I've done whatever I want

The sunny July ushered in the summer vacation that students longed for, and it was also the best time for talented students to travel and get in touch with society.

On the China Eastern Airlines flight from Shanghai to New York, one-third of the passengers were members of international study tour groups and parent representatives, one-third of the passengers were retired square dancers, and one-third of the passengers were business people. .

When Xin Tong checked in, he chose a seat near the window. Sitting next to him was a middle-aged man wearing a gray suit and glasses. He looked very gentleman. He was sprayed with French perfume, which smelled good. The smell drifted over faintly.

Sitting in the same row of seats near the aisle was a well-dressed old woman with wavy curly hair bearing the mark of the times.

Beautiful flight attendants and handsome flight attendants walk around the cabin from time to time, helping passengers operate the audio and video systems, and answering passengers' questions about the aircraft, weather, routes, services, etc. Their beautiful figures always attract the attention of passengers and make everyone Forget about the hardships of the journey.

"Little handsome boy, let's change seats!" After the plane shook a little in the clouds, the old lady sitting by the aisle pulled Xin Tong's clothes and shouted.

Xin Tong was admiring the scenery outside the window. After the plane passed through the clouds, the floating clouds under the wings caught Xin Tong's attention.

"Okay!" Xin Tong turned around and smiled slightly.

It seems that the beautiful scenery outside the window has also aroused the interest of the old lady.

After they exchanged positions, the old man, as soon as he sat firmly, reached out and pulled the porthole, sighed, and said to the middle-aged uncle beside him: "Young man, how do you open this window? I want to get some air!"

"Mom, this is an airplane, not a bus, and the portholes cannot be opened!" said the middle-aged man.

"What? Can't open it?"

"Yes!"

"Little handsome boy, I'd better sit by the aisle, it's too crowded in here!"

Xin Tong smiled slightly, stood up, and returned to his position by the porthole.

Judging from the old lady's behavior, Xin Tong felt that this was her first time flying.

Many old people have worked in factories all their lives and have never traveled by plane. When they reach retirement age and their children have grown up, they no longer have the pressure of life and have plenty of time. They dress up, go on tours with groups, and do some funny things. Nothing surprising.

They went back and forth, but it was difficult for the gentleman sitting in the middle. After Xin Tong sat down, he turned to the middle-aged man next to him and said, "I'm sorry!"

"It's okay!" Xin Tong's friendly and courteous behavior endeared him to the middle-aged man.

"Sir, are you on a business trip to New York?" Xin Tong asked curiously.

"I am returning to the United States. I immigrated to the United States ten years ago. This time I came to Shanghai for academic exchange!" the middle-aged man replied.

"Academic exchange?"

"Yes, I went to Fudan University to do academic exchanges on semiconductor chips!" Upon hearing that the other party was doing academic exchanges on chips, Xin Tong immediately revealed his identity: "I am also engaged in the semiconductor industry. I just entered Tsinghua University this year. Graduate student from the School of Microelectronics!”

"Then I am your alumnus. My name is Lu Wei. I obtained my bachelor's degree from the Department of Physics of Tsinghua University in 1996. Then I went to the United States and obtained my doctorate from Rice University. I am now a professor in the Department of Electrical Engineering and Computer Science at the University of Michigan. , the main research direction is memory chips." The middle-aged man introduced himself.

"Nice to meet you!"

"When you study chip design, pay more attention to memory chips. Memory chips are a very important type of product in the semiconductor industry, accounting for more than [-]%. Memory chips are basically used in all our daily electronic devices." Professor Lu Wei said Seeing that the person sitting next to him was his schoolmate, he started talking.

When Xin Tong heard that the other party was a professor at the University of Michigan, he thought that he must not miss this great opportunity to ask for advice.

"When I enter the Tsinghua University campus, I will definitely study memory chips."

"To achieve control and computing, electronic products need to store information, which is inseparable from memory chips. Memory chips can be simply divided into flash memory and memory. Flash memory includes NAND FLASH and NOR FLASH, and memory is mainly DRAM. In order to more conveniently understand storage We can use eating peanuts to illustrate the role of the chip. Eating peanuts in your mouth is like the CPU processing data. The size of the hard drive is the size of your pocket (how many peanuts can be put in it), and the size of the memory is the size of your hand (one time). Catch as many as you can)."

Xin Tong nodded.

"The storage industry emerged in the 60s and has gone through three stages of development so far. Before 1990, DRAM (Dynamic Random Access Memory) was the main product in the memory chip market. However, DRAM has great limitations. Data can only be kept for a short period of time. In order to solve the problem of long-term data storage, the American Intel Corporation developed NOR flash technology in 1988. By 1990, NOR Flash had non-volatility, ultra-high reading speed, and on-chip Features such as execution have been recognized by the market.”

"However, NOR Flash also has obvious shortcomings, such as slow writing speed and high price, which determines that it is not suitable for large-capacity storage. When it comes to NOR flash, I have to mention GigaDevice. Zhu Yiming, the boss of this company He is also a graduate of Tsinghua University. Last year they launched the first SPI NOR Flash product in China and it is now in mass production!"

Hearing this, Xin Tong gave a thumbs up.

"However, NOR flash is obviously not the best solution. Scientists are constantly exploring. As early as 1989, Japan's Toshiba Corporation published the NAND flash structure. Nand-flash memory has the advantages of large capacity and fast rewriting speed. It is a high-data An ideal solution for storage density, suitable for the storage of large amounts of data. After 2000, NAND Flash began to grow explosively."

"So mankind has found the best way to store data!" After listening to Professor Lu's talk about the development history of memory chips, Xin Tong felt as if he was enlightened.

"No! I don't think so yet!" Professor Lu shook his head.

"Why?"

"The current mainstream computing systems all adopt the von Neumann architecture. The von Neumann architecture eliminates the situation that the original computer system can only rely on hardware to control the program. The program code is stored in the memory and is separated from the arithmetic control unit. It realizes programmable computer functions and greatly promotes the development of computers."

Xin Tong nodded.

"The current architecture also has certain limitations. In order to meet the needs of speed and capacity, modern computing systems usually adopt a three-level storage structure of cache (SRAM), main memory (DRAM), and external storage (NAND Flash). SRAM responds Time is usually on the nanosecond level, DRAM is generally on the 100 nanosecond level, and NAND Flash is as high as 100 microseconds. When data is transmitted between these three levels of storage, the difference in response time forms a 'storage wall'."

"The DRAM and NAND Flash currently widely used on the market are limited by their physical characteristics and are difficult to break through the 'storage wall'. This requires us to find a new type of special material and structure for storage. If this new type of memory chip also has DRAM The read and write speed and lifespan of NAND Flash, as well as the non-volatile characteristics of NAND Flash, can theoretically simplify the storage architecture and merge the current memory and external memory into persistent memory, which is expected to eliminate or reduce the 'storage wall' between memory and external memory."

Professor Lu said this and fell into deep thought.

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