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68 votes

Can a CPU function with nothing more than a power supply and a ROM, using only the internal cache as RAM?

See this extremely detailed account of the PC boot sequence: http://www.drdobbs.com/parallel/booting-an-intel-architecture-system-par/232300699?pgno=2 Since no DRAM is available at this point, code ...
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60 votes

How is 255 Tbit/s processed in optical fiber communication?

It seems like you're referring specifically to http://www.nature.com/nphoton/journal/v8/n11/full/nphoton.2014.243.html . It can be read here: https://www.researchgate.net/publication/269099858_Ultra-...
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47 votes
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Why do microcontrollers have so little RAM?

There are several reasons for this. First of all, memory takes up a lot of silicon area. This means that increasing the amount of RAM directly increases the silicon area of the chip and hence the ...
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46 votes
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Can we not simply connect a battery to a RAM to prevent data loss during power cuts?

Short answer: You might be able to "connect a battery to a RAM to prevent data loss during power outage", but this depends on the type of RAM. SRAM (Static Random Access Memory) is not DRAM, ...
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44 votes
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How is 255 Tbit/s processed in optical fiber communication?

Rather than worrying about a research paper that's pushing things to the limit first start by understanding the stuff sitting in front of you. How does an SATA 3 hard drive in a home computer put 6 ...
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35 votes
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Why use DDR instead of increasing clock speed?

With SDR, there are two clock edges per bit, but only at most one edge on the data line. With high frequency communication, the analog bandwidth limits how close you can put edges together on any ...
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34 votes
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Why does RAM (any type) access time decrease so slowly?

It's because it's easier and cheaper to increase the bandwidth of the DRAM than to decrease the latency. To get the data from an open row of ram, a non trivial amount of work is necessary. The ...
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33 votes

Why execute code from RAM?

In addition to the speed & other features which others have already mentioned, executing code from RAM can be useful in bootloaders where you need to reprogram your micro's flash - you can't ...
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31 votes
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72-pin SIMM RAM: how do ground (Vss) and supply (Vcc) voltage pins work

The board is a multi-layer stackup, probably 4-layer. This means that there are more layers inside the PCB on which other connections are routed. You can tell this from the seemingly disappearing ...
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25 votes
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Enable vendor-disabled hardware on tablet?

Each of those modules is 4Gb (Gigabit). So you have 8Gb of RAM. A byte is typically 8-bits, so that equates to 1GB of RAM, which is what your OS is reporting. There is no disabled memory that you can ...
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24 votes

Can we not simply connect a battery to a RAM to prevent data loss during power cuts?

Can we not simply connect a battery to a RAM to prevent data loss during power cuts? So can you tell me why is this not done? Of course we do! It's called battery-backed SRAM, and it's widely used in ...
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23 votes
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How can I access more than 15 addresses of data from my 8 bit incomplete computer?

the half of the byte in memory is the opcode and other half is the address Well, that's one way of doing it, but there are lots of ways of doing it. Instructions don't have to contain the address. ...
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20 votes

Does a CPU completely freeze when using a DMA?

If there is a single memory interface, there would be hardware to arbitrate between requests. Typically a processor would be given priority over I/O without starving I/O, but even with I/O always ...
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20 votes

Is there a lower limit on operating frequency of CMOS SRAM?

No, there is no minimum frequency because it's static RAM. Any digital circuit that's described as "static" doesn't have a minimum frequency - it can operate with all signals held static for an ...
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19 votes

72-pin SIMM RAM: how do ground (Vss) and supply (Vcc) voltage pins work

The PCB most probalby has 4 layers. The outer 2 which you can see are used to carry the signals and the inner 2 are power layers - 1 for ground and 1 for Vcc. This is the most common approach when ...
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19 votes

Why execute code from RAM?

I didn't look at the datasheet for that micro. However, it is often the case in this situation that fetching from RAM is faster than fetching from the flash the program memory is implemented from. ...
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19 votes

Why use DDR instead of increasing clock speed?

The real problem is bandwidth. The highest frequency that a data line can generate (well, not counting slew rate) is when it's sending a 101010 data pattern, which occurs at half of the data rate. ...
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18 votes
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Does a CPU completely freeze when using a DMA?

You are correct that the CPU cannot be accessing the memory during a DMA transfer. However there are two factors which in combination allow apparent parallel memory access by the CPU and the device ...
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18 votes

Why execute code from RAM?

When you want to execute in RAM because it is faster, it's usually because that RAM is on-chip SRAM. This is a scarce resource, which you will probably want for data that requires read/write access. ...
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16 votes
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What do hardware address pins do?

These "hardware addressing" pins are not for addressing words in the RAM, but rather to select the address of the whole device on the IIC bus. The manufacturer realizes that you might want to have ...
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16 votes

Which is faster, Executing a program from ROM or RAM?

The datasheet should tell you how long each instruction takes, and what differences there are, if any, between executing from RAM or ROM. For microcontroller that offer the option of executing from ...
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15 votes

Why do microcontrollers have so little RAM?

Memory probably takes up the most silicon space, and RAM being very fast to use is volatile - and uses power constantly to keep its state. Unless you need lots of RAM, it's not useful for many other ...
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15 votes
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What happens when microcontrollers run out of RAM?

In general the stack and the heap crash in to each other. At that point it all gets messy. Depending on the MCU one of several things may (or will) happen. Variables get corrupted The stack gets ...
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15 votes

How is 255 Tbit/s processed in optical fiber communication?

Ignoring the details of the specific transmission in question (which @alex.forencich has already discussed in considerable detail), it seems like it's probably useful to consider the more general case....
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  • 2,903
14 votes

Why do microcontrollers have so little RAM?

Besides the excellent points brought up in the other answers, another reason for limited RAM is the architecture of the microcontroller. For example, take the the Microchip PIC10LF320, which has only ...
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14 votes
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Why are the semiconductor chips (DRAM) volatile

DRAM is built as a capacitor and a switch for each bit - the data is stored as a charge on the capacitor. It is pretty much impossible to make a perfect capacitor and a perfect transistor, certainly ...
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14 votes

Can a CPU function with nothing more than a power supply and a ROM, using only the internal cache as RAM?

Generally, the cache memory is not addressable. A program cannot store or retrieve data intentionally from it.
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14 votes

Why execute code from RAM?

In addition to all good answers: Why wouldn't I just always execute from RAM if the benefit is only increased speed? Because in an embedded system, usually you don't have the required amount of ...
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