# Tag Info

142

GPUs are bad at doing one thing at a time. A modern high-end GPU may have several thousand cores, but these are organized into SIMD blocks of 16 or 32. If you want to compute 2+2, you might have 32 cores each compute an addition operation, and then discard 31 of the results. GPUs are bad at doing individual things fast. GPUs only recently topped the one-...

105

Addition is fast because CPU designers have put in the circuitry needed to make it fast. It does take significantly more gates than bitwise operations, but it is frequent enough that CPU designers have judged it to be worth it. See https://en.wikipedia.org/wiki/Adder_(electronics). Both can be made fast enough to execute within a single CPU cycle. They'...

100

Computers have a "real-time clock" -- a special hardware device (e.g., containing a quartz crystal) on the motherboard that maintains the time. It is always powered, even when you shut your computer off. Also, the motherboard has a small battery that is used to power the clock device even when you disconnect your computer from power. The battery doesn't ...

83

First, not all processor architectures stopped at 32 registers. Almost all the RISC architectures that have 32 registers exposed in the instruction set actually have 32 integer registers and 32 more floating point registers (so 64). (Floating point "add" uses different registers than integer "add".) The SPARC architecture has register windows. On the ...

79

"Back in the day" computers were defined more by their word size, for example the PDP-8 had 12-bit words composed of two 6-bit "bytes". A "nibble" was half a byte, or 3 bits in this case (and here the op codes were 3 bits). It is only in recent decades that 8-bit bytes became so prevalent as to make them the default. Calling the NES 8-bit is less ambiguous ...

78

A byte of data is eight bits, there may be more bits per byte of data that are used at the OS or even the hardware level for error checking (parity bit, or even a more advanced error detection scheme), but the data is eight bits and any parity bit is usually invisible to the software. A byte has been standardized to mean 'eight bits of data'. The text isn't ...

77

As stated by user120366, 16 possible 2-input logic gates exist, I've tabulated them for you here: A|B||0|1|2|3|4|5|6|7|8|9|a|b|c|d|e|f -+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 0|0||0|0|0|0|0|0|0|0|1|1|1|1|1|1|1|1 0|1||0|0|0|0|1|1|1|1|0|0|0|0|1|1|1|1 1|0||0|0|1|1|0|0|1|1|0|0|1|1|0|0|1|1 1|1||0|1|0|1|0|1|0|1|0|1|0|1|0|1|0|1 A and B are the inputs, 0 through f ...

64

Branching One piece of hardware that pretty much no GPU has is a Branch Predictor. That's because their primary function is to compute simple functions over large sets of data. The only "branching" that a typical GPU does is the jump at the end of a loop body. CPUs, on the other hand, typically run code that executes branches quite frequently (about 20% ...

56

if you ran an electrical current through a material today, it would travel at the same speed as if you did it with the same material 50 years ago. With that in mind, how is it computers have become faster? What main area of processor design is it that has given these incredible speed increases? You get erroneous conclusions because your initial ...

48

This is a broad question that does not have an easy answer; it's a long way from electrons skittering along copper wires to rendering a website in Firefox. I will attempt to give you an overview from bottom to top and point you towards the right things to look up. Encoding Numbers The basic motivation is to compute things, as in doing arithmetics¹. The ...

48

Traditionally, a byte can be any size, and is just the smallest addressable unit of memory. These days, 8 bit bytes have pretty much been standardized for software. As JustAnotherSoul said, the hardware may store more bits than the 8 bits of data. If you're working on programmable logic devices, like FPGAs, you might see that their internal memory is often ...

47

Assembly language is a way to write instructions for the computer's instruction set, in a way that's slightly more understandable to human programmers. Different architectures have different instruction sets: the set of allowed instructions is different on each architecture. Therefore, you can't hope to have a write-once-run-everywhere assembly program. ...

44

A) Historically, machines have been characterized by number of bits per 'machine word'. Why should NES be handled differently? B) Calling it a 'byte' is not as clear since historically a 'byte' has not always been composed of eight bits (e.g some early machines had six bits per byte). Admittedly this is not so strong a point anymore. C) On a side note: I ...

40

There are several aspects. The relative cost of a bitwise operation and an addition. A naive adder will have a gate-depth which depend linearly of the width of the word. There are alternative approaches, more costly in terms of gates, which reduce the depth (IIRC the depth then depend logarithmically of the width of the word). Others have given ...

39

Let me see if I can clear up a few potential misconceptions here. Sometimes people think that when they write a research paper they have to use fancy language: it's not enough to just say what they mean, but rather, it has to be written in academic code with more complex-sounding language. Or, they think that using bigger words will make them sound more ...

38

The best answer I can give is, it doesn't really "look" like anything. The instruction currently being executed by the CPU is represented by a series of wires, some of which have a high voltage, some of which have a low voltage. You can interpret the high and low voltages as zeroes and ones, but you can equally well interpret groups of high and low voltages ...

32

That text is extremely poorly worded. He is almost certainly talking about ECC (error-correcting code) RAM. ECC ram will commonly store 8-bits worth of information using 9-bits. The extra bit-per-byte is used to store error correction codes. (In both cases, every byte is spread across every chip. Image courtesy of Puget Systems) This is all completely ...

31

First your integer numbers are converted into binary numbers. For example, the integer 2 is converted to 0010. The CPU uses a digital comparator: A digital comparator or magnitude comparator is a hardware electronic device that takes two numbers as input in binary form and determines whether one number is greater than or less than or equal to the ...

31

Since we're in Computer Science, I'll answer this way: they don't. What do we mean by a "computer?" There are many definitions, but in computer science as a science, the most common is the Turing machine. A turing machine is defined by several aspects: a state-set, a transition table, a halting set, and important for our discussion, an alphabet. This ...

29

Memory latency is one of the fundamental problems studied in computer architecture research. Speculative Execution Speculative execution with out-of-order instruction issue is often able to find useful work to do to fill the latency during an L1 cache hit, but usually runs out of useful work after 10 or 20 cycles or so. There have been several attempts to ...

27

That means, one year of computation time on a single GPU (or half a year on two GPUs, or a quarter of a year on four GPUs, etc.). If you are thinking of using this term in your own writing, I encourage you to also specify what type of GPU you are using. One-GPU year on a Tesla V100 GPU is a lot more computation than one-GPU year on a K520 GPU. The notion ...

27

First, not every problem is easily amenable to a parallel solution. If it's not possible to formulate your problem as such, you might not gain anything from using a GPU or any parallel approach to begin with. Second, it takes time to move data from the host to the device (i.e., the GPU). You can waste time doing many such transfers and the potentially fast ...

26

It can be software, or hardware, or both, or none. There are two kinds of overflows: overflow when growing the stack (when entering a function), and overflow when accessing an array on the stack. Overflows when growing the stack can be detected by making a bounds check on function entry, to verify that there is enough room (and either signal an error or ...

25

The complete picture is fairly complicated. There are many layers built on top of one another that collectively implement high-level abstractions on top of electrical voltages. There is no simple explanation of how everything is put together, especially considering that computer hardware and software has evolved dramatically in the past fifty years. If ...

25

It's easiest to think of $1$ representing a true statement and $0$ representing a false statement. The logic gates then act as truth functions. Say you put two statements, $p,q$, together to form a new statement, $r$. In the case of and (logical conjunction), both $p$ and $q$ must be true for $r$ to be true. In the case of or (logical disjunction), $r$ ...

24

CPUs operate in cycles. At each cycle, something happens. Usually, an instruction takes more cycles to execute, but multiple instructions are executed at the same time, in different states. For example, a simple processor might have 3 steps for each instruction: fetch, execute and store. At any time, 3 instructions are being processed: one is being fetched, ...

23

I think I see your confusion. The TLB and the data cache are two separate mechanisms. They are both caches of a sort, but they cache different things: The TLB is a cache for the virtual address to physical address lookup. The page tables provide a way to map virtualaddress $\mapsto$ physicaladdress, by looking up the virtual address in the page tables. ...

23

Some other things to consider: Part of the reason for using a binary number system is that it's the lowest-base number system that can represent numbers in logarithmic, rather than linear, space. To uniquely distinguish between $n$ different numbers in unary, the average length of representations must be proportional to at least $n$, since there is only ...

23

GPUs are really good at doing the same simple calculation many times over in parallel. They're usually good at spawning millions of short-lived "threads" that perform the same instruction on multiple bits of data (Same Instruction, Multiple Data, or SIMD). They excel at SIMD situations. They have less memory than the CPU has access to and are not meant as ...

23

I think the questioner has it backwards. If we have a logical function such that A | B | result ---+---+------- 0 | 0 | 0 0 | 1 | 0 1 | 0 | 0 1 | 1 | 1 then we decide to call that function and because it is obvious that the result is 1 only when A and B are both 1. Similarly for or, exclusive-or, etc. There are 16 ...

Only top voted, non community-wiki answers of a minimum length are eligible