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Does Linux not use segmentation but only paging?
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The Linux Programming Interface shows the layout of a virtual address space of a process. Is each region in the diagram a segment?
From Understanding The Linux Kernel,
is it correct that the following means that the segmentation unit in MMU maps the segments and offsets within segments into the virtual memory address, and the paging unit then maps the virtual memory address to the physical memory address?
The Memory Management Unit (MMU) transforms a logical address into a linear
address by means of a hardware circuit called a segmentation unit; subsequently, a second hardware circuit called a paging unit transforms the linear address into a physical address (see Figure 2-1).
Then why does it say that Linux doesn't use segmentation but only paging?
Segmentation has been included in 80x86 microprocessors to encourage
program- mers to split their applications into logically related
entities, such as subroutines or global and local data areas. However,
Linux uses segmentation in a very limited way. In fact, segmentation and paging are somewhat redundant, because both can be
used to separate the physical address spaces of processes:
segmentation can assign a different linear address space to each
process, while paging can map the same linear address space into
different physical address spaces. Linux prefers paging to
segmentation for the following reasons:
• Memory management is simpler when all processes use the same segment
regis- ter values—that is, when they share the same set of linear
addresses.
• One of the design objectives of Linux is portability to a wide range
of architectures; RISC architectures in particular have limited
support for segmentation.
The 2.6 version of Linux uses segmentation only when required by the
80x86 architecture.
linux kernel virtual-memory
edited 12 mins ago
Rui F Ribeiro
36.5k1271116
asked 32 mins ago
Tim
23k66225408
add a comment |Â
up vote
3
down vote
favorite
The Linux Programming Interface shows the layout of a virtual address space of a process. Is each region in the diagram a segment?
From Understanding The Linux Kernel,
is it correct that the following means that the segmentation unit in MMU maps the segments and offsets within segments into the virtual memory address, and the paging unit then maps the virtual memory address to the physical memory address?
The Memory Management Unit (MMU) transforms a logical address into a linear
address by means of a hardware circuit called a segmentation unit; subsequently, a second hardware circuit called a paging unit transforms the linear address into a physical address (see Figure 2-1).
Then why does it say that Linux doesn't use segmentation but only paging?
Segmentation has been included in 80x86 microprocessors to encourage
program- mers to split their applications into logically related
entities, such as subroutines or global and local data areas. However,
Linux uses segmentation in a very limited way. In fact, segmentation and paging are somewhat redundant, because both can be
used to separate the physical address spaces of processes:
segmentation can assign a different linear address space to each
process, while paging can map the same linear address space into
different physical address spaces. Linux prefers paging to
segmentation for the following reasons:
• Memory management is simpler when all processes use the same segment
regis- ter values—that is, when they share the same set of linear
addresses.
• One of the design objectives of Linux is portability to a wide range
of architectures; RISC architectures in particular have limited
support for segmentation.
The 2.6 version of Linux uses segmentation only when required by the
80x86 architecture.
linux kernel virtual-memory
edited 12 mins ago
Rui F Ribeiro
36.5k1271116
asked 32 mins ago
Tim
23k66225408
Can you specify the editions please. It might also be helpful to specify author names. I know at least the first is from a prominent figure. However, both of the title names are a bit generic, it was not clear to me at first that you were talking about books :-).
– sourcejedi
23 mins ago
add a comment |Â
up vote
3
down vote
favorite
up vote
3
down vote
favorite
The Linux Programming Interface shows the layout of a virtual address space of a process. Is each region in the diagram a segment?
From Understanding The Linux Kernel,
is it correct that the following means that the segmentation unit in MMU maps the segments and offsets within segments into the virtual memory address, and the paging unit then maps the virtual memory address to the physical memory address?
The Memory Management Unit (MMU) transforms a logical address into a linear
address by means of a hardware circuit called a segmentation unit; subsequently, a second hardware circuit called a paging unit transforms the linear address into a physical address (see Figure 2-1).
Then why does it say that Linux doesn't use segmentation but only paging?
Segmentation has been included in 80x86 microprocessors to encourage
program- mers to split their applications into logically related
entities, such as subroutines or global and local data areas. However,
Linux uses segmentation in a very limited way. In fact, segmentation and paging are somewhat redundant, because both can be
used to separate the physical address spaces of processes:
segmentation can assign a different linear address space to each
process, while paging can map the same linear address space into
different physical address spaces. Linux prefers paging to
segmentation for the following reasons:
• Memory management is simpler when all processes use the same segment
regis- ter values—that is, when they share the same set of linear
addresses.
• One of the design objectives of Linux is portability to a wide range
of architectures; RISC architectures in particular have limited
support for segmentation.
The 2.6 version of Linux uses segmentation only when required by the
80x86 architecture.
linux kernel virtual-memory
edited 12 mins ago
Rui F Ribeiro
36.5k1271116
asked 32 mins ago
Tim
23k66225408
The Linux Programming Interface shows the layout of a virtual address space of a process. Is each region in the diagram a segment?
From Understanding The Linux Kernel,
is it correct that the following means that the segmentation unit in MMU maps the segments and offsets within segments into the virtual memory address, and the paging unit then maps the virtual memory address to the physical memory address?
The Memory Management Unit (MMU) transforms a logical address into a linear
address by means of a hardware circuit called a segmentation unit; subsequently, a second hardware circuit called a paging unit transforms the linear address into a physical address (see Figure 2-1).
Then why does it say that Linux doesn't use segmentation but only paging?
Segmentation has been included in 80x86 microprocessors to encourage
program- mers to split their applications into logically related
entities, such as subroutines or global and local data areas. However,
Linux uses segmentation in a very limited way. In fact, segmentation and paging are somewhat redundant, because both can be
used to separate the physical address spaces of processes:
segmentation can assign a different linear address space to each
process, while paging can map the same linear address space into
different physical address spaces. Linux prefers paging to
segmentation for the following reasons:
• Memory management is simpler when all processes use the same segment
regis- ter values—that is, when they share the same set of linear
addresses.
• One of the design objectives of Linux is portability to a wide range
of architectures; RISC architectures in particular have limited
support for segmentation.
The 2.6 version of Linux uses segmentation only when required by the
80x86 architecture.
linux kernel virtual-memory
linux kernel virtual-memory
edited 12 mins ago
Rui F Ribeiro
36.5k1271116
asked 32 mins ago
Tim
23k66225408
edited 12 mins ago
Rui F Ribeiro
36.5k1271116
asked 32 mins ago
Tim
23k66225408
edited 12 mins ago
Rui F Ribeiro
36.5k1271116
edited 12 mins ago
Rui F Ribeiro
36.5k1271116
edited 12 mins ago
Rui F Ribeiro
36.5k1271116
36.5k1271116
asked 32 mins ago
Tim
23k66225408
asked 32 mins ago
Tim
23k66225408
asked 32 mins ago
Tim
23k66225408
23k66225408
Can you specify the editions please. It might also be helpful to specify author names. I know at least the first is from a prominent figure. However, both of the title names are a bit generic, it was not clear to me at first that you were talking about books :-).
– sourcejedi
23 mins ago
add a comment |Â
Can you specify the editions please. It might also be helpful to specify author names. I know at least the first is from a prominent figure. However, both of the title names are a bit generic, it was not clear to me at first that you were talking about books :-).
– sourcejedi
23 mins ago
Can you specify the editions please. It might also be helpful to specify author names. I know at least the first is from a prominent figure. However, both of the title names are a bit generic, it was not clear to me at first that you were talking about books :-).
– sourcejedi
23 mins ago
Can you specify the editions please. It might also be helpful to specify author names. I know at least the first is from a prominent figure. However, both of the title names are a bit generic, it was not clear to me at first that you were talking about books :-).
– sourcejedi
23 mins ago
add a comment |Â
3 Answers
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up vote
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The x86-64 architecture does not use segmentation in long mode (64-bit mode) [...] https://en.wikipedia.org/wiki/X86_memory_segmentation#Later_developments
Do not worry about the details which would only apply to the legacy 32-bit modes. Linux for the 32-bit modes is not used as much and may be considered "in a state of benign neglect for several years". See 32-Bit x86 support in Fedora [LWN.net, 2017].
answered 15 mins ago
sourcejedi
19.8k42682
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up vote
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As the x86 has segments, it is not possible to not use them. But both cs (code segment) and ds(data segment) base addresses are set to zero, so the segmentation is not really used. An exception is thread local data, one of the normally unused segment registers points to thread local data. But that is mainly to avoid reserving one of the general purpose registers for this task.
It doesn't say that Linux doesn't use segmentation on the x86, as that would not be possible. You already highlighted one part, Linux uses segmentation in a very limited way. The second part is Linux uses segmentation only when required by the 80x86 architecture
You already quoted the reasons, paging is easier and more portable.
answered 14 mins ago
RalfFriedl
3,4601522
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up vote
4
down vote
In one short sentence: The linux kernel is able to deal with segments and paging to some extend.
Now, let's first clarify the difference between paging vs segmentation on a processor level.
A processor that segments the memory actually uses two pointers to tell exactly what byte is addressed.
First there is the segment which is an array of memory with a specific size. The first pointer just points to this segment and by this addresses the whole segment. To specify what byte is meant in that segment a second pointer is needed which is only local to the segment. So those two combined can address a singly byte. For example you could have a 16 bit pointer for the segments to address 65536 different segments and a 16 bit local pointer in them to address the bytes. The disadvantage of this is that the local pointer determines the maximum size of a continuous memory block. You might be able to use the whole memory, but each block you use can only be a fraction of the maximum memory you can address. Especially for programs that work on large data sets (move encoders, 3d renderer, databases...) this is a serious problem.
Paging is somewhat the combination of the advantages of a flat memory model (only one pointer addressing the whole memory space), and the ability to define blocks of memory that are somewhat isolated from others. In a paging system you have one pointer addressing the whole memory space. So a 32 bit system with a 32 bit pointer is able to address 4GB of memory with each address pointing exactly to one byte. But that memory area is still broken down to "pages". A Page is just a block of memory of a defined size. And this is where the "magic" starts. The processor has a second mapping that tell exactly how each of those pages are mapped into the "flat" memory space. A program using the memory sees it as completely flat. It can address every byte exactly with only one pointer. But the kernel is able to shift memory blocks, define special attributes for them..and such things. For example the kernel is able to define such a memory block "read-only" and terminate every program that tries to write to it. Also swapping is based on this: Let's assume a program tries to access a memory address outside of the range of the physical available memory. The kernel can catch this, choose one memory block that wasn't used for some time an write it to the swap space, then take that memory page and map it to the place the application tired to write to and magically there is more memory for applications available than the machine has.
This is all very low-level and what is meant when the linux kernel prefers paging over segmentation. But just to mention it, the linux kernel is able to deal with both to some extend, and sometimes even combine the approaches (32 bit linux PAE kernel for example).
Now let's talk about segments in programs: While this is still pretty low level it is far higher than the segments on processor level, and again all a bit simplified just to give the idea. When you have an executable that is basically just a memory dump that needs to be loaded back in memory then executed. But not everything in the executable is code. Executables can contain data as well like the text on buttons and such things. So to make this easier, programs are divided into segments, for code, for data. When an executable is loaded those parts of it end up in different memory parts with different abilities. To make programs more secure it is for example good practise to only allow code in the code segment to be executed. That prevents any attacks that try to manipulate program data to valid binary code and then having it executed somehow. The kernel will notice that something in the data segment should be executed and will prevent that.
More details can be found here
answered 14 mins ago
Goro
2,06441846
add a comment |Â
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
4
down vote
The x86-64 architecture does not use segmentation in long mode (64-bit mode) [...] https://en.wikipedia.org/wiki/X86_memory_segmentation#Later_developments
Do not worry about the details which would only apply to the legacy 32-bit modes. Linux for the 32-bit modes is not used as much and may be considered "in a state of benign neglect for several years". See 32-Bit x86 support in Fedora [LWN.net, 2017].
answered 15 mins ago
sourcejedi
19.8k42682
add a comment |Â
up vote
4
down vote
The x86-64 architecture does not use segmentation in long mode (64-bit mode) [...] https://en.wikipedia.org/wiki/X86_memory_segmentation#Later_developments
Do not worry about the details which would only apply to the legacy 32-bit modes. Linux for the 32-bit modes is not used as much and may be considered "in a state of benign neglect for several years". See 32-Bit x86 support in Fedora [LWN.net, 2017].
answered 15 mins ago
sourcejedi
19.8k42682
add a comment |Â
up vote
4
down vote
up vote
4
down vote
The x86-64 architecture does not use segmentation in long mode (64-bit mode) [...] https://en.wikipedia.org/wiki/X86_memory_segmentation#Later_developments
Do not worry about the details which would only apply to the legacy 32-bit modes. Linux for the 32-bit modes is not used as much and may be considered "in a state of benign neglect for several years". See 32-Bit x86 support in Fedora [LWN.net, 2017].
answered 15 mins ago
sourcejedi
19.8k42682
The x86-64 architecture does not use segmentation in long mode (64-bit mode) [...] https://en.wikipedia.org/wiki/X86_memory_segmentation#Later_developments
Do not worry about the details which would only apply to the legacy 32-bit modes. Linux for the 32-bit modes is not used as much and may be considered "in a state of benign neglect for several years". See 32-Bit x86 support in Fedora [LWN.net, 2017].
answered 15 mins ago
sourcejedi
19.8k42682
answered 15 mins ago
sourcejedi
19.8k42682
answered 15 mins ago
sourcejedi
19.8k42682
answered 15 mins ago
sourcejedi
19.8k42682
19.8k42682
add a comment |Â
add a comment |Â
up vote
4
down vote
As the x86 has segments, it is not possible to not use them. But both cs (code segment) and ds(data segment) base addresses are set to zero, so the segmentation is not really used. An exception is thread local data, one of the normally unused segment registers points to thread local data. But that is mainly to avoid reserving one of the general purpose registers for this task.
It doesn't say that Linux doesn't use segmentation on the x86, as that would not be possible. You already highlighted one part, Linux uses segmentation in a very limited way. The second part is Linux uses segmentation only when required by the 80x86 architecture
You already quoted the reasons, paging is easier and more portable.
answered 14 mins ago
RalfFriedl
3,4601522
add a comment |Â
up vote
4
down vote
As the x86 has segments, it is not possible to not use them. But both cs (code segment) and ds(data segment) base addresses are set to zero, so the segmentation is not really used. An exception is thread local data, one of the normally unused segment registers points to thread local data. But that is mainly to avoid reserving one of the general purpose registers for this task.
It doesn't say that Linux doesn't use segmentation on the x86, as that would not be possible. You already highlighted one part, Linux uses segmentation in a very limited way. The second part is Linux uses segmentation only when required by the 80x86 architecture
You already quoted the reasons, paging is easier and more portable.
answered 14 mins ago
RalfFriedl
3,4601522
add a comment |Â
up vote
4
down vote
up vote
4
down vote
As the x86 has segments, it is not possible to not use them. But both cs (code segment) and ds(data segment) base addresses are set to zero, so the segmentation is not really used. An exception is thread local data, one of the normally unused segment registers points to thread local data. But that is mainly to avoid reserving one of the general purpose registers for this task.
It doesn't say that Linux doesn't use segmentation on the x86, as that would not be possible. You already highlighted one part, Linux uses segmentation in a very limited way. The second part is Linux uses segmentation only when required by the 80x86 architecture
You already quoted the reasons, paging is easier and more portable.
answered 14 mins ago
RalfFriedl
3,4601522
As the x86 has segments, it is not possible to not use them. But both cs (code segment) and ds(data segment) base addresses are set to zero, so the segmentation is not really used. An exception is thread local data, one of the normally unused segment registers points to thread local data. But that is mainly to avoid reserving one of the general purpose registers for this task.
It doesn't say that Linux doesn't use segmentation on the x86, as that would not be possible. You already highlighted one part, Linux uses segmentation in a very limited way. The second part is Linux uses segmentation only when required by the 80x86 architecture
You already quoted the reasons, paging is easier and more portable.
answered 14 mins ago
RalfFriedl
3,4601522
answered 14 mins ago
RalfFriedl
3,4601522
answered 14 mins ago
RalfFriedl
3,4601522
answered 14 mins ago
RalfFriedl
3,4601522
3,4601522
add a comment |Â
add a comment |Â
up vote
4
down vote
In one short sentence: The linux kernel is able to deal with segments and paging to some extend.
Now, let's first clarify the difference between paging vs segmentation on a processor level.
A processor that segments the memory actually uses two pointers to tell exactly what byte is addressed.
First there is the segment which is an array of memory with a specific size. The first pointer just points to this segment and by this addresses the whole segment. To specify what byte is meant in that segment a second pointer is needed which is only local to the segment. So those two combined can address a singly byte. For example you could have a 16 bit pointer for the segments to address 65536 different segments and a 16 bit local pointer in them to address the bytes. The disadvantage of this is that the local pointer determines the maximum size of a continuous memory block. You might be able to use the whole memory, but each block you use can only be a fraction of the maximum memory you can address. Especially for programs that work on large data sets (move encoders, 3d renderer, databases...) this is a serious problem.
Paging is somewhat the combination of the advantages of a flat memory model (only one pointer addressing the whole memory space), and the ability to define blocks of memory that are somewhat isolated from others. In a paging system you have one pointer addressing the whole memory space. So a 32 bit system with a 32 bit pointer is able to address 4GB of memory with each address pointing exactly to one byte. But that memory area is still broken down to "pages". A Page is just a block of memory of a defined size. And this is where the "magic" starts. The processor has a second mapping that tell exactly how each of those pages are mapped into the "flat" memory space. A program using the memory sees it as completely flat. It can address every byte exactly with only one pointer. But the kernel is able to shift memory blocks, define special attributes for them..and such things. For example the kernel is able to define such a memory block "read-only" and terminate every program that tries to write to it. Also swapping is based on this: Let's assume a program tries to access a memory address outside of the range of the physical available memory. The kernel can catch this, choose one memory block that wasn't used for some time an write it to the swap space, then take that memory page and map it to the place the application tired to write to and magically there is more memory for applications available than the machine has.
This is all very low-level and what is meant when the linux kernel prefers paging over segmentation. But just to mention it, the linux kernel is able to deal with both to some extend, and sometimes even combine the approaches (32 bit linux PAE kernel for example).
Now let's talk about segments in programs: While this is still pretty low level it is far higher than the segments on processor level, and again all a bit simplified just to give the idea. When you have an executable that is basically just a memory dump that needs to be loaded back in memory then executed. But not everything in the executable is code. Executables can contain data as well like the text on buttons and such things. So to make this easier, programs are divided into segments, for code, for data. When an executable is loaded those parts of it end up in different memory parts with different abilities. To make programs more secure it is for example good practise to only allow code in the code segment to be executed. That prevents any attacks that try to manipulate program data to valid binary code and then having it executed somehow. The kernel will notice that something in the data segment should be executed and will prevent that.
More details can be found here
answered 14 mins ago
Goro
2,06441846
add a comment |Â
up vote
4
down vote
In one short sentence: The linux kernel is able to deal with segments and paging to some extend.
Now, let's first clarify the difference between paging vs segmentation on a processor level.
A processor that segments the memory actually uses two pointers to tell exactly what byte is addressed.
First there is the segment which is an array of memory with a specific size. The first pointer just points to this segment and by this addresses the whole segment. To specify what byte is meant in that segment a second pointer is needed which is only local to the segment. So those two combined can address a singly byte. For example you could have a 16 bit pointer for the segments to address 65536 different segments and a 16 bit local pointer in them to address the bytes. The disadvantage of this is that the local pointer determines the maximum size of a continuous memory block. You might be able to use the whole memory, but each block you use can only be a fraction of the maximum memory you can address. Especially for programs that work on large data sets (move encoders, 3d renderer, databases...) this is a serious problem.
Paging is somewhat the combination of the advantages of a flat memory model (only one pointer addressing the whole memory space), and the ability to define blocks of memory that are somewhat isolated from others. In a paging system you have one pointer addressing the whole memory space. So a 32 bit system with a 32 bit pointer is able to address 4GB of memory with each address pointing exactly to one byte. But that memory area is still broken down to "pages". A Page is just a block of memory of a defined size. And this is where the "magic" starts. The processor has a second mapping that tell exactly how each of those pages are mapped into the "flat" memory space. A program using the memory sees it as completely flat. It can address every byte exactly with only one pointer. But the kernel is able to shift memory blocks, define special attributes for them..and such things. For example the kernel is able to define such a memory block "read-only" and terminate every program that tries to write to it. Also swapping is based on this: Let's assume a program tries to access a memory address outside of the range of the physical available memory. The kernel can catch this, choose one memory block that wasn't used for some time an write it to the swap space, then take that memory page and map it to the place the application tired to write to and magically there is more memory for applications available than the machine has.
This is all very low-level and what is meant when the linux kernel prefers paging over segmentation. But just to mention it, the linux kernel is able to deal with both to some extend, and sometimes even combine the approaches (32 bit linux PAE kernel for example).
Now let's talk about segments in programs: While this is still pretty low level it is far higher than the segments on processor level, and again all a bit simplified just to give the idea. When you have an executable that is basically just a memory dump that needs to be loaded back in memory then executed. But not everything in the executable is code. Executables can contain data as well like the text on buttons and such things. So to make this easier, programs are divided into segments, for code, for data. When an executable is loaded those parts of it end up in different memory parts with different abilities. To make programs more secure it is for example good practise to only allow code in the code segment to be executed. That prevents any attacks that try to manipulate program data to valid binary code and then having it executed somehow. The kernel will notice that something in the data segment should be executed and will prevent that.
More details can be found here
answered 14 mins ago
Goro
2,06441846
add a comment |Â
up vote
4
down vote
up vote
4
down vote
In one short sentence: The linux kernel is able to deal with segments and paging to some extend.
Now, let's first clarify the difference between paging vs segmentation on a processor level.
A processor that segments the memory actually uses two pointers to tell exactly what byte is addressed.
First there is the segment which is an array of memory with a specific size. The first pointer just points to this segment and by this addresses the whole segment. To specify what byte is meant in that segment a second pointer is needed which is only local to the segment. So those two combined can address a singly byte. For example you could have a 16 bit pointer for the segments to address 65536 different segments and a 16 bit local pointer in them to address the bytes. The disadvantage of this is that the local pointer determines the maximum size of a continuous memory block. You might be able to use the whole memory, but each block you use can only be a fraction of the maximum memory you can address. Especially for programs that work on large data sets (move encoders, 3d renderer, databases...) this is a serious problem.
Paging is somewhat the combination of the advantages of a flat memory model (only one pointer addressing the whole memory space), and the ability to define blocks of memory that are somewhat isolated from others. In a paging system you have one pointer addressing the whole memory space. So a 32 bit system with a 32 bit pointer is able to address 4GB of memory with each address pointing exactly to one byte. But that memory area is still broken down to "pages". A Page is just a block of memory of a defined size. And this is where the "magic" starts. The processor has a second mapping that tell exactly how each of those pages are mapped into the "flat" memory space. A program using the memory sees it as completely flat. It can address every byte exactly with only one pointer. But the kernel is able to shift memory blocks, define special attributes for them..and such things. For example the kernel is able to define such a memory block "read-only" and terminate every program that tries to write to it. Also swapping is based on this: Let's assume a program tries to access a memory address outside of the range of the physical available memory. The kernel can catch this, choose one memory block that wasn't used for some time an write it to the swap space, then take that memory page and map it to the place the application tired to write to and magically there is more memory for applications available than the machine has.
This is all very low-level and what is meant when the linux kernel prefers paging over segmentation. But just to mention it, the linux kernel is able to deal with both to some extend, and sometimes even combine the approaches (32 bit linux PAE kernel for example).
Now let's talk about segments in programs: While this is still pretty low level it is far higher than the segments on processor level, and again all a bit simplified just to give the idea. When you have an executable that is basically just a memory dump that needs to be loaded back in memory then executed. But not everything in the executable is code. Executables can contain data as well like the text on buttons and such things. So to make this easier, programs are divided into segments, for code, for data. When an executable is loaded those parts of it end up in different memory parts with different abilities. To make programs more secure it is for example good practise to only allow code in the code segment to be executed. That prevents any attacks that try to manipulate program data to valid binary code and then having it executed somehow. The kernel will notice that something in the data segment should be executed and will prevent that.
More details can be found here
answered 14 mins ago
Goro
2,06441846
In one short sentence: The linux kernel is able to deal with segments and paging to some extend.
Now, let's first clarify the difference between paging vs segmentation on a processor level.
A processor that segments the memory actually uses two pointers to tell exactly what byte is addressed.
First there is the segment which is an array of memory with a specific size. The first pointer just points to this segment and by this addresses the whole segment. To specify what byte is meant in that segment a second pointer is needed which is only local to the segment. So those two combined can address a singly byte. For example you could have a 16 bit pointer for the segments to address 65536 different segments and a 16 bit local pointer in them to address the bytes. The disadvantage of this is that the local pointer determines the maximum size of a continuous memory block. You might be able to use the whole memory, but each block you use can only be a fraction of the maximum memory you can address. Especially for programs that work on large data sets (move encoders, 3d renderer, databases...) this is a serious problem.
Paging is somewhat the combination of the advantages of a flat memory model (only one pointer addressing the whole memory space), and the ability to define blocks of memory that are somewhat isolated from others. In a paging system you have one pointer addressing the whole memory space. So a 32 bit system with a 32 bit pointer is able to address 4GB of memory with each address pointing exactly to one byte. But that memory area is still broken down to "pages". A Page is just a block of memory of a defined size. And this is where the "magic" starts. The processor has a second mapping that tell exactly how each of those pages are mapped into the "flat" memory space. A program using the memory sees it as completely flat. It can address every byte exactly with only one pointer. But the kernel is able to shift memory blocks, define special attributes for them..and such things. For example the kernel is able to define such a memory block "read-only" and terminate every program that tries to write to it. Also swapping is based on this: Let's assume a program tries to access a memory address outside of the range of the physical available memory. The kernel can catch this, choose one memory block that wasn't used for some time an write it to the swap space, then take that memory page and map it to the place the application tired to write to and magically there is more memory for applications available than the machine has.
This is all very low-level and what is meant when the linux kernel prefers paging over segmentation. But just to mention it, the linux kernel is able to deal with both to some extend, and sometimes even combine the approaches (32 bit linux PAE kernel for example).
Now let's talk about segments in programs: While this is still pretty low level it is far higher than the segments on processor level, and again all a bit simplified just to give the idea. When you have an executable that is basically just a memory dump that needs to be loaded back in memory then executed. But not everything in the executable is code. Executables can contain data as well like the text on buttons and such things. So to make this easier, programs are divided into segments, for code, for data. When an executable is loaded those parts of it end up in different memory parts with different abilities. To make programs more secure it is for example good practise to only allow code in the code segment to be executed. That prevents any attacks that try to manipulate program data to valid binary code and then having it executed somehow. The kernel will notice that something in the data segment should be executed and will prevent that.
More details can be found here
answered 14 mins ago
Goro
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edited 7 mins ago
answered 14 mins ago
Goro
2,06441846
answered 14 mins ago
Goro
2,06441846
answered 14 mins ago
Goro
2,06441846
2,06441846
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Can you specify the editions please. It might also be helpful to specify author names. I know at least the first is from a prominent figure. However, both of the title names are a bit generic, it was not clear to me at first that you were talking about books :-).
– sourcejedi
23 mins ago