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{{short description|Memory segmentation on Intel x86}}
{{Use dmy dates|date=May 2019|cs1-dates=y}}
'''x86 memory segmentation''' is a term for the kind of [[memory segmentation]] characteristic of the Intel [[x86]] computer [[instruction set architecture]]. The x86 architecture has supported memory segmentation since the original [[Intel 8086]] (1978), but ''x86 memory segmentation'' is a plainly descriptive [[retronym]]. The introduction of memory segmentation mechanisms in this architecture reflects the legacy of earlier 80xx processors, which initially<ref>in the [[Intel 8008]]</ref> could only address 16, or later<ref>from the [[Intel 8080]]</ref> 64 KB of memory (16,384 or 65,536 [[byte]]s), and whose instructions and registers were optimised for the latter. Dealing with larger addresses and more memory was thus comparably slower, as that capability was somewhat grafted-on in the Intel 8086. Memory segmentation could keep programs compatible, relocatable in memory, and by confining significant parts of a program's operation to 64 KB segments, the program could still run faster.
In 1982, the [[Intel 80286]] added support for [[virtual memory]] and [[memory protection]]; the original mode was renamed '''[[real mode]]''', and the new version was named '''[[protected mode]]'''. The [[x86-64]] architecture, introduced in 2003, has largely dropped support for segmentation in 64-bit mode.
In both real and protected modes, the system uses 16-bit ''segment registers'' to derive the actual memory address. {{anchor|Extra segment}}In real mode, the registers CS, DS, SS, and ES point to the currently used program [[code segment]] (CS), the current [[data segment]] (DS), the current [[stack segment]] (SS), and one ''extra'' segment determined by the system programmer (ES). The [[Intel 80386]], introduced in 1985, adds two additional segment registers, FS and GS, with no specific uses defined by the hardware. The way in which the segment registers are used differs between the two modes.<ref name=Arch />
The choice of segment is normally defaulted by the processor according to the function being executed. Instructions are always fetched from the code segment. Any data reference to the stack, including any stack push or pop,
== Real mode ==
[[Image:Overlapping realmode segments.svg|thumb|right|300px|Three segments in [[real mode]] memory (click on image to enlarge). There is an overlap between segment 2 and segment 3; the bytes in the turquoise area can be used from both segment selectors. If however the program code dealing with segment 2 never uses offsets large enough to reach 0x77D0, then it can be thought of as a shorter, non-overlapping, and at most 30,672-byte segment.]]
In [[real mode]] or [[Virtual 8086 mode|V86 mode]], the
The 16-bit segment selector in the segment register is interpreted as the most significant 16 bits of a linear 20-bit address, called a segment address, of which the remaining four least significant bits are all zeros. The segment address is always added to a 16-bit offset in the instruction to yield a ''linear'' address, which is the same as [[physical address]] in this mode. For instance, the segmented address 06EFh:1234h (here the suffix "h" means [[hexadecimal]]) has a segment selector of 06EFh, representing a segment address of 06EF0h, to which the offset is added, yielding the linear address 06EF0h + 1234h = 08124h.
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{|
|-
! style=width:18em | <code>
| '''Segment'''
| 16 bits, shifted 4 bits left (or multiplied by 0x10)
|-
! style=width:18em | <code>+
| '''Offset'''
| 16 bits
|- style="text-decoration:line-through"
! style=width:18em | <code>
|
|-
! style=width:18em | <code>
| '''Address'''
| 20 bits
|}
Because of the way the segment address and offset are added, a single linear address can be mapped to up to 2<sup>12</sup> = 4096 distinct segment:offset pairs. For example, the linear address 08124h can have the segmented addresses 06EFh:1234h, 0812h:0004h, 0000h:8124h, etc. This could be confusing to programmers accustomed to unique addressing schemes, but it can also be used to advantage, for example when addressing multiple nested data structures.
The effective 20-bit [[address space]] of
{{anchor|Paragraph}}
=== End-of-address-space quirkiness ===
In 16-bit real mode, enabling applications to make use of multiple memory segments (in order to access more memory than available in any one 64K-segment) is quite complex, but was viewed as a necessary evil for all but the smallest tools (which could do with less memory). The root of the problem is that no appropriate address-arithmetic instructions suitable for flat addressing of the entire memory range are available.{{Citation needed|date=July 2011}} Flat addressing is possible by applying multiple instructions, which however leads to slower programs.▼
{{main article|A20 line}}
The last segment, FFFFh (65535), begins at linear address FFFF0h (1048560), 16 bytes before the end of the 20-bit address space, and thus can access, with an offset of up to 65,536 bytes, up to 65,520 (65536−16) bytes past the end of the 20-bit address space of the 8086 or 8088 CPU. A further 4,094 next-highest 64K-segments also still cross that 1MB-threshold, but by less and less. On the 8086 and 8088 CPUs, these address accesses were wrapped around to the beginning of the address space such that 65535:16 would access address 0, and e.g. 65533:1000 would access address 952 of the linear address space. The fact that some programs written for the 8088 and 8086 relied on this quirky wrap-around as a feature led to the [[Gate A20]] compatibility issues in later CPU generations, with the [[Intel 286]] and above, where the linear address space was expanded past 20 bits.
▲In 16-bit real mode, enabling applications to make use of multiple memory segments for a single data structure (in order to access more memory than available in any one 64K-segment) is quite complex, but was viewed as a necessary evil for all but the smallest tools (which could do with less memory). The root of the problem is that no appropriate address-arithmetic instructions suitable for flat addressing of the entire memory range are available.{{Citation needed|date=July 2011}} Flat addressing is possible by applying multiple instructions, which however leads to slower programs.
The ''[[x86 memory models|memory model]]'' concept derives from the setup of the segment registers. For example, in the ''tiny model'' CS=DS=SS, that is the program's code, data, and stack are all contained within a single 64 KB segment. In the ''small'' memory model DS=SS, so both data and stack reside in the same segment; CS points to a different code segment of up to 64 KB.
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{{refimprove section|date=August 2015}}
[[Image:Protected mode segments.svg|thumb|300px|left|Three segments in [[protected mode]] memory (click on image to enlarge), with the '''local descriptor table'''
=== 80286 protected mode ===
The [[Intel 80286|80286]]'s [[protected mode]] extends the processor's address space to 2<sup>24</sup> bytes (16 megabytes), but not by adjusting the shift value used to calculate a segment address from the value in a segment register. Instead,
286 protected mode was seldom used as it would have excluded the large body of users with 8086/88 machines. Moreover, it still necessitated dividing memory into 64k segments like was done in real mode. This limitation can be worked around on 32-bit CPUs which permit the use of memory pointers greater than 64k in size, however as the Segment Limit field is only 24-bit long, the maximum segment size that can be created is 16MB (although paging can be used to allocate more memory, no individual segment may exceed 16MB). This method was commonly used on Windows 3.x applications to produce a flat memory space, although as the OS itself was still 16-bit, API calls could not be made with 32-bit instructions. Thus, it was still necessary to place all code that performs API calls in 64k segments.
Once 286 protected mode is invoked, it could not normally be exited except by performing a hardware reset. Machines following the rising [[IBM PC/AT]] standard could feign a reset to the CPU via the standardised keyboard controller, but this was significantly sluggish. Windows 3.x worked around both of these problems by intentionally triggering a [[triple fault]] in the interrupt-handling mechanisms of the CPU, which would cause the IBM AT-compatible hardware to reset the CPU, nearly instantly, thus causing it to drop back into real mode
=== Detailed segmentation unit workflow ===
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In the [[Intel 80386]] and later, protected mode retains the segmentation mechanism of 80286 protected mode, but a [[paging]] unit has been added as a second layer of address translation between the segmentation unit and the physical bus. Also, importantly, address offsets are 32-bit (instead of 16-bit), and the segment base in each segment descriptor is also 32-bit (instead of 24-bit). The general operation of the segmentation unit is otherwise unchanged. The paging unit may be enabled or disabled; if disabled, operation is the same as on the 80286. If the paging unit is enabled, addresses in a segment are now virtual addresses, rather than physical addresses as they were on the 80286. That is, the segment starting address, the offset, and the final 32-bit address the segmentation unit derived by adding the two are all virtual (or logical) addresses when the paging unit is enabled. When the segmentation unit generates and validates these 32-bit virtual addresses, the enabled paging unit finally translates these virtual addresses into physical addresses. The physical addresses are 32-bit on the [[Intel 80386|386]], but can be larger on newer processors which support [[Physical Address Extension]].
A 386 CPU can be put back into real mode by clearing a bit in the CR0 control register, however this is a privileged operation in order to enforce security and robustness. By way of comparison, a 286 could only be returned to real mode by forcing a processor reset, e.g. by a [[triple fault]] or using external hardware.
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== Practices ==
Logical addresses can be explicitly specified in [[x86 assembly language]], e.g. (AT&T syntax):
{{codett|movl $42, %fs:(%eax) ; Equivalent to M[fs:eax]<-42) in|asm}} [[Register Transfer Language|RTL]]
or in [[Intel syntax]]:
<syntaxhighlight lang="
mov dword [fs:eax], 42
</syntaxhighlight>
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* All CPU instructions are implicitly fetched from the ''[[code segment]]'' specified by the segment selector held in the CS register.
* Most memory references come from the ''[[data segment]]'' specified by the segment selector held in the DS register. These may also come from the extra segment specified by the segment selector held in the ES register, if a segment-override prefix precedes the instruction that makes the memory reference. Most, but not all, instructions that use DS by default will accept an ES override prefix.{{fact|date=April 2025}}
* Processor [[run-time stack|stack]] references, either implicitly (e.g. '''push''' and '''pop''' instructions) or explicitly ([[stack-based memory allocation|memory accesses using the (E)SP or (E)BP registers]]) use the ''stack segment'' specified by the segment selector held in the SS register. For explicit references, the segment can be overridden.
* [[x86 string instructions|String instructions]] (e.g. '''stos''', '''movs'''), along with data segment, also use the ''extra segment'' specified by the segment selector held in the ES register.
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