Newbie charging: The story of the "Bandwidth" Family

One day, cainiao ran into the doctor's clinic with red and swollen eyes and said "beaten". Dr. qimmediately filled his mouth with indignation, saying that he would leave this breath for cainiao, but cainiao was dummies: "Even when people play CS with each other, the speed and accuracy of the screen is always inferior to that of others. The result is that every time BT (burst), there is a flash of screen. After several rounds, my eyes can't stand it ...... We all use MX440 graphics cards !" Dr. Q looked up with a smile: "haha! It turns out that 'bandwidth' has bullied you. Let me give you this lesson ."

I,Ubiquitous bandwidth

Cainiao stupid: The term "Bandwidth" sounds very abstract. What is bandwidth? What is the relationship between it and hardware performance?

Dr. q: here I give a metaphor that the bandwidth is actually like the road width and the maximum allowed speed, while the data flow is equivalent to the traffic flow on the road (figure 1 ). When the number of vehicles is small, the road surface is wider and narrower, which has no impact on the speed of vehicles. As the traffic increases gradually until a critical point, the influence of road width on traffic flow suddenly becomes apparent. If the road surface is narrow, more vehicles cannot pass through at the same time, or even cause congestion. Of course, we can also increase the maximum speed allowed, so that we can pass more cars within the unit time.

Figure 1

With the above image metaphor, I believe it is not difficult for you to understand the bandwidth. From a professional perspective, the bandwidth refers to the wavelength, frequency, or energy band range, the upper and lower boundary frequency difference of the frequency band is expressed by the number of weeks per second. That is to say, bandwidth is used to describe the bandwidth. However, in digital transmission, bandwidth is also often used to measure the ability to transmit data, for example, it is used to indicate the size and throughput of data transmitted per unit of time.

For many accessories, bandwidth is one of the important parameters that determine the performance. bandwidth is everywhere from the bus frequency, memory, video card, display outside the chassis, and even the network of the motherboard in the chassis.

II,System Bus and memory bandwidth

Cainiao stupid: Let me explain the bandwidth of the system bus and memory first.

Q: OK. First, let's look at the system's working process. When the CPU receives a user's command, it first searches for the relevant data from the CPU's level-1 Cache (L1 Cache) and does not have the data in the level-1 Cache, the CPU continues to search for level-1 second-level Cache (L2 Cache). Because the capacity of level-1 and level-2 Cache is limited and the hit rate is low, the CPU reaches the memory level-1 and the hard disk level-1. Here we can regard the memory as a bridge or warehouse between the memory controller (usually in the North Bridge Chip) and the CPU. Obviously, the memory capacity determines the size of the "warehouse", while the memory bandwidth determines the width of the "bridge". The two are indispensable, this is what we often call "memory capacity" and "memory speed ".

Because the amount of data processed by the system is very large, almost every step of the operation has to go through the memory, it is also the most frequently used part of the system. Therefore, when the memory capacity is fixed, the memory bandwidth determines the performance of the system to a certain extent. High bandwidth with only memory is not enough. Since the motherboard is the carrier of CPU and memory, it has a "high limit" on memory bandwidth ", this is what we usually call the external bus frequency, or the front-end bus (FSB ).

Cainiao stupid: Oh, that is to say, the frequency of the front-end bus (FSB) directly affects the speed of data exchange between the CPU and memory. I bought a high-bandwidth memory slide, you still need to have a "high-bandwidth" motherboard to support it, right?

Q: smart! This involves the combination of CPU, motherboard, and memory. Currently, the frontend bus frequency on a PC is 266 MHz, 333 MHz, 400 MHz, 533 MHz, and MHz. If the FSB bandwidth is equal to the memory bandwidth, there is no bottleneck problem, if the memory bandwidth is less than FSB, the memory bandwidth bottle diameter is formed, and the system performance cannot be fully utilized (figure 2 ).

Figure 2

Therefore, for mainstream Intel platforms such as Sai Yang 4 and old P4, the external frequency is 100 MHz, the FSB is 400 MHz, and the data bandwidth is 3.2 GB/s. DDR400 or dual-channel DDR200 or above should be used; the external frequency of P4B and D is 133 MHz, the FSB is 533 MHz, and the data bandwidth is 4.2 GB/s. DDR533 or dual-channel DDR266 or above should be used; p4C, P4E, and new P4A have an external frequency of 200 MHz. The FSB is 800 MHz, and the data bandwidth is 6.4 GB/s. A dual-channel DDR400 or above should be used. For mainstream AMD platforms, the external frequency of Athlon XP is divided into 133 MHz, 166 MHz, and 200 MHz. DDR266, DDR333, and DDR400 memory should be used respectively.
Cainiao stupid: As you mentioned above, when FSB is 3.2 MHz, the data bandwidth is GB/s. How is it calculated?

Q: Here is a formula for calculating the bandwidth of data: bandwidth = working frequency × bus width/8 (bandwidth time and byte conversion ). Both the SDR and DDR are 64-bit bus widths, but the DDR can transmit data at both the rising and falling edges of the clock signal. Therefore, the calculated bandwidth is × 2. Therefore, we can get the following results:

DDR266, operating frequency is 133 MHz, bandwidth is 133 × 2 × 64/8 = 2100 MB/s = 2.1 GB/s

DDR333, operating frequency is 166 MHz, bandwidth is 166 × 2 × 64/8 = 2700 MB/s = 2.7 GB/s

DDR400, operating frequency is 200 MHz, bandwidth is 200 × 2 × 64/8 = 3200 MB/s = 3.2 GB/s

In addition, the so-called dual-channel DDR (Figure 3) means that the chipset can separately address and read data on two different data channels. The two memory channels that work independently of each other are attached to two memory controllers that work independently in parallel and have a bit width of 64bit. Therefore, the general DDR memory can reach a bit width of bit. Therefore, the memory bandwidth is twice the bandwidth of a single channel. Therefore:

Figure 3

The dual-channel DDR266 bandwidth is 133 × 2 × 64/8 × 2 = 4200 MB/s = 4.2 GB/s

The dual-channel DDR333 bandwidth is 166 × 2 × 64/8 × 2 = 5400 MB/s = 5.4 GB/s

The dual-channel DDR400 bandwidth is 200 × 2 × 64/8 × 2 = 6400 MB/s = 6.4 GB/s

Cainiao stupid: I understand a lot after you say this! But how do I identify the product's memory bandwidth when I buy memory?

Dr. q: this is relatively simple. For example, for the current mainstream DDR memory, it can be divided into DDR266, DDR333, DDR400, till DDR533. The last three digits represent the frequency of operation. The specification can be easily identified by the logo on the memory stick (figure 4. It should be noted that non-standard memory specifications exceed MHz, such as DDR533. Currently, there is no CPU available to work with, and it can only be achieved through overclocking.

Figure 4

III,Memory bandwidth

Cainiao stupid: Let's go back to my troubles. Why is it always me that I get hit? Is this also related to bandwidth?

Q: Ha! This is what I want to talk about. The bandwidth calculation method shown above shows that the bandwidth is related not only to the clock frequency, but also to the number of data bus digits of the storage unit. Video memory plays an important role in the video card, and the number of digits supported by various video card chips is also different. Now let's take a look at your video card ...... Well, the graphics chip is GeForce 4 MX440 (Figure 5). Theoretically, it supports-bit video memory, ......

Figure 5

Cainiao: What is it? Let's talk about it!

Q: Don't be angry if you say so. In order to lower the market price of products and attract the attention of consumers who are greedy for low prices, some manufacturers will "shrink" The Display memory ", launched a "budget" video card with only half of the standard product. For example, if you use this video card, although the standard version uses the 6ns video memory, but because the video memory is 64-bit, the video memory bandwidth of the video card is only 166 MHz x 64-bit/8 = 1.3 GB/s. Therefore, its performance is theoretically only half of the standard products. When running a 3D game with high requirements, the rendering speed of textures and materials cannot be kept up, and the image delay is quite strange.

Cainiao stupid: Who! It turned out to be a JS player, but now everything is too late. Dr. qquick taught me how to identify this shrinking graphics card in the future.

Q: It's not too late. Here we will teach you how to understand two different kinds of video memory packages: the video memory particles in the MicroBGA package are basically square, and no pins are exposed (figure 6 ). The display particles encapsulated by TSOP are rectangular and the pins are exposed (figure 7), so we can easily distinguish these two types of memory from the appearance. For the current products on the market, the memory size of each MicroBGA package is generally 32 bits, and the bandwidth of each card is generally bits; the Display memory in the TSOP package is generally 16 bits, so that a total of 8 to reach the standard bits, usually evenly distributed on both sides of the PCB. Like your video card, you use the TSOP encapsulation of the video memory, but only the front is integrated with four, so a total of only 16 bit x 4 = 64bit.

Figure 6

Figure 7

IV,Bandwidth of the monitor

Cainiao dumb: I found my screen was flashing. I felt tired when I looked at it for a while. Why?

Q: This is generally caused by a low screen update rate. Excluding the reason set by the user, the update rate is mainly restricted by the display bandwidth.

What is the bandwidth of the monitor? It refers to the number of pixels scanned by the monitor per second. That is to say, the total number of pixels displayed on each scan line per unit of time, in MHz ). Bandwidth represents a comprehensive indicator of a display and an important indicator to measure the performance of a display. This mainly refers to the CRT display. Because the LCD uses different display principles, the importance of bandwidth is not highlighted by the CRT.

Cainiao stupid: What are the advantages of high bandwidth for our monitor?

Dr. q: high bandwidth means that a higher screen update rate can be achieved at the same resolution (for example, 1024 × 768. As you know, when using CRT