Bandwidth - the basics of memory bandwidth
The word bandwidth is commonly used in the field of electronics, and it means the range of wavelengths, frequencies, or bands of energy, specifically the difference between the upper and lower boundaries of the frequency band in terms of the number of cycles per second. It is clear that bandwidth is used to describe the width of a frequency band, but it is also commonly used in digital transmission to measure the ability to transmit data. It is used to indicate the size of the transmitted data capacity per unit of time, indicating the ability to throughput data.
In many articles, we tend to see various descriptions of bandwidth, so how to calculate the bandwidth of the memory in question? For the memory bandwidth calculation there is the following method:
B represents the bandwidth, F represents the memory clock frequency, D represents the number of memory data bus bits, then the bandwidth is:
B=F×D/8
For example, the SDRAM bandwidth of the PC-100 is calculated as follows:
100MHZ×64BIT/8=800MB per S
Of course, this calculation method is for SDRAM that only relies on the rising edge signal to transmit data, for DDR that transmits data on both the rising and falling edges the calculation method is a little bit different, and it should be multiplied by 2 at the end, because its transmission efficiency is doubled, which is an important reason why DDR can have such high performance.
The same method can be used to calculate the bandwidth of buses that have a strong relationship with memory bandwidth, such as PCI and AGP buses. For example, PCI bandwidth = 33MHz x 32BIT/8 = 133MB/S, AGP 1X bus bandwidth is 66MHz x 64BIT/8 = 528MB/S, and AGP 4X bandwidth = 528MHz x 4 = 2.1GB/sec.
Through such calculations, we can easily see that the development of the bus is accompanied by the expansion of the bandwidth, and only a high-bandwidth bus can constantly meet the requirements of the current data transmission of various hardware. For example, the graphics card from the PCI bus to AGP, it is because the 133MB / S transmission rate of the PCI bus has long been unable to meet the requirements of a variety of graphics processing. Instead, AGP1X, AGP4X, and AGP8X have expanded the transmission bandwidth.
The calculated bandwidth is the theoretical value of the maximum peak bandwidth they can reach, and by comparing the peak bandwidth we can understand the performance of the various types of memory, the following table gives the peak bandwidth of common memory.
Common Memory Peak Bandwidth Table
PC-66 SDRAM
528 MB/s
PC-100 SDRAM
800 MB/s
PC-133 SDRAM
1064 MB/s
PC-150 SDRAM
1200 MB/s
PC-600 RDRAM
1200 MB/s
PC-800 RDRAM
1600 MB/s
PC-1600 DDR
1600 MB/s
PC- 2100 DDR
2100 MB/s
In practice, these memory cells may not reach their peak bandwidth, and there are many other factors that affect bandwidth. For example, because there is always a delay time between writing data to and reading data from the storage unit. In addition to the delay time affecting bandwidth, the hit rate of the stored data also has an important relationship. When these factors are taken into account, the actual bandwidth of the PC100's SDRAM is only 40% of the peak bandwidth, even with a 100% hit rate.
From the bandwidth calculations given above, it is clear that the bandwidth is not only related to the clock frequency, but also to the number of bits in the memory unit's data bus. While we focus on the clock frequency when dealing with various graphics cards, calculating bandwidth also requires the number of bits in the graphics memory. Graphics memory plays an important role in graphics cards, and various graphics chips support the number of bits of graphics memory is also different, manufacturers are also expanding the number of bits of graphics memory to achieve the purpose of increasing the bandwidth of graphics memory. The following table shows the number of bits of memory supported by common graphics chips.
Chip model
Supported memory bits (BIT)
Chip model
Supported memory bits
VOODOO3-2000\3000\3500
128
RADEON VE
64(DDR)
VOODOO4-4500\5500\6000
256
G400\G400MAX
128
TNT2\TNT2pro\TNT2 Ultra
128
G450\G550
128(DDR )
TNT2M64\TNT2Vanta
64
Savage4GT/PRO/PRO+
128
GeForce256
128
Savage2000/ 2000+
128<
GeForceMX
GeForceMX400
64/128 SDRAM or 64(DDR)
Kyro
KyroII
128
GeForceMX200
64
SIS300/SIS305
128
GeForceGTS\Pro\Ultra
GeForce3
128(DDR)
SIS315
128(DDR)
RADEON\SELE
128 or 128 (DDR)
BLADE XP
128
BANDWIDTH OUTLET - BANDWIDTH OF MONITORS
We often see the term bandwidth when we buy monitors as well, and in this case it's differentiated from bandwidth in the storage world, which is is closer to the traditional definition of bandwidth in electronics. Bandwidth, a parameter of a monitor, represents a comprehensive indicator of a monitor and is an important indicator of how good a monitor is. It refers to the number of pixels scanned by the monitor per second, that is, the total number of pixels displayed on each scanning line per unit of time, in Hz. The bandwidth of a monitor is also calculated in a certain way, so you can calculate the bandwidth based on a number of parameters or calculate some parameters based on the bandwidth when you are choosing a monitor. This can be a very clear understanding of the bottom of the monitor, JS want to hide can not.
The detailed monitor bandwidth calculation method is as follows:
With r(x) that the number of pixels on each horizontal scanning line; r(y) that the number of horizontal scanning lines per frame; V that the refresh rate of the screen per second; B that the bandwidth. The formula for calculating the bandwidth of a monitor is:
B = r(x) × r(y) × V
But in practice, in order to avoid signal attenuation at the scanning edges, and to ensure the clarity of the image, in fact, the number of pixels scanned horizontally by the e-beam, as well as the scanning frequency of the rows, should be a little higher than the theoretical value. Therefore, in practice, the bandwidth calculation formula added a 1.3 parameter:
B = r(x) × r(y) × V × 1.3
Based on the above formula, we can more clearly understand the actual meaning of the bandwidth. When the refresh rate of a monitor is increased a little bit, its bandwidth will have to be increased a lot. For example, if a user selects a monitor with a resolution of 1024 x 768 and a refresh rate of 85Hz, we can calculate the actual bandwidth of the monitor as follows:
B = 1024 x 768 x 85 x 1.3 = 87MHz
The size of the bandwidth is very important when selecting a monitor.
Well friends, the story of bandwidth is told here, whether it is the main story or anecdotal, I hope to help you have a complete understanding of the bandwidth.