Week 4 Flashcards by Franklin Hole

What is the Octal System?

Base 8

symbols: 0,1,2,3,4,5,6,7

4096 512 64 8 1

8^4 8^3 8^2 8^1 8^0

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What is the Binary System?

Base 10

10000 1000 100 10 1

10^4 10^3 10^2 10^1 10^0

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What is the Hexadecimal System?

Base 16

0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F

65536 4096 256 16 1

16^4 16^3 16^2 16^1 16^0

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What is the purpose of memory?

Stores both code and data

Memory is divided into units of 1 byte (8 bits)

Memory with 2^n addressable units requires n-bit addresses

8 bits = 256 addresses

32 bit = 2^32 addresses (roughly 4gb)

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What is the CPU (Central Processing Unit)

Controls the execution of instructions

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What are Registers (hint: memory locations)?

Small, fast memory locations within CPU

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What is the purpose of General Purpose Registers?

Data storage, address calculations and arithmetic operations

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What is the purpose of Special Purpose Registers?

One that has a specific control or data handling task to carry out

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What is the Stack Pointer?

Points to the top of the stack

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What is the Base Pointer (hint: current frame)?

Points to the start (base) of the current frame

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What is the Link Register (LR) (hint: function completion)?

Stores return address for after function completion

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What is a Program Counter (PC)?

Stores the address of the next instruction to be executed

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What is the Arithmetic Logic Unit (ALU)?

The ALU handles arithmetic and logical operations

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What is the Status Register (hint: contains)?

Contains status and mode bits

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What are Assembly Languages?

Simple Instructions

Difficult programming language

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What is Branching (Branch Instructions)?

Branch instructions will “jump” to a different instruction

They do so by changing the program counter

What is Temporal Locality?

Items accessed recently are likely to be accessed again soon

If you want to discard data, start with the one that has not been requested in a long time

What is Spatial Locality?

Items located near each other in memory are likely to be accessed within a short period of time

Most programs exhibit spatial location

Copy additional memory locations into the cache

What are the 2 outcomes of a memory request?

Cache hit

Cache miss

What is a Cache Hit?

Cache contains the data (faster data return)

What is a Cache Miss?

Cache does not contain requested data - first read from memory

What are the 3 Cache Designs?

Direct Mapping

Fully-associative Mapping

Set-associative Mapping

What is a Direct-Mapped Cache?

Each block of main memory maps to one unique cache line

What are the pros and cons of a Direct-Mapped Cache?

Pros:

Simple, easy to implement and search

Cons:

Inefficient use of the cache space

What is a Fully-Associative Cache?

Each block of main memory can map to any line of the cache

What are the pros and cons of a Fully-Associative Cache?

Pros: Flexible Cons: Takes a long time to search

What is a Set-Associative Cache?

Cache divided into N cache sets Each cache set consists of S lines Each block of main memory maps to one unique cache set

What are the 2 options with dealing with a Write Hit?

Write-Back - initially write only to cache, write into memory if being replaced Write-Through - Write into both cache and memory

What are the pros and cons of Write-Back?

Pros: Fast writes Cons: Mechanism for cache coherence

What are the pros and cons of Write-Through?

Pros: Consistency between the cache and memory Cons: Slow writes

What are the 2 options with dealing with a Write-Miss?

Write around - write into memory only Write allocate - fetch from memory and write into cache

What are the pros and cons of Write around?

Pros: Avoids unnecessary cache access and cache pollution Cons: Does not utilise temporal locality

What are the pros and cons of Write allocate?

Pros: Exploits spatial and temporal locality Cons: Potential unnecessary cache access and cache pollution

What is Virtual Memory?

Use disk storage as if it was main memory analogy: Main memory is the cache of the disk

What is Demand Paging?

Only load a page when there is a request for it

What are the pros and cons of Demand Paging?

Pros: More processes can be loaded into memory Cons: CPU spends too much time swapping pages - Thrashing

What is Pre-Paging?

Guess which pages will be requested and pre-load them

What are the pros and cons of Pre-Paging?

Pros: Less overhead when page predictions are accurate Cons: Resources wasted on loading unused pages