i386/i386.reference/s02_02.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Section 2.2</TITLE>
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   8 Chapter 2 -- Basic Programming Model</A><BR>
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  11 <P>
  12 <HR>
  13 <P>
  14 <H1>2.2  Data Types</H1>
  15 Bytes, words, and doublewords are the fundamental data types (refer to
  16
  17 <A HREF="#fig2-2">Figure 2-2</A>
  18   ). A byte is eight contiguous bits starting at any logical
  19 address. The bits are numbered 0 through 7; bit zero is the least
  20 significant bit.
  21 <P>
  22 A word is two contiguous bytes starting at any byte address. A word thus
  23 contains 16 bits. The bits of a word are numbered from 0 through 15; bit 0
  24 is the least significant bit. The byte containing bit 0 of the word is
  25 called the low byte; the byte containing bit 15 is called the high byte.
  26 <P>
  27 Each byte within a word has its own address, and the smaller of the
  28 addresses is the address of the word. The byte at this lower address
  29 contains the eight least significant bits of the word, while the byte at the
  30 higher address contains the eight most significant bits.
  31 <P>
  32 A doubleword is two contiguous words starting at any byte address. A
  33 doubleword thus contains 32 bits. The bits of a doubleword are numbered from
  34 0 through 31; bit 0 is the least significant bit. The word containing bit 0
  35 of the doubleword is called the low word; the word containing bit 31 is
  36 called the high word.
  37 <P>
  38 Each byte within a doubleword has its own address, and the smallest of the
  39 addresses is the address of the doubleword. The byte at this lowest address
  40 contains the eight least significant bits of the doubleword, while the byte
  41 at the highest address contains the eight most significant bits.
  42 <A HREF="#fig2-3">Figure 2-3</A>
  43
  44 illustrates the arrangement of bytes within words anddoublewords.
  45 <P>
  46 Note that words need not be aligned at even-numbered addresses and
  47 doublewords need not be aligned at addresses evenly divisible by four. This
  48 allows maximum flexibility in data structures (e.g., records containing
  49 mixed byte, word, and doubleword items) and efficiency in memory
  50 utilization. When used in a configuration with a 32-bit bus, actual
  51 transfers of data between processor and memory take place in units of
  52 doublewords beginning at addresses evenly divisible by four; however, the
  53 processor converts requests for misaligned words or doublewords into the
  54 appropriate sequences of requests acceptable to the memory interface. Such
  55 misaligned data transfers reduce performance by requiring extra memory
  56 cycles. For maximum performance, data structures (including stacks) should
  57 be designed in such a way that, whenever possible, word operands are aligned
  58 at even addresses and doubleword operands are aligned at addresses evenly
  59 divisible by four. Due to instruction prefetching and queuing within the
  60 CPU, there is no requirement for instructions to be aligned on word or
  61 doubleword boundaries. (However, a slight increase in speed results if the
  62 target addresses of control transfers are evenly divisible by four.)
  63 <P>
  64 Although bytes, words, and doublewords are the fundamental types of
  65 operands, the processor also supports additional interpretations of these
  66 operands. Depending on the instruction referring to the operand, the
  67 following additional data types are recognized:
  68 <DL>
  69 <DT>
  70 Integer:
  71 <DD>
  72 A signed binary numeric value contained in a 32-bit doubleword, 16-bit word,
  73 or 8-bit byte. All operations assume a 2's complement representation. The
  74 sign bit is located in bit 7 in a byte, bit 15 in a word, and bit 31 in a
  75 doubleword. The sign bit has the value zero for positive integers and one
  76 for negative. Since the high-order bit is used for a sign, the range of an
  77 8-bit integer is -128 through +127; 16-bit integers may range from -32,768
  78 through +32,767; 32-bit integers may range from -2^(31) through +2^(31) -1.
  79 The value zero has a positive sign.
  80 <DT>
  81 Ordinal:
  82 <DD>
  83 An unsigned binary numeric value contained in a 32-bit doubleword,
  84 16-bit word, or 8-bit byte. All bits are considered in determining
  85 magnitude of the number. The value range of an 8-bit ordinal number
  86 is 0-255; 16 bits can represent values from 0 through 65,535; 32 bits
  87 can represent values from 0 through 2^(32) -1.
  88 <DT>
  89 Near Pointer:
  90 <DD>
  91 A 32-bit logical address. A near pointer is an offset within a segment.
  92 Near pointers are used in either a flat or a segmented model of memory
  93 organization.
  94 <DT>
  95 Far Pointer:
  96 <DD>
  97 A 48-bit logical address of two components: a 16-bit segment selector
  98 component and a 32-bit offset component. Far pointers are used by
  99 applications programmers only when systems designers choose a
 100 segmented memory organization.
 101 <DT>
 102 String:
 103 <DD>
 104 A contiguous sequence of bytes, words, or doublewords. A string may
 105 contain from zero bytes to 2^(32) -1 bytes (4 gigabytes).
 106 <DT>
 107 Bit field:
 108 <DD>
 109 A contiguous sequence of bits. A bit field may begin at any bit position
 110 of any byte and may contain up to 32 bits.
 111 <DT>
 112 Bit string:
 113 <DD>
 114 A contiguous sequence of bits. A bit string may begin at any bit position
 115 of any byte and may contain up to 2^(32) -1 bits.
 116 <DT>
 117 BCD:
 118 <DD>
 119 A byte (unpacked) representation of a decimal digit in the range 0 through 9.
 120 Unpacked decimal numbers are stored as unsigned byte quantities. One
 121 digit is stored in each byte. The magnitude of the number is determined from
 122 the low-order half-byte; hexadecimal values 0-9 are valid and are
 123 interpreted as decimal numbers. The high-order half-byte must be zero for
 124 multiplication and division; it may contain any value for addition and
 125 subtraction.
 126 <DT>
 127 Packed BCD:
 128 <DD>
 129 A byte (packed) representation of two decimal digits, each in the range
 130 0 through 9. One digit is stored in each half-byte. The digit in the
 131 high-order half-byte is the most significant. Values 0-9 are valid in each
 132 half-byte. The range of a packed decimal byte is 0-99.
 133 </DL>
 134
 135 <A HREF="#fig2-4">Figure 2-4</A>
 136   graphically summarizes the data types supported by the 80386.
 137 <P>
 138 <A NAME="fig2-1">
 139 <IMG align=center SRC="fig2-1.gif" border=0>
 140 <P>
 141 <HR>
 142 <P>
 143 <A NAME="fig2-2">
 144 <IMG align=center SRC="fig2-2.gif" border=0>
 145 <P>
 146 <HR>
 147 <P>
 148 <A NAME="fig2-3">
 149 <IMG align=center SRC="fig2-3.gif" border=0>
 150 <P>
 151 <HR>
 152 <P>
 153 <A NAME="fig2-4">
 154 <IMG align=center SRC="fig2-4.gif" border=0>
 155 <P>
 156 <HR>
 157 <P>
 158 <B>up:</B> <A HREF="c02.htm">
 159 Chapter 2 -- Basic Programming Model</A><BR>
 160 <B>prev:</B> <A HREF="s02_01.htm">2.1  Memory Organization and Segmentation</A><BR>
 161 <B>next:</B> <A HREF="s02_03.htm">2.3  Registers</A>
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