2012:
segment can only be located at an address that is a multiple of 16, starting from address 0. Page, which means a segment can only be located at an address that is a multiple of 256, starting from address 0. Inpage, which means a segment can be located at whichever of the preceding attributes apply plus must be located so that it does not cross a page boundary The alignment codes are: B - byte W - word G - paragraph xR - inpage P - page A - absolute the x in the inpage alignment code can be any other alignment code. a segment can have the inpage attribute, meaning it must reside within a 256 byte page and can have the word attribute, meaning it must reside on an even numbered byte.
139:
542:
a minimal amount of padding is required. The minimal amount of padding required is always less than the largest alignment in the structure. Computing the maximum amount of padding required is more complicated, but is always less than the sum of the alignment requirements for all members minus twice the sum of the alignment requirements for the least aligned half of the structure members.
530:) normally allocates individual data items on aligned boundaries, data structures often have members with different alignment requirements. To maintain proper alignment the translator normally inserts additional unnamed data members so that each member is properly aligned. In addition, the data structure as a whole may be padded with a final unnamed member. This allows each member of an
77:
36:
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access to pa=0x12345ABC. Here, the 20/12-bit split luckily matches the hexadecimal representation split at 5/3 digits. The hardware can implement this translation by simply combining the first 20 bits of the physical address (0x12345) and the last 12 bits of the virtual address (0xABC). This is also referred to as virtually indexed (ABC) physically tagged (12345).
1557:(4096 Bytes) page is not just an arbitrary 4 KiB chunk of data. Instead, it is usually a region of memory that's aligned on a 4 KiB boundary. This is because aligning a page on a page-sized boundary lets the hardware map a virtual address to a physical address by substituting the higher bits in the address, rather than doing complex arithmetic.
2011:
A segment can have one (and in the case of the inpage attribute, two) of five alignment attributes: Byte, which means a segment can be located at any address. Word, which means a segment can only be located at an address that is a multiple of two, starting from address 0H. Paragraph, which means a
541:
member is followed by a member with a larger alignment requirement or at the end of the structure. By changing the ordering of members in a structure, it is possible to change the amount of padding required to maintain alignment. For example, if members are sorted by descending alignment requirements
513:
When a single memory word is accessed the operation is atomic, i.e. the whole memory word is read or written at once and other devices must wait until the read or write operation completes before they can access it. This may not be true for unaligned accesses to multiple memory words, e.g. the first
557:
One use for such "packed" structures is to conserve memory. For example, a structure containing a single byte (such as a char) and a four-byte integer (such as uint32_t) would require three additional bytes of padding. A large array of such structures would use 37.5% less memory if they are packed,
545:
Although C and C++ do not allow the compiler to reorder structure members to save space, other languages might. It is also possible to tell most C and C++ compilers to "pack" the members of a structure to a certain level of alignment, e.g. "pack(2)" means align data members larger than a byte to a
1560:
Example: Assume that we have a TLB mapping of virtual address 0x2CFC7000 to physical address 0x12345000. (Note that both these addresses are aligned at 4 KiB boundaries.) Accessing data located at virtual address va=0x2CFC7ABC causes a TLB resolution of 0x2CFC7 to 0x12345 to issue a physical
504:
Some processor designs deliberately avoid introducing such complexity, and instead yield alternative behavior in the event of a misaligned memory access. For example, implementations of the ARM architecture prior to the ARMv6 ISA require mandatory aligned memory access for all multi-byte load and
492:
If the highest and lowest bytes in a datum are not within the same memory word, the computer must split the datum access into multiple memory accesses. This requires a lot of complex circuitry to generate the memory accesses and coordinate them. To handle the case where the memory words are in
505:
store instructions. Depending on which specific instruction was issued, the result of attempted misaligned access might be to round down the least significant bits of the offending address turning it into an aligned access (sometimes with additional caveats), or to throw an
514:
word might be read by one device, both words written by another device and then the second word read by the first device so that the value read is neither the original value nor the updated value. Although such failures are rare, they can be very difficult to identify.
1407:. If an array is partitioned for more than one thread to operate on, having the sub-array boundaries unaligned to cache lines could lead to performance degradation. Here is an example to allocate memory (double array of size 10) aligned to cache of 64 bytes.
701:
If the type "short" is stored in two bytes of memory then each member of the data structure depicted above would be 2-byte aligned. Data1 would be at offset 0, Data2 at offset 2, and Data3 at offset 4. The size of this structure would be 6 bytes.
705:
The type of each member of the structure usually has a default alignment, meaning that it will, unless otherwise requested by the programmer, be aligned on a pre-determined boundary. The following typical alignments are valid for compilers from
285:, which generally means that the data's memory address is a multiple of the data size. For instance, in a 32-bit architecture, the data may be aligned if the data is stored in four consecutive bytes and the first byte lies on a 4-byte boundary.
1130:
It is possible to change the alignment of structures to reduce the memory they require (or to conform to an existing format) by reordering structure members or changing the compiler's alignment (or “packing”) of structure members.
803:(ten bytes with C++Builder and DMC, eight bytes with Visual C++, twelve bytes with GCC) will be 8-byte aligned with C++Builder, 2-byte aligned with DMC, 8-byte aligned with Visual C++, and 4-byte aligned with GCC.
569:, it may also be used to format a data structure for transmission using a standard protocol. However, in this usage, care must also be taken to ensure that the values of the struct members are stored with the
509:
exception (if MMU hardware is present), or to silently yield other potentially unpredictable results. The ARMv6 and later architectures support unaligned access in many circumstances, but not necessarily all.
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padding = (align - (offset & (align - 1))) & (align - 1) = -offset & (align - 1) aligned = (offset + (align - 1)) & ~(align - 1) = (offset + (align - 1)) & -align
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between structure elements or after the last element of a structure. For example, on a 32-bit machine, a data structure containing a 16-bit value followed by a 32-bit value could have 16 bits of
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structure to be aligned to a one byte boundary will cause the pre-processor to discard the pre-determined alignment of the structure members and thus no padding bytes would be inserted.
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For example, the padding to add to offset 0x59d for a 4-byte aligned structure is 3. The structure will then start at 0x5a0, which is a multiple of 4. However, when the alignment of
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Although data structure alignment is a fundamental issue for all modern computers, many computer languages and computer language implementations handle data alignment automatically.
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The last member is padded with the number of bytes required so that the total size of the structure should be a multiple of the largest alignment of any structure member (
1391:, if the project packing is smaller than this. For this reason, setting the project packing to any value other than the default of 8 bytes would break the
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On some
Microsoft compilers, particularly for RISC processors, there is an unexpected relationship between project default packing (the /Zp directive) and the
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members are stored sequentially in memory so that, in the structure below, the member Data1 will always precede Data2; and Data2 will always precede Data3:
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Note that the definitions above assume that each primitive datum is a power of two bytes long. When this is not the case (as with 80-bit floating-point on
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This is how a dynamic allocator that has no knowledge of alignment, can be used to provide aligned buffers, at the price of a factor two in space loss.
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the packing size of a structure from the project default packing. This leads to interoperability problems with library headers which use, for example,
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On modern computers where the target alignment is a power of two. This might not be true, for example, on a system using 9-bit bytes or 60-bit words.
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directives used in library headers and result in binary incompatibilities between structures. This limitation is not present when compiling for x86.
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Alignment concerns can affect areas much larger than a C structure when the purpose is the efficient mapping of that area through a hardware
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different memory pages the processor must either verify that both pages are present before executing the instruction or be able to handle a
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is a power of 2). In this context, a byte is the smallest unit of memory access, i.e. each memory address specifies a different byte. An
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padding = (align - (offset mod align)) mod align aligned = offset + padding = offset + ((align - (offset mod align)) mod align)
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supported by the computer, aligned accesses will always access a single memory word. This may not be true for misaligned data accesses.
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After compilation the data structure will be supplemented with padding bytes to ensure a proper alignment for each of its members:
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is the aligning of elements according to their natural alignment. To ensure natural alignment, it may be necessary to insert some
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between the 16-bit value and the 32-bit value to align the 32-bit value on a 32-bit boundary. Alternatively, one can
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The following formulas provide the number of padding bytes required to align the start of a data structure (where
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allow at least partial control of data structure padding, which may be useful in certain special circumstances.
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While there is no standard way of defining the alignment of structure members (while C and C++ allow using the
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the structure, omitting the padding, which may lead to slower access, but uses three quarters as much memory.
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In this case 3 bytes are added to the last member to pad the structure to the size of 12 bytes (
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specifier for this purpose it can be used only for specifying a stricter alignment), some compilers use
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in modern computer hardware performs reads and writes to memory most efficiently when the data is
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although accessing each structure might take longer. This compromise may be considered a form of
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A block of data of size 2 - 1 always has one sub-block of size 2 aligned on 2 bytes.
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792:(eight bytes) will be 8-byte aligned on Windows and 4-byte aligned on Linux (8-byte with
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is no longer necessary as the structure is already aligned to the size of a long word.
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two-byte boundary so that any padding members are at most one byte long. Likewise, in
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451:. A memory pointer that refers to a data aggregate (a data structure or array) is
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Data structures can be stored in memory on the stack with a static size known as
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at a time. As long as the memory word size is at least as large as the largest
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8086 Family
Utilities - User's Guide for 8080/8085-Based Development Systems
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1679:#define aligntonext(p, bits) alignto(((p) + (1 << bits) - 1), bits)
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on a 32-bit system. The above directives are available in compilers from
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The compiled size of the structure now matches the pre-compiled size of
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Although use of "packed" structures is most frequently used to conserve
1991:. Revision E (A620/5821 6K DD ed.). Santa Clara, California, USA:
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directives to specify packing inside source files. Here is an example:
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1572:// Example: get 4096 bytes aligned on a 4096 byte buffer with malloc()
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assuming that the address where structure begins is an even number */
428:-byte aligned. When a memory access is not aligned, it is said to be
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105:. Statements consisting only of original research should be removed.
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if (and only if) each primitive datum in the aggregate is aligned.
432:. Note that by definition byte memory accesses are always aligned.
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will return zero, therefore the original value is left unchanged.
1676:#define alignto(p, bits) (((p) >> bits) << bits)
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Way in which data is arranged and accessed in computer memory
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Here is a structure with members of various types, totaling
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2052:- discusses stack alignment for x86-64 calling conventions
1995:. May 1982 . pp. 1-6, 3-5. Order Number: 9800639-04.
1660:) works by adding an aligned increment, then clearing the
1009:/* 3 bytes to make total size of the structure 12 bytes */
1017:
The compiled size of the structure is now 12 bytes.
637:) -- providing the offset is unsigned or the system uses
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The following formulas produce the correct values (where
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a power of two, the modulo operation can be reduced to a
810:(four bytes) will be 4-byte aligned. (e.g.: char*, int*)
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on any memory access during the instruction execution.
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A memory pointer that refers to primitive data that is
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1963:(2003 ed.). Upper Saddle River, New Jersey, USA:
1813:"Attributes - D Programming Language: Align Attribute"
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For instance, on a 32-bit operating system, a 4
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It would be beneficial to allocate memory aligned to
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Some data types are dependent on the implementation.
2056:
The Lost Art of
Structure Packing by Eric S. Raymond
818:
64-bit system when compared to a 32-bit system are:
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to eliminate all padding except around bit strings.
443:
if it is only allowed to contain addresses that are
262:. It consists of three separate but related issues:
2184:
2099:
163:. Unsourced material may be challenged and removed.
1853:"LayoutKind Enum (System.Runtime.InteropServices)"
1796:"The Programming Language Pascal (Revised Report)"
814:The only notable differences in alignment for an
1833:"The Rustonomicon - Alternative Representations"
1116:In this example the total size of the structure
1068:In this example the total size of the structure
1538:Hardware significance of alignment requirements
2040:Article on data alignment and data portability
1075:, not 5 (so that the size is a multiple of 4 (
382:-byte aligned address would have a minimum of
2076:
2050:Stack Alignment in 64-bit Calling Conventions
1878:"The curious case of unaligned access on ARM"
1673:// Assume `uint32_t p, bits;` for readability
1290:This structure would have a compiled size of
1024:in this case, which = 4 on linux-32bit/gcc).
931:/* After compilation in 32-bit x86 machine */
8:
2025:IBM developerWorks article on data alignment
1960:Computer Systems: A Programmer's Perspective
469:or on the heap with a dynamic size known as
258:is the way data is arranged and accessed in
1285:/* restore original alignment from stack */
1200:has been replaced (and thus eliminated) by
64:Learn how and when to remove these messages
2083:
2069:
2061:
1723:GNAT Reference Manual 7.4.0w documentation
447:-byte aligned, otherwise it is said to be
424: bytes long and the datum address is
394:least-significant zeros when expressed in
2030:Article on data alignment and performance
1546:mechanism (PCI remapping, operation of a
241:Learn how and when to remove this message
223:Learn how and when to remove this message
121:Learn how and when to remove this message
1780:. July 1966. pp. 55–56. C28-6571-3.
1719:"Ada Representation Clauses and Pragmas"
1399:Allocating memory aligned to cache lines
1211:The alternative method of enforcing the
985:/* 4 bytes - largest structure member */
1710:
1690:
1982:"1. Introduction: Segment Alignment".
1240:/* set alignment to 1 byte boundary */
609:(align - (offset mod align)) mod align
1234:/* push current alignment to stack */
854:(eight bytes) will be 8-byte aligned.
840:(eight bytes) will be 8-byte aligned.
833:(eight bytes) will be 8-byte aligned.
826:(eight bytes) will be 8-byte aligned.
648:Typical alignment of C structs on x86
7:
774:(four bytes) will be 4-byte aligned.
767:(four bytes) will be 4-byte aligned.
760:(four bytes) will be 4-byte aligned.
481:The CPU accesses memory by a single
161:adding citations to reliable sources
1745:SPARCompiler Ada Programmer's Guide
753:(two bytes) will be 2-byte aligned.
1575:// unaligned pointer to large area
746:(one byte) will be 1-byte aligned.
413:aligned is 8 bytes aligned).
25:
1924:"Working with Packing Structures"
738:) when compiling for 32-bit x86:
45:This article has multiple issues.
1912:6.58.8 Structure-Packing Pragmas
1617:// well-aligned pointer to 4 KiB
615:
573:required by the protocol (often
537:Padding is only inserted when a
420:when the data being accessed is
137:
75:
34:
2002:from the original on 2020-02-29
1668:. A possible implementation is
439: bytes long is said to be
148:needs additional citations for
53:or discuss these issues on the
2035:MSDN article on data alignment
1876:Kurusa, Levente (2016-12-27).
1742:"F.8 Representation Clauses".
1383:directive can only be used to
416:A memory access is said to be
1:
2092:Application binary interface
1957:; David, O'Hallaron (2003).
603:is already equal to that of
550:a structure may be declared
2045:Byte Alignment and Ordering
101:the claims made and adding
2266:
2197:Foreign function interface
1664:least significant bits of
614:Since the alignment is by
172:"Data structure alignment"
2192:Binary-code compatibility
2161:Position-independent code
1439://create array of size 10
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534:to be properly aligned.
256:Data structure alignment
1932:. Microsoft. 2007-07-09
607:, the second modulo in
1142:/* after reordering */
1127:on linux-32bit/gcc)).
518:Data structure padding
401:The alternate wording
268:data structure padding
2245:Compiler construction
796:compile time option).
785:compile time option).
629:is a bitwise AND and
407:b/8 byte aligned
2250:Composite data types
2176:Virtual method table
1368:Default packing and
1282:#pragma pack(pop)
1237:#pragma pack(1)
1231:#pragma pack(push)
1121:(FinalPadShort) == 6
865:before compilation:
157:improve this article
2141:Memory segmentation
1544:address translation
1306:, and many others.
1125:alignof(short) == 2
560:space–time tradeoff
532:array of structures
487:primitive data type
2114:Calling convention
1857:docs.microsoft.com
1652:where aligntonext(
575:network byte order
86:possibly contains
2232:
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2224:Year 2038 problem
1993:Intel Corporation
1965:Pearson Education
1955:Bryant, Randal E.
1309:Another example:
581:Computing padding
370:is a multiple of
354:A memory address
344:assembly language
330:implementations,
283:naturally aligned
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2166:Relocation
2119:Call stack
2006:2020-02-29
1936:2011-01-11
1887:2019-08-07
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1818:2012-04-13
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1705:References
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712:Visual C++
616:definition
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