Direct Memory Mapping
Summary
TLDRThis educational video script explains the concept of direct memory mapping in computer systems. It begins with an introduction to the organization of secondary and main memory, both divided into pages and frames of equal size. The script delves into the process of bringing elements from main memory into cache memory, highlighting the similarities in organization between blocks and lines. It discusses the significance of physical address bits, block and line offset, and tag bits in the mapping process. The explanation of round-robin mapping and the direct mapping technique's strictness concludes the session, promising numerical problems in upcoming lessons to solidify understanding.
Takeaways
- 😀 The video introduces different cache memory mapping techniques, starting with direct memory mapping.
- 🔍 The secondary memory and main memory are conceptually diagrammed, showing how programs are converted into processes and subdivided into pages and frames of equal size.
- 💾 The operating system is responsible for managing the subdivision of processes into pages and their loading into main memory, as detailed in a separate course on operating systems.
- 📚 Cache and main memory organization is similar, with main memory parts termed as 'blocks' and cache parts as 'lines', both having the same size.
- 📝 The smallest addressable memory unit is a 'word', and in a byte-addressable memory, each word is one byte in size.
- 📉 The script explains how to calculate the number of blocks in main memory and how they are numbered, using an example with 64-word size and block size of four words.
- 📍 It details the concept of physical address bits (PA bits), which are used to address locations in the main memory, split into block numbers and word offsets within blocks.
- 🔄 The mapping of main memory blocks to cache lines is done in a round-robin manner, which is a many-to-one relationship.
- 🔑 The PA bits are split into block or line offset, block numbers, and tag bits, with the tag bits identifying which block is present in the cache.
- 🔍 Direct mapping is a strict memory mapping technique where main memory blocks are directly mapped to cache lines, as opposed to other mapping techniques that may allow more flexibility.
- 📈 The video concludes with an explanation of why the tag bits are called so, as they act as identifiers or 'tags' for the blocks present in the cache.
Q & A
What is the main focus of the session described in the transcript?
-The main focus of the session is to explain different cache memory mapping techniques, starting with direct memory mapping.
How is the secondary memory related to the main memory in terms of program execution?
-In terms of program execution, programs permanently reside in secondary storage and during execution, they turn into processes. The operating system is responsible for subdividing these processes into equal-sized pages and bringing them into the main memory.
What is the term used for the smallest addressable memory unit?
-The smallest addressable memory unit is called a 'word'.
What does the size of each block and frame represent in the context of the script?
-The size of each block and frame represents the equal-sized subdivisions of the main memory and secondary memory, respectively, which are used for efficient memory management and access.
How are the main memory and cache memory organized in terms of blocks and lines?
-In the script, parts of the main memory are termed as 'blocks', and parts of the cache are named as 'lines'. Both the block size and line size are the same, facilitating a direct correspondence between them.
What is the significance of the term 'byte addressable memory' in the script?
-The term 'byte addressable memory' signifies that the size of each word is one byte, which is a standard unit for measuring memory capacity and addressing within the memory.
How many bits are required to address 64 words in the main memory?
-To address 64 words in the main memory, 6 bits are required, as log2(64) equals 6.
What are the different parts of a physical address according to the script?
-A physical address is split into the most significant bits for identifying blocks, and the least significant bits for addressing each word within a block.
What is the purpose of the round-robin mapping technique mentioned in the script?
-The round-robin mapping technique is used to assign main memory blocks to cache lines in a cyclic order, ensuring that all blocks have an opportunity to be mapped to each cache line over time.
What is the role of 'tag bits' in direct memory mapping?
-Tag bits are used to identify which block is present in the cache. They work as identifiers or 'tags' for the blocks within a cache line.
What is the significance of the least significant two bits in block numbering?
-The least significant two bits in block numbering dictate which cache line a particular block will be mapped onto in the direct memory mapping technique.
Outlines
📚 Introduction to Cache Memory Mapping Techniques
This paragraph introduces the concept of cache memory mapping techniques, starting with direct mapping. It explains the process of how programs stored in secondary storage are executed as processes and subdivided into pages, with the main memory also divided into frames of the same size. The operating system is responsible for managing this subdivision. The paragraph also delves into the organization of cache and main memory, where parts are termed as blocks and lines respectively, with the line size equal to the block size. The smallest addressable memory unit, a word, and the concept of byte addressable memory are introduced. An example is given with a 64-word main memory and block size of four words, explaining how blocks and words are addressed using physical address bits. The significance of the PA split in identifying blocks and words within them is also highlighted.
🔄 Direct Memory Mapping and Round Robin Technique
This paragraph discusses the direct memory mapping technique, where main memory blocks are mapped onto cache lines in a round-robin fashion due to the limitation of cache size. It explains how blocks are mapped onto cache lines sequentially and then cycle back to the beginning once the end is reached. The paragraph clarifies the significance of the least significant bits in determining which cache line a block maps onto, creating a many-to-one relationship. It also introduces the concept of block or line offset, line number, and tag bits in the context of physical address bits. The tag bits are particularly important as they serve as identifiers for which block is present in the cache, functioning as 'tags'. The explanation includes an analysis of different block numbers and their corresponding tag bits to illustrate the direct mapping process. The paragraph concludes with a summary of the direct mapping technique and a preview of solving numerical problems in upcoming sessions.
Mindmap
Keywords
💡Cache Memory
💡Direct Memory Mapping
💡Secondary Memory
💡Process
💡Page
💡Frame
💡Operating System
💡Block
💡Line
💡Round Robin Mapping
💡Tag Bits
💡Offset
Highlights
Introduction to different cache memory mapping techniques starting with direct memory mapping.
Conceptual block diagrams of secondary memory and main memory are presented for understanding.
Explanation of how programs in secondary storage turn into processes and are subdivided into pages.
Main memory is split into frames of equal size to match the page size.
Operating system's role in subdividing processes into pages and bringing them into main memory.
Cache and main memory organization similarities, with main memory parts termed as blocks and cache parts as lines.
Cache line size is the same as the block size, ensuring consistency between cache and main memory.
Introduction of the smallest addressable memory unit, a word, and its byte addressability.
Assumption of a main memory with 64 word size and block size of four words, leading to 16 blocks.
Explanation of how memory blocks are organized and addressed using physical address bits (PA bits).
PA bits split into block number identification and word addressing within each block.
Illustration of how a physical address points to a specific word in memory using bit place values.
Assumption of a 16-word cache with a block size of four words, resulting in four cache lines.
Description of the round-robin mapping technique used in direct memory mapping.
Explanation of how the least significant bits of the block number dictate cache line mapping.
PA bits split into block or line offset, block number, and tag bits for direct mapping.
Tag bits' role in identifying which block is present in the cache, serving as a tag for each block.
Conclusion summarizing the direct mapping technique and its strict mapping procedure.
Announcement of future sessions focusing on solving numerical problems related to direct memory mapping.
Transcripts
[Music]
hello
everyone welcome back in our previous
session we learned about the cashier
memory
from this session onwards we will focus
on different cache memory mapping
techniques starting with the direct
memory mapping
so gear up and let's get to learning so
as you can see in here this is the
conceptual block diagram of the
secondary memory
and this is a conceptual block diagram
of main memory
now we already know programs in the
computer permanently reside in the
secondary storage
during execution the same programs turn
into processes
just like our friend mr clark joseph
can't hear
now every process is subdivided into
equal sized pages
likewise the main memory is also split
into equi-sized frames
and the size of each frame is as same as
the size of each page
the process of subdividing the processes
into the pages and then bringing them
into the main memory
is the job of the operating system and
for the details of that
you can refer to our beautifully
presented operating systems course
but for today we are here to learn about
how the elements are brought from the
main memory into the cache memory
the organization of the cache and the
main memory is almost similar to this
organization
in this the parts of the main memory are
termed as blocks
and the parts of the cache are named as
lines
also the line size is as same as the
block size
now remember these all are concepts we
won't just go on drawing the lines and
the blocks
on the caches and the rams these
illustrations are just for the sake of
understanding
now a smallest addressable memory unit
is called a word
and a byte addressable memory means the
size of each word is one byte
let's assume we have a main memory with
64 word size and the size of each block
is given as
four words hence the number of blocks in
the main memory
is 64 by 4 that is 16 so that blocks are
numbered as
0 1 2 3 up to 15.
now coming to the 64 main memory words
that is starting from 0 up until 63
they are organized in the main memory
somewhat like this
because each and every memory block is
supposed to have only four words
now we know using one bit place we can
address two locations
that is zero to the memory location zero
and one to the memory cell one
similarly with two bit places four
locations can be addressed
zero zero for m zero zero 1 for m 1
1 0 for m 2 and 1 1 for m 3
so for 8 memory cells we will be needing
log 8 base 2 that is log 2 cube base 2
that is 3 bit
places similarly in order to address
0 to 63 that is 64 words
we will be needing log 64 base 2
which actually results in 6 bits
now these 6 bits are called pa bits or
physical address bits
and the reason behind that is the main
memory is sometimes referred to as
physical address space
and in this particular physical address
space there are 0 to 15 that is 16
blocks
and in order to locate each one of them
we will be needing
log 16 base 2 that is logs 2 to the
power 4
base 2 which is 4 bits so the pa bits
are split like
the most significant 4 bits are used for
identifying the blocks
and the least significant two bits are
used for addressing
each word in each block so the zero zero
will be addressing to the zeroth word
zero one for the first word one zero for
the second word and similarly one one
for the third word
now let me show you how meaningful the
pa split is
suppose the processor generates the
physical address zero followed by five
ones
now following our ps plate the most
significant four bits that is zero
triple one which is nothing but seven
will be referring to the
block number seven and the least
significant two bits one one
will refer to the last word of that
particular block
let's analyze the generated physical
address
if we consider all the bit places
magnitudes and
add up all the values which has ones
underneath of them
we get the value as 31 and that is the
exact value of the word
which was being pointed out by the
physical address
isn't it beautiful now let's assume we
have a cache of 16 words
and the block size was already given to
us as four words
now we already know both the block and
the line are equal in sizes
in that case line size is also going to
be 4 words
therefore number of lines in the cache
is 16 by 4
that is 4 which is 0 1
2 and 3. and in order to identify four
different lines
we will be needing log 4 base 2 that is
log 2 square base 2 which is 2 bits
well it's pretty obvious that all the
main memory blocks can't really be
assigned to all the cache aligns at once
therefore we have to perform something
called mapping now the mapping takes
place in round robin manner
so these are the blocks of the main
memory and these are the cache lines
the zeroth block will be mapped onto the
zeroth line the first block will be
mapped onto the first line
the second block is going to be mapped
onto the second line and the third block
will be mapped onto the third line at
this point we might think that for the
fourth block and the rest there are no
available cache lines
but there the round robin manner comes
at rescue
so the fourth block will be mapped onto
the zeroth line and the fifth one is
going to be mapped onto the first line
and this keeps on following
now this is the complete mapping if we
observe closely
the least significant two bits of the
block number is actually dictating which
cache a line to map onto
like the block number zero four eight
twelve they are going to get mapped on
to the line number zero
the block numbers one five nine and
thirteen
mapped on to the line number one for
blocks
two six ten and fourteen they are going
to get mapped onto the line number
two and finally block number 3 7
11 and 15 these are going to get mapped
onto the
line number three so this is a many to
one relation
so finally the pa bits are split like
this
the least significant two bits are
called block or line offset
they determine each word inside either
block or line
now the rest are called block numbers
now from the block numbers the last two
bits
are known as line number because they
actually dictate
which cache a line that particular block
will be mapping onto
so the remaining bits are known as tag
bits
now let's try and understand why these
are called tag bits
now let's select block number 3 and
analyze its contents
12 13 14 15 and these are their 6-bit
binary equivalents
also we witnessed that like block number
three block number seven
eleven and fifteen are also mapped onto
the same cache line that is
client number three so let's observe
their contents as well
block number seven's got 28 29 30 and 31
11 has 44 45 46 and 47
and for 15 there is 60 61 62 and 63.
now observe this for block number three
the tag bits are
0 0 for block number 7 the tag bits are
0
1 for 11 the tag bits are 1 0
and 15 it's 1 1. so do you understand
the pattern
so actually these bits will identify
which one of the blocks
is present in the caching basically they
work as the tags
and thus the naming so to conclude
this memory mapping technique is called
direct mapping as the main memory blocks
are mapped directly onto the cache
alliance
and the mapping procedure is strict
so that was all for this session i hope
now you have a lucid understanding about
the entire concept of direct memory
mapping technique
from the next session onwards we will
solve interesting numerical problems
related to this concept hope to see you
in the next one
thank you all for watching
[Music]
you
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