存技术最大的突破点其实不在于用户们所认为的两倍于DDR的传输能力，而是在采用更低发热量、更低功耗的情况下，DDR2可以获得更快的频率提升，突破标准DDR的400MHZ限制。

    DDR内存通常采用TSOP芯片封装形式，这种封装形式可以很好的工作在200MHz上，当频率更高时，它过长的管脚就会产生很高的阻抗和寄生电容，这会影响它的稳定性和频率提升的难度。这也就是DDR的核心频率很难突破275MHZ的原因。而DDR2内存均采用FBGA封装形式。不同于目前广泛应用的TSOP封装形式，FBGA封装提供了更好的电气性能与散热性，为DDR2内存的稳定工作与未来频率的发展提供了良好的保障。

    DDR2内存采用1.8V电压，相对于DDR标准的2.5V，降低了不少，从而提供了明显的更小的功耗与更小的发热量，这一点的变化是意义重大的。

DDR2采用的新技术：

    除了以上所说的区别外，DDR2还引入了三项新的技术，它们是OCD、ODT和Post CAS。

    OCD（Off-Chip Driver）：也就是所谓的离线驱动调整，DDR II通过OCD可以提高信号的完整性。DDR II通过调整上拉（pull-up）/下拉（pull-down）的电阻值使两者电压相等。使用OCD通过减少DQ-DQS的倾斜来提高信号的完整性；通过控制电压来提高信号品质。

    ODT：ODT是内建核心的终结电阻器。我们知道使用DDR SDRAM的主板上面为了防止数据线终端反射信号需要大量的终结电阻。它大大增加了主板的制造成本。实际上，不同的内存模组对终结电路的要求是不一样的，终结电阻的大小决定了数据线的信号比和反射率，终结电阻小则数据线信号反射低但是信噪比也较低；终结电阻高，则数据线的信噪比高，但是信号反射也会增加。因此主板上的终结电阻并不能非常好的匹配内存模组，还会在一定程度上影响信号品质。DDR2可以根据自已的特点内建合适的终结电阻，这样可以保证最佳的信号波形。使用DDR2不但可以降低主板成本，还得到了最佳的信号品质，这是DDR不能比拟的。

    Post CAS：它是为了提高DDR II内存的利用效率而设定的。在Post CAS操作中，CAS信号（读写/命令）能够被插到RAS信号后面的一个时钟周期，CAS命令可以在附加延迟（Additive Latency）后面保持有效。原来的tRCD（RAS到CAS和延迟）被AL（Additive Latency）所取代，AL可以在0，1，2，3，4中进行设置。由于CAS信号放在了RAS信号后面一个时钟周期，因此ACT和CAS信号永远也不会产生碰撞冲突。

总的来说，DDR2采用了诸多的新技术，改善了DDR的诸多不足，虽然它目前有成本高、延迟慢能诸多不足，但相信随着技术的不断提高和完善，这些问题终将得到解决。

Section I   Listening Comprehension

Directions:

This section is designed to test your ability to understand spoken English. You will hear a selection of recorded materials and you must answer the questions that accompany them. There are three parts in this section, Part A, Part B and Part C.

Remember, while you are doing the test, you should first put down your answers in your test booklet. At the end of the listening comprehension section, you will have 5 minutes to transfer all your answers from your test booklet to ANSWER SHEET 1.

Now look at Part A in your test booklet.

Part A

Directions:

For questions 1 - 5, you will hear a talk about the geography of Belgium. While you listen, fill out the table with the information you have heard. Some of the information has been given to you in the table. Write only 1 word or number in each numbered box. You will hear the recording twice. You now have 25 seconds to read the table below. (5 points)

Part B

Directions:

For Questions 6-10, you will hear an interview with Mr. Saffo from the Institute for the Future. While you listen, complete the sentences or answer the questions. Use not more than 3 words for each answer. You will hear the recording twice. You now have 25 seconds to read the sentences and questions below. (5 points)

What is Saffo according to himself?

The Institute for the Future provides services to private companies and

The Institute believes that to think systematically about the long-range future is

To succeed in anything, one should be flexible, curious and

What does Saffo consider to be essential to the work of a team?

Part C

Directions:

You will hear three pieces of recorded material. Before listening to each one, you will have time to read the questions related to it. While listening, answer each question by choosing A, B, C or D. After listening, you will have time to check your answers. You will hear each piece once only. (10 points)

Questions 11-13 are based on the following talk about naming newborns. You now have 15 seconds to read Questions 11-13.

11. What do we often do with the things we love?

[A] Ask for their names.

[B] Name babies after them.

[C] Put down their names.

[D] Choose names for them.

12. The unpleasant meaning of an old family name is often overlooked if

[A] the family tree is fairly limited.

[B] the family tie is strong enough.

[C] the name is commonly used.

[D] nobody in the family complains.

13. Several months after a baby’s birth, its name will

[A] show the beauty of its own.

[B] develop more associations.

[C] lose the original meaning.

[D] help form the baby’s personality.

Questions 14 - 16 are based on the biography of Bobby Moore, an English soccer player. You now have 15 seconds to read Questions 14 - 16.

14. How many matches did Moore play during his professional career?

[A] 90.

[B] 108.

[C] 180.

[D] 668.

15. In 1964, Bobby Moore was made

[A] England’s footballer of the year.

[B] a soccer coach in West Germany.

[C] a medalist for his sportsmanship.

[D] a number of the Order of the British Empire.

16. After Moore retired from playing, the first thing he did was

[A] editing Sunday Sport.

[B] working for Capital Radio.

[C] managing professional soccer teams.

[D] developing a sports marketing company.

Questions 17 - 20 are based on the following talk on the city of Belfast. You now have 20 seconds to read Questions 17 - 20.

17. Belfast has long been famous for its

[A] oil refinery.

[B] linen textiles.

[C] food products.

[D] deepwater port.

18. Which of the following does Belfast chiefly export?

[A] Soap.

[B] Grain.

[C] Steel.

[D] Tobacco.

19. When was Belfast founded?

[A] In 1177.

[B] In 1315.

[C] In the 16^th century.

[D] In the 17^th century.

20. What happened in Belfast in the late 18^th century?

[A] French refugees arrived.

[B] The harbor was destroyed.

[C] Shipbuilding began to flourish.

[D] The city was taken by the English.