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1

I think you are correct, but I can't see for sure which output of the decoder will be active, because it has no lables on its outputs. And I would avoid talking about the 'first' bit (which is open to multiple interpretations), better call it the 'lowest' bit.


1

All of the other popular answers presented here talk about literal differences between FPGAs and CPUs. They point out the parallel nature of the FPGA vs the sequential nature of a CPU, or give examples of why certain algorithms might work well on an FPGA. All of those are good and true, but I would suggest however that there is a more fundamental ...


1

Well in general for two chips to talk to each other there's some agreement for what frequency the interface will run at. For your memory case the memory interface would be running at 1333MHz, and while the CPU only runs at 1Ghz, the memory interface on that CPU chip is designed to run at 1333MHz. From there you have two interesting problems. Imagine if ...


0

The problems you describe in your original question could only be avoided if ALL of the memory in the machine were included on the CPU. Any additional memory added to the machine via slots on the main board would be subject to the same delays you describe and would require marshaling and logic control devices between the CPU / RAM and onboard RAM. RAM is ...


1

In addition to the answers already given there is one additional aspect: The waste due to production faults: Let's say 1/100 of all CPUs of a certain model produced are faulty (in reality it is less, of course; 1/100 is easier to calculate) and 1/100 of all RAMs produced are faulty. If both components would be combined on one single chip then 1/100 of all ...


1

While all the previous answers are correct in pointing out why it's so difficult to add more memory to the CPU, it's also true that there is quite a lot of memory in modern CPUs. In real-time operations when deterministic delays are important it is not unheard of to use the on-chip cache as addressable memory, for code and/or data. The advantage is fast ...


2

Almost all of the above + one more additional problem: the heat. The DRAM cells are essentially leaky capacitors. And the dielectric here is the SiO2 layer itself. As the temperature increases, the leakage currents increase proportionately. These discharge the DRAM cells far faster which would require much faster refresh rates, which would increase the ...


1

In addition to the other reasons mentioned, many systems have more than one CPU core. At times when information which is stored in main DRAM is consistent with all cached copies, all processors which do not have the information cached will have equal access to it. Some architectures are designed around the assumption that each CPU core will "own" a range ...


3

In addition to the other answers, there's more that can be said about such a system. Moving memory to the main die would include a host of other engineering problems. You'd have to reroute buses, build a DMA controller into the main processor, restructure the IRQ bus, and determine how to get rid of all that extra heat you'd be putting out in a concentrated ...


67

Intel's Haswell (or at least those products that incorporate the Iris Pro 5200 GPU) and IBM's POWER7 and POWER8 all include embedded DRAM, "eDRAM". One important issue that has led eDRAM not to be common until recently is that the DRAM fabrication process is not inherently compatible with logic processes, so that extra steps must be included (which increase ...


15

The main reasons larger memory (GB's of DRAM) isn't included on the CPU die itself is primarily about cost. CPU die space is significantly more expensive because of the manufacturing process required to make the very small features. It may also not be possible to manufacture the two on the same die, though I don't know enough about the details to give any ...


5

There are several reasons why adding large amounts of DRAM to a CPU could be infeasible. The process and fab may not be set up for DRAM. DRAM requires special circuit elements that take extra manufacturing steps to produce. This increases the cost of manufacturing. All that memory has to be tested. Memory testing increases your test time. That's another ...


1

There are actually two types of RAM. They are static RAM and Dynamic RAM. Static memory are very fast but it comes in a higher cost. Meanwhile the dynamic RAM is slow compared to static RAM but comes in a cheap cost compared to static RAM. Cache memory falls into static RAM. you can see that they comes in KB or MB sizes. They are fast. But high cost.



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