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98 votes
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Why not make one big CPU core?

The problem lies with the assumption that CPU manufacturers can just add more transistors to make a single CPU core more powerful without consequence. To make a CPU do more, you have to plan what ...
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86 votes
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What limits CPU speed?

Practically, what limits CPU speed is both the heat generated and the gate delays, but usually, the heat becomes a far greater issue before the latter kicks in. Recent processors are manufactured ...
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85 votes
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Why is RAM not put on the CPU chip?

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 ...
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76 votes
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Why don't we make CPUs with 1000s of layers to make use of space in the third dimension?

The two killer reasons are yield, and heat. Yield. Every time you do a process step, you get less than 100% perfection. Let's say you get 99% perfection per step. In a process with 20 steps, you would ...
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73 votes
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Why do CPUs need so much current?

CPUs are not 'simple' by any stretch of the imagination. Because they have a few billion transistors, each one of which will have some small leakage at idle and has to charge and discharge gate and ...
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66 votes
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Why is there no `nand` instruction in modern CPUs?

http://www.ibm.com/support/knowledgecenter/ssw_aix_61/com.ibm.aix.alangref/idalangref_nand_nd_instrs.htm : POWER has NAND. But generally modern CPUs are built to match automated code generation by ...
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55 votes

How can an FPGA outperform a CPU?

CPU's are sequential processing devices. They break an algorithm up into a sequence of operations and execute them one at a time. FPGA's are (or, can be configured as) parallel processing devices. ...
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51 votes
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How can a CPU deliver more than one instruction per cycle?

First, as Keelan's comment and Turbo J's answer point out, the measurement was 113,093 Dhrystone MIPS not native MIPS. The Ivy Bridge microarchitecture of the i7 3630QM can only commit 4 fused µops ...
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51 votes
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Is transistor the only electronic component on a CPU?

The logical blocks and memories can be made out of only transistors. The important question is: are all of the circuits on CPUs logical blocks and memories, or is there anything else? The answer ...
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49 votes
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When Intel / AMD choose their Nanometer Processes, why were the specific numbers, 5, 7, 10, 14, 22, 32, 45, etc chosen?

There are a number of different reasons for this. The numbers aren't chosen Modern CPU manufacturing processes, at least for top-of-the line mainstream CPUs such as Intel Xeon and Core, AMD Epyc and ...
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43 votes

Why do CPU's typically connect to only one bus?

The approach which you show is quite an old topology for motherboards - it predates PCIe which really puts it back somewhere in the '00s. The reason is primarily due to difficulties of integration. ...
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43 votes
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Why don't we see faster 7400 series chips?

As technology size decreases, wire resistance/capacitance cannot scale proportionally to the propagation delay of the now faster/smaller transistors. Because of that, the delay becomes largely wire ...
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41 votes
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What is the lowest level of CPU programming above transistors?

In my CS program, my professor has claimed that NAND gates are the most basic gate to engineer, and so every other gate and higher-level circuits found in CPUs are made from NAND gates “Yeah? Well, ...
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  • 40.2k
40 votes

What limits CPU speed?

The heat issue is well covered by fuzzyhair. To summarize the transmission delays, consider this: The time needed for an electrical signal to cross the motherboard is now more than one clock cycle of ...
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  • 2,073
40 votes

Why not make one big CPU core?

In addition to the other answers, there is another element: chip yields. A modern processor has several billion transistors in them, each and every one of those transistors have to work perfectly in ...
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  • 1,424
40 votes

Why does Intel's Haswell chip allow floating point multiplication to be twice as fast as addition?

This possibly answers the title of the question, if not the body: Floating point addition requires aligning the two mantissa's before adding them (depending on the difference between the two ...
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  • 4,451
40 votes

For mainstream computing what are the practical advantages of 64-bit register size CPUs given the needs of today and the near future?

With 48bits you can address 256TiB of RAM, plenty of space to be useful Its not about address space (*). In fact most 64-bit desktop processors have a 48-bit address bus. There is little point ...
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37 votes

How can an FPGA outperform a CPU?

Markt has this mostly right, but I'm going to throw in my 2 cents here: Imagine that I told you that I wanted to write a program which reversed the order of bits inside of a 32-bit integer. ...
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  • 3,239
37 votes
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How can cache be that fast?

This CPU has... 2 cores A 32-KB instruction and 32-KB data first-level cache (L1) for each core Since there are two cores, we can expect the benchmark to run two threads in parallel. Their ...
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37 votes

Why aren't CPUs cooled from below as well as above?

They aren't cooled from below because they have pins on the bottom, and FR4 below that. Due to having a much lower thermal conductivity, $$ \begin{array}{rrl} \text{Copper:} & 385\phantom{.25} &...
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37 votes
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How does the current processor technology with low clock rates (<10 GHz) deals with mmWave (>10 GHz) technology used in 5G?

So, how do processors deal with the frequencies in the range of 20-100 GHz? They don't. Even if we consider a demodulation of data to lower frequencies from carrier frequencies, there needs to be a ...
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  • 6,361
33 votes
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What stops an assembly program from crashing the operating system?

In the end, all programs are machine code, regardless of whether the source language was assembler or a high-level language. The important thing is that there are hardware mechanisms that limit what ...
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  • 164k
32 votes
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How can an FPGA outperform a CPU?

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 ...
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32 votes

Why is there no `nand` instruction in modern CPUs?

The cost of such an ALU functions is 1) the logic that performs the function itself 2) the selector that selects this function result instead of the others out of all ALU functions 3) the cost of ...
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32 votes

Using CPU heat to generate electricity

The issue with thermoelectric generators is they are horrendously inefficient. For a CPU you HAVE to get rid of the heat they produce or they melt down. You could hook up a peltier module and ...
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31 votes

How can cache be that fast?

@peufeu's answer points out that these are system-wide aggregate bandwidths. L1 and L2 are private per-core caches in Intel Sandybridge-family, so the numbers are 2x what a single core can do. But ...
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30 votes

Why is there no `nand` instruction in modern CPUs?

Turn it around - first see why Nand was popular in hardware logic design - it has several useful properties there. Then ask whether those properties still apply in a CPU instruction... TL/DR - they ...
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30 votes
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FPGA CPUs, how to find the max speed?

The speed of a design is limited by several things. The biggest will most likely be the propagation delay through the combinatorial logic in your design, called the critical path. If you use a fast ...
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30 votes
Accepted

Can you make a CPU out of electronic components drawn by hand on paper?

If the gain of a single inverter is less than unity, then it will not be possible to combine any significant number of gates together to build a larger circuit. The signal levels will just peter out. ...
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