Common technical indicators about memory
Next, let's talk about various technical indicators of memory that everyone is generally concerned about, including pin number, capacity, speed, parity and so on. Usually there are 8MB, 16MB, 32MB, 64MB, 128MB, 256MB, 5 12MB, 1GMB, of which 5 12 GB memory becomes the mainstream configuration. Access time of memory chip is another important index of memory, and its unit is expressed in nanosecond (ns). Whether there are parity bits in memory chips is a problem that people often ignore. Parity plays a key role in ensuring the correct reading and writing of data, especially when calculating very large data. Some standard memory chips have parity bits, while others don't. Non-standard memory chips all have parity bits.
For SDRAM, we should evaluate its performance through at least three parameters.
(1) System clock period-it represents the maximum frequency at which SDRAM can run. For example, an SDRAM chip with a system clock frequency of 10ns can run at the frequency of 100MHz. Most SDRAM chips can meet this requirement. Obviously, the smaller this number is, the higher the frequency at which SDRAM chips can run. For modern PC-100 SDRAM,-10 engraved on its chip means that its clock cycle is 10ns, and it can run the external frequency of 100MHz. According to the modern product data sheet, we can know that the data access time of this chip (discussed below) is 6ns.
(2) Access time-similar to Edo /FPM DRAM, indicating the delay time of reading data. The access time of most SDRAM chips is 6, 7, 8 or 10ns. But don't confuse it with the system clock frequency. Many people regard the access time as the external frequency that this SDRAM chip can run. For Gao Shida PC- 100 SDRAM, the -7 engraved on the chip means its access time is 7ns. But the clock frequency of his system is still 10ns, and the external frequency is 100MHz.
(3)CAS (longitudinal addressing pulse) reaction time-CAS delay time. Some SDRAM can run in CAS delay (CL)2 or 3 mode. That is, the delay of reading data can be two clock cycles or three clock cycles. We can write this performance into the EEPROM of SDRAM, so that the BIOS of PC will check this content and run at a faster speed in CL=2 mode.
However, the above three performance indicators are mutually restrictive. In other words, when you have fast access time, you must sacrifice the performance of CAS latency. Therefore, when evaluating and comparing the performance of SDRAM, we must comprehensively consider the above three indicators, not just from the -6, -7, -8 or-10 engraved on the chip.
That is, once the chip manufacturer calls its product PC- 100. Its chip conforms to PC- 100 standard. Then -6, -7,-10 is just a symbol. Doesn't mean -6 is faster than -7. So in order to prevent people from misunderstanding, Samsung no longer uses this data to represent the access time, but uses letters to represent the access time.
The following is a simple example of evaluating SDRAM performance.
For 100MHz system, the system clock period is 10ns.
Rough calculation: the total delay time of reading regional data =CAS delay+access time.
For example: modern PC- 100 SDRAM, access time 8ns, CL2 mode. Therefore, the total delay time =2 x period+access time =2 x 10 ns+8 ns = 28 ns.
If SDRAM runs in CL3 mode, the access time is 6ns. In this way, the total delay =3 x 10 ns+6 ns = 36 ns.
Obviously, when SDRAM runs in CL2 mode, its speed is completely faster than that in CL3 mode.
Buy memory
Common SDRAM brands in the market include Hyundai, Samsung, LG, NEC, Toshiba, Siemens, TI (Texas Instruments) and so on. When buying, pay attention to whether the words printed on the surface of the chip are clear, what is the nominal speed and where is the place of origin. It should be noted that the brand mentioned above only refers to the memory stick, not the whole memory stick, and the work of packaging the memory stick on the circuit board to make the memory stick is completed by other manufacturers. For example, the famous American Kingston memory only encapsulates the high-quality memory chips of other manufacturers, and does not produce memory chips itself. Therefore, even the same brand of memory chips, due to different packaging manufacturers, the quality will be very different, as can be seen from the circuit board technology. A good circuit board looks uniform in color, smooth in surface, neat in edge, burr-free, six-layer board structure and heavy in hand. The mainstream LGS memory has been replaced by HY memory. Although the brand has changed, the things are still the same. The common model is GM72V6684 1ET7J, which is 8×8 particles. I remember that the memory granule model I bought a year ago is GM72V6684 1CT7J, which is the same as the current memory. At present, the CAS parameters of PC- 133 on the market are basically 3, and it will take some time to buy a PC- 133 memory stick with a real CAS parameter of 2. KINGMAX memory and gold bars are also good choices, but the memory adopts patented packaging technology and will not encounter fakes.