Questions tagged [cpu-cache]
A fast memory limited in space close to the CPU. A cache is designed to reduce the average time to access memory.
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AMAT using Local miss rate, global miss rate, local hit rate, global hit rate
Consider the following scenario as shown in image:
I have summarized the above Memory layout in terms of Miss rates and hit rates:
...
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Need help understanding set-associative cache
The problem I'm trying to solve is:
A set-associative cache consists of 64 lines, or slots, divided into four-line sets. Main memory contains 4K blocks of 128 words each. Show the format of main ...
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37 views
Slowdown when accessing data at page boundaries?
Hi I have a program which accesses memory words that are located X bytes apart in virtual address space. For instance, ...
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44 views
Why isn't a valid bit used for associative cache in processors
Direct map cache uses a valid bit to effectively know if any data is present to a specific cache-slot (aka line/index). If this is the only use of this bit, then I believe, once a line has v-bit set, ...
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3answers
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Is CPU Registers part of Primary Memory?
A friend of mine appeared in an exam recently, and one of the question asked was regarding CPU Registers, which has two points:
(a) CPU Registers are part of Primary Memory
(b) They are volatile
And ...
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1answer
29 views
Clarification on interplay between cache line size and read/write sizes
Say that you have cache lines with the size of 64 bytes and a set-associative or directly mapped cache. Let's also say that the word size is 8 bytes.
According to my understanding, we use a number ...
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4answers
87 views
Compiler instructions to sync core caches: are they really needed?
I have read reviews of this book, and quote the following from one of the reviews (emphasis mine):
Other than straining your eyes with old-styled C++, you can read such
misconceptions in the book ...
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1answer
171 views
What uses have been proposed for overlaid skewed associativity?
In "Concurrent Support of Multiple Page Sizes On a Skewed Associative TLB" (2004; PDF), André Seznec proposed using overlaid ways with different indexing functions with guaranteed avoidance of bank ...
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76 views
Total bits required for a direct-mapped cache
I'm taking a course in computer architecture in which the main reference is the Computer Organization and Design by Patterson and Hennessy. I came across an example which I couldn't grasp its answer: ...
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1answer
276 views
L1 and Ln cache: when are they written?
I have been following the "High Performance Computer Architecture" course from Georgia Tech (also on YouTube), and unless I've missed something, I cannot see where the following has been explained:
...
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1answer
38 views
Finding size of cache in blocks
Let's say I have a 64 kB Cache provided (i,e 2^18) which is direct mapped and block size is 16 byte = 2^5
Now, in the solution it says
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1answer
291 views
Calculate number of cache lines per set or cache size
How can I calculate the number of cache lines per set or the cache size with the given information?
m (number of physical address bits): 32
C (cache size): unknown
B (Block size in bytes): 32
E (...
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1answer
382 views
How to calculate a direct mapped chace capacity with tag and valid bits?
I've seen some very useful posts about this, but none took into consideration both the tag and valid bits. This is a question I took from a notebook in my computer engineering course.
Consider a ...
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1answer
484 views
What's the difference between cache miss penalty and latency to memory?
Can I say that cache miss penalty includes latency to memory? My current understanding is that cache miss penalty is the time moving data from the layer closer to main memory to it. But I'm not sure ...
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Associative cache finding the tag and word number
An associative cache has a block size of 16 words. The capacity of the cache is 32 Kbytes and main memory can store 4 Mbytes. The word (the addressable unit) size is 2 bytes.
I'm unsure how to find ...
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1answer
99 views
Number of MUX required for Cache Mapping
I have read that the number of multiplexers required is equal to the number of bits in the TAG field. Is it true? If yes then why?
I know that the size of each multiplexer has to be S to 1, where S ...
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45 views
Relating L1 cache, TLB and main memory
Earlier while solving problems from computer organization book by Stallings, I came across the fact that
$T_a=(H_{L1})(T_{C1})+(1-H_{L1})(T_{C1}+T_M)$
where,
$T_a \rightarrow$ Average access ...
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1answer
21 views
How to count stores in cache analysis of matrix multiplication
I'm trying to understand cache misses/iter and came across this that I couldn't understand or reason out.
For ijk iteration, my slides say that there are 2 loads and 0 stores.
...
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63 views
cache coherence: false sharing vs. memory coalescing CPU vs GPU
False sharing is defined on CPU as when atleast two cores write to the same cache line. If the two cores have their own private caches, one will invalidate the other's cache line when they write to ...
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2answers
208 views
How many words of memory map to the same cache entry?
I am going over some practice questions for the Major field exam and it asks:
A processor with a word-addressable memory has a two-way set-associative cache. A cache line is one word, so a cache ...
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1answer
778 views
Architecture - calculating miss penalty
I know that AVG Memory Access Time = Hit time + Miss Rate * Miss Penalty
If I am given the AMAT and miss rate, aswell as the latency to access memory(call this x) ...
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1answer
4k views
Cache effective access time calculation
In order to calculate the effective access time of a memory sub-system, I see some different approaches, a.k.a formulas. All are reasonable, but I don't know how they differ and what is the correct ...
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2answers
252 views
what is the meaning of hit time?
Average memory access time = Hit time + Miss rate * miss penalty
Assume a computer with only one cache level. What is the exact meaning of hit time? Is it the ...
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3answers
174 views
Loading a word from byte-addressable cache
I have asked a similar question at stack-overflow, but then I found this question here, and figured it should go here instead..
So, my question is pretty much the same as the one in the question I ...
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228 views
Calculating miss rate for 2 way set associative cache
From my homework:
Consider a 2-way set-associative cache with eight 32-byte blocks. Instructions and operands are 32-bits. There are an 8- bit data bus and a 16-bit address bus. A sample code is ...
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Cache mapping problem
Okay.I have problem about cache mapping. Here is the problem .
Memory size is 1 MB
Byte addresable
Cache block size is 16 Bytes.
Cache size is 64kb
Since memory is 1 mb=2**20 Bytes.
So we need ...
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1answer
42 views
How does cache partitioning prevent covert/side-channel attacks?
In a report on an open-source separation kernel (Muen kernel) I was reading,
in the future work section, it says that cache coloring can be implemented to prevent covert/side-channel attacks.
It is ...
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289 views
Cache Hit or miss
Asume 512 Bytes direct-mapped cache with 64 Byte cache blocks (cache line size), is empty at the beginning and below given set of physical addresses are referred by CPU in the given order. At each ...
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590 views
Set Associative Cache Exercises
I am trying to solve an exercise on set-associative cache , i struggled with it for a while but i think that i figured out the answer , would be helpful if someone could check if my solution is ...
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1answer
426 views
Byte/word addressing in cache
When dealing with caches, the address is split in three parts - offset, index and tag. Question is: if we are given a word address, do we first have to shift left by 2 bits by adding 00 at the end of ...
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87 views
Computer Architecture: Hit ratio with address
There is a problem that as much as I am trying to understand it, I fail. There are theoretically three cache manufacturers that each make 16 byte cache. The first has 16 blocks, 8 blocks the second, ...
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99 views
MESI protocol and write to Main Memory
I am studying cache coherence MESI protocol with "intervention" (cache can send to other cache without use the Main Memory).
On my notes I wrote that in case of a processor has a block in M (modified ...
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1answer
34 views
Word arrangement in caches
Why are the words in a cache line adjacent to one another?
Does this arrangement improve reading/writing performance or are there other reasons to justify this choice?
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Trouble with Direct Mapping for Caches
I have an online homework assignment that is asking me for the tag and index in a direct-mapped cache, for a series of memory addresses. The cache is specified to have 16 one word blocks.
One of the ...
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How to find number of blocks in cache?
Suppose a computer using direct mapped cache has 512KB of main memory and a cache of 4 Bytes, where each cache size is 64KB.
1)Find Number of blocks in cache.
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1answer
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A cache memory has a line size of eight 64-bit words and a capacity of 4K words
A cache memory has a line size of eight 64-bit words and a capacity of 4K words. The main memory size that is cacheable is 1024 Mbits. Assuming that the addressing is done at the byte level, show ...
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43 views
TLB failure in a memory access
I have come across a question which says
If there is a TLB failure in a memory access, the disk must be accessed to load a new page into main memory.
The answer is NO, but i dont understand why. Is ...
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2answers
83 views
How is an OS page stored in a k-way set-associative cache?
I have been reading about set-associative caches. As far as I have read, in case of n-way set-associative cache each way stores, a block (let's say 16 bytes) and therefore each set will be of size 16n ...
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1answer
154 views
Is finding cache size possible with information given?
An 8-way set-associative cache is used in a computer in which the real memory size is 222 bytes. The line size is 16 bytes, and there are 26 lines per set.
A) What is the size of the cache in bytes?
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1answer
902 views
2-way set associative cache exercise
In the exercise I have a 32 bit processor with a 2-way set associative cache. I have 32 bit addresses: 31-14 tag, 13-5 index, 4-0 offset.
Calculate:
1) Cache line size in number of words (1 word = 4 ...
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2answers
75 views
structure of cache when CPU uses words smaller than the main memory
As the title says, i have the following excersice:
An associative mapping cache with four lines in each set, can store in each line two words of 16 bits each. It can store 4K words, that they have ...
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1answer
2k views
Calculating Tag Bits in a Direct-Mapped Cache
The following comes from Patterson & Hennessy Computer Org. and Design (5th ed., p. 390):
How many total bits are required for a direct-mapped cache with 16 KiB
of data and 4-word blocks, ...
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2answers
46 views
Whats the point of caching if the minimum single clock cycle time is the prorogation delay of the slowest component (fetching from DRAM)?
I know that the clock speed is determined by the slowest stage within the processor (usually fetch) because one clock cycle will take as much time as the slowest pipeline stage to ensure everything is ...
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150 views
How is there a CPU read misses on exclusive caches?
If the lower level cache contains blocks that are not present in the higher level cache or other caches then it is said to be exclusive.
It is exclusive because it is only there. How can there be ...
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1answer
236 views
When LRU is NOT the optimal replacement policy
I've been googling this topic but still couldnt get a legit answer. So can someone explain with some examples that if we were to use cache memory instead of LRU we would have lower miss rate (better ...
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37 views
Through how many levels must cache writeback propagate?
Suppose that my multicore computer system has the diagrammed memory caches.
...
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2answers
488 views
Why page size = size of one cache way?
Why is that the page size equals the size of one cache way?
My book states "A direct-mapped cache cannot be bigger than a page".
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378 views
Physically indexed virtually tagged cache
PIVT cache is indexed physically so address translation using TLB is needed to get into cache and we use virtual address as the tag for comparison .
I read that homonym is a problem which is caused ...
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1answer
2k views
What is the difference between caching, buffering and paging? Expecting a detailed answer on the OS level
I have been reading about caching and buffering it seems about the same to me can't get hold of the differences clearly and paging, for now the only difference I understand between paging and caching ...
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1answer
392 views
CPU cache - retrieving data from memory
Regarding CPU cache, if the CPU does not find the data it needs in the cache, I understand it then looks for it in the main memory (RAM). (Let's assume we have only one level of cache in order to keep ...
