Intelligent knowledge Rom Ram

Semiconductor memory is widely used in various computing devices such as computers. Compared with hard disk, floppy disk, tape, and other magnetic memory, CD, VCD, DVD, and PD optical memory, and MD magnetic/Optical Memory, semiconductor memory has obvious advantages: fast speed and small size, durable. However, the cost of semiconductor memory is relatively high, and the capacity is not large.

Rom and ram refer to semiconductor memory. The original meaning is: Rom refers to read-only memory, that is, this type of memory can only be read and cannot be written. Ram is short for random access memory. This term originated from the fact that some computers used magnetic drums as memory, and magnetic drums and tapes were typical sequential read/write devices. Ram can be read and written randomly, hence its name.

However, this concept was introduced 30 years ago. The meaning of semiconductors and computing technologies has changed over the past 30 years.

Rom usually refers to non-volatile memory, or does not power down. When the system stops power supply, they can still maintain data. So the CD also has a CD-ROM or DVD-ROM. Ram usually does not have data after it is powered. It is typically like the memory of a computer. It needs to be reloaded from the hard disk when the system restarts. Sometimes, if the data can be erased, will also borrow the concept of RAM, such as DVD-RAM, in fact, only can erase the DVD disc, not the real semiconductor memory.

There are many types of Rom, suitable for different use needs. Common examples include:
Mask-Rom
The Rom data is written during production. In fact, it is similar to the CD principle and is written into the data state in the Process of semiconductor lithography. In this case, Rom data cannot be lost, and its cost is very low. Mask Rom is widely used in devices that do not require data updates. However, it cannot be erased at all, making it less popular on computing devices. For example, in the handheld computer, I only know the Palm OS PDA of the visor series produced by Handspring, and the low-end clie SL-10 produced by Sony uses this ROM.

Flrom/fusible link Rom
Writing data to the chip manufacturer is sometimes not so flexible, and there are still confidentiality issues. So there is also a rom called fusible link. This rom uses a fusing link internally. device manufacturers can use high voltage to write data. After the data is written, some internal connections are fused, and the data is permanently kept inside, it is the same as the mask Rom. This one-time writing Rom is not very common.

EPROM/Erasible programmable Rom
This is a ROM that can be written multiple times. Because it can be written multiple times, it is called programmable. The write of EPROM requires a dedicated device, while the Data erasure requires removing the protection label on the Rom, exposing the internal chip to ultraviolet light for a period of time. In fact, this ROM is called programmable rather barely, because the write circuit is dedicated and usually not integrated into the computing device, so it is usually not updated frequently and is a plug-and-play method. When an update is required, write the update to a dedicated Write Device. This rom was used for the BIOS of some early computer boards.

EEPROM/E2PROM (E square ROM)/electrically Erasible programmable Rom
Different from EPROM, E2PROM can be erased using a circuit instead of ultraviolet rays. The write/Erase ratio is higher than the read voltage, usually greater than 20 V. The write speed is also faster than EPROM, in milliseconds. The EEPROM was used a lot at one time, but it is currently replaced by an improved Rom-Flash ROM.

Flash ROM/flash EEPROM
This is currently the most common erasable Rom, which is widely used in BIOS storage for expansion cards such as the motherboard and video card and sound card. And now a variety of Stamp-sized semiconductor memory cards, including Compact Flash/CF, smart media/SM, security digital/SD, multimedia card/MMC, memory stick/MS, as well as the new standard vCard from Fuji, as well as USB mobile hard drive/USB flash disks of various key chain sizes, Flash ROM is used internally. Most PDA/handheld computers also use it to store operating systems and built-in programs. There are also digital cameras, digital cameras, and the fireware inside the MD/MP3 player (used to store DSP/ASIC programs), and most of them use Flash ROM. Compared with EEPROM, Flash ROM has the advantages of fast writing speed and low writing voltage. However, the cost is also high, so in many low-end devices, there will still be manufacturers using mask Rom to reduce costs and improve price competitiveness.

Ram, the most common is the computer memory. Ram is also used in PDAs and PDAs to store programs and data. However, in PDAs and PDAs, the data in Ram is retained as long as the battery has power, no loss. In the MD/MP3 player and DC/DV, Ram is also used to save program data or buffer data.
Ram has two categories: static RAM and static RAM/SRAM. The speed of SRAM is very fast. It is currently the fastest reading and writing storage device, but it is also very expensive. Therefore, it can only be used in demanding scenarios, such as CPU Level 1 buffering and level 2 buffering. The other is dynamic RAM, dynamic RAM/dram. Dynamic RAM is slower than SRAM, but faster than any Rom. The computer memory is dram.

There are too many DRAM types to list them one by one. Here we only list the most common ones:
DRAM, if used together with other DRAM types, it refers to the oldest DRAM memory, which is not optimized in addressing and is slow, it was only available on computers earlier than 386.
Fpram/fastpage Ram, fast page memory, reads data in the form of pages, faster than dram, 486 used.

Edoram, extended data output Ram, which is a faster reading method than the fast-page memory, widely seen in computers in the 486 and early Pentium era, the Edo RAM used on the PC is usually 72-pin on one side. Now you can find them on older computers. In PDA/handheld computers, this Edo memory is still widely used.

SDRAM, sychronous dram, synchronous memory. Early PC memory clock and CPU external clock were not synchronized, which resulted in a collaboration time each time data was read and written, with low efficiency, while SDRAM can run in sync with the external clock of the CPU to improve read and write efficiency. The Pentium to Pentium III era has been the master of sdram pc, the 168pin dual-sided pins of memory is still common. This memory is often used on low-end display cards.

Ddr ram, ddr sdram, double date-rate Ram. This improved Ram and SDRAM are basically the same. The difference is that it can read and write data twice at a clock, which doubles the data transmission speed. This is the most widely used memory in computers, and it has a cost advantage. In fact, it beat Intel's other memory standard-Rambus DRAM. In many high-end graphics cards, high-speed ddr ram is also provided to increase the bandwidth, which can greatly improve the pixel rendering capability of the 3D accelerator card.

RDRAM and Rambus DRAM, which are patented by Intel, are very different from the original memory read/write method. It uses a high-speed serial method, which is very advantageous for continuous read/write, however, compared with ddr ram, random read/write is not obvious. Moreover, the cost is high, and it has not become the mainstream with the powerful impetus of Intel. It is only used on some high-end p iv computers and servers.

VRAM and video RAM are dual-port RAM. The advantage of dual-port is that when one end is written, the other end can be read. The most common application is to use the video card, one end can be written into the screen data, and the other end can be read and converted into video signals and output to the monitor by ramdac (digital/analog signal converter. For graphic designers who use high-resolution displays, high-speed VRAM is essential to the video card. VRAM costs a lot.

Sgram is an improved model of SDRAM with a low cost, but it can work in a way similar to VRAM dual port. Matrox used it to arm the mystique series low-end graphics card.

Wram, Windows Ram, is a simplified VRAM model. matrox used it to arm millennium series of high-end graphics cards.
Among other devices except computers, Edo Ram and SDRAM still have a large market.

There are a wide variety of ROM and Ram, there are a lot of products designed for specific applications and devices, but the most common is so much.

Finally, let's summarize the information about the PDA/handheld devices.

Usually we will see the nominal xxm ROM and xxm RAM on the PDA/handheld computer. Here the ROM refers to the storage space of the operating system and the built-in program. when the power is down, the data still exists. In addition to the low-end devices that use the mask Rom, all the other devices use Flash ROM, which means that the operating system can be upgraded, and some programs can change the system so that common programs can use the remaining space in the Rom. Ram is the space where these devices can store user programs and data. However, when the device is completely powered off, the data will be lost, when the battery voltage is low, it will be forced to shut down and use the remaining power to keep the data, or use the backup battery to complete the work.

New models of handheld computers or PDA devices also allow users to use external expansion cards to save more programs and data. These types of external expansion cards are mentioned in the above post. They are incompatible with each other (SD and MMC can use the same slot), and the performance and price vary depending on the user's needs. These expansion cards use Flash ROM.