i386/i386.reference/s08_02.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Section 8.2</TITLE>
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   7 <B>up:</B> <A HREF="c08.htm">
   8 Chapter 8 -- Input/Output</A><BR>
   9 <B>prev:</B> <A HREF="s08_01.htm">8.1  I/O Addressing</A><BR>
  10 <B>next:</B> <A HREF="s08_03.htm">8.3  Protection and I/O</A>
  11 <P>
  12 <HR>
  13 <P>
  14 <H1>8.2  I/O Instructions</H1>
  15 The I/O instructions of the 80386 provide access to the processor's I/O
  16 ports for the transfer of data to and from peripheral devices. These
  17 instructions have as one operand the address of a port in the I/O address
  18 space. There are two classes of I/O instruction:
  19 <OL>
  20 <LI> Those that transfer a single item (byte, word, or doubleword) located
  21 in a register.
  22 <LI> Those that transfer strings of items (strings of bytes, words, or
  23 doublewords) located in memory. These are known as "string I/O
  24 instructions" or "block I/O instructions".
  25 </OL>
  26
  27 <H2>8.2.1  Register I/O Instructions</H2>
  28 The I/O instructions <A HREF="IN.htm">IN</A> and
  29 <A HREF="OUT.htm">OUT</A>are provided to move data between I/O ports
  30 and the EAX (32-bit I/O), the AX (16-bit I/O), or AL (8-bit I/O) general
  31 registers. <A HREF="IN.htm">IN</A> and
  32 <A HREF="OUT.htm">OUT</A>instructions address I/O ports either directly, with
  33 the address of one of up to 256 port addresses coded in the instruction, or
  34 indirectly via the DX register to one of up to 64K port addresses.
  35 <P>
  36 <A HREF="IN.htm">IN</A> (Input from Port)
  37 transfers a byte, word, or doubleword from an input
  38 port to AL, AX, or EAX. If a program specifies AL with the
  39 <A HREF="IN.htm">IN</A> instruction,
  40 the processor transfers 8 bits from the selected port to AL. If a program
  41 specifies AX with the
  42 <A HREF="IN.htm">IN</A> instruction, the processor transfers 16 bits from
  43 the port to AX. If a program specifies EAX with the
  44 <A HREF="IN.htm">IN</A> instruction, the
  45 processor transfers 32 bits from the port to EAX.
  46 <P>
  47 <A HREF="OUT.htm">OUT</A>(Output to Port)
  48 transfers a byte, word, or doubleword to an output
  49 port from AL, AX, or EAX. The program can specify the number of the port
  50 using the same methods as the <A HREF="IN.htm">IN</A> instruction.
  51 <P>
  52 <A NAME="fig8-1">
  53 <IMG align=center SRC="fig8-1.gif" border=0>
  54
  55 <H2>8.2.2  Block I/O Instructions</H2>
  56 The block (or string) I/O instructions
  57 <A HREF="INS.htm">INS</A> and
  58 <A HREF="OUTS.htm">OUTS</A> move blocks of data
  59 between I/O ports and memory space. Block I/O instructions use the DX
  60 register to specify the address of a port in the I/O address space.
  61 <A HREF="INS.htm">INS</A> and
  62 <A HREF="OUTS.htm">OUTS</A>use DX to specify:
  63 <UL>
  64 <LI> 8-bit ports numbered 0 through 65535
  65 <LI> 16-bit ports numbered 0, 2, 4, . . . , 65532, 65534
  66 <LI> 32-bit ports numbered 0, 4, 8, . . . , 65528, 65532
  67 </UL>
  68 Block I/O instructions use either SI or DI to designate the source or
  69 destination memory address. For each transfer, SI or DI are automatically
  70 either incremented or decremented as specified by the direction bit in the
  71 flags register.
  72 <P>
  73 <A HREF="INS.htm">INS</A> and <A HREF="OUTS.htm">OUTS</A>,
  74 when used with repeat prefixes, cause block input or output
  75 operations.
  76 <A HREF="REP.htm">REP</A>, the repeat prefix, modifies
  77 <A HREF="INS.htm">INS</A> and <A HREF="OUTS.htm">OUTS</A> to provide a means
  78 of transferring blocks of data between an I/O port and memory. These block
  79 I/O instructions are string primitives (refer also to
  80 <A HREF="c03.htm">Chapter 3</A>
  81    for more on
  82 string primitives). They simplify programming and increase the speed of data
  83 transfer by eliminating the need to use a separate
  84 <A HREF="LOOP.htm">LOOP</A> instruction or an
  85 intermediate register to hold the data.
  86 <P>
  87 The string I/O primitives can operate on byte strings, word strings, or
  88 doubleword strings. After each transfer, the memory address in ESI or EDI is
  89 updated by 1 for byte operands, by 2 for word operands, or by 4 for
  90 doubleword operands. The value in the direction flag (DF) determines whether
  91 the processor automatically increments ESI or EDI (DF=0) or whether it
  92 automatically decrements these registers (DF=1).
  93 <P>
  94 <A HREF="INS.htm">INS</A>(Input String from Port)
  95 transfers a byte or a word string element from
  96 an input port to memory. The mnemonics
  97 <A HREF="INS.htm">INSB</A>,
  98 <A HREF="INS.htm">INSW</A>, and
  99 <A HREF="INS.htm">INSD</A> are variants
 100 that explicitly specify the size of the operand. If a program specifies
 101 <A HREF="INS.htm">INSB</A>,
 102 the processor transfers 8 bits from the selected port to the memory
 103 location indicated by ES:EDI. If a program specifies
 104 <A HREF="INS.htm">INSW</A>, the processor
 105 transfers 16 bits from the port to the memory location indicated by ES:EDI.
 106 If a program specifies
 107 <A HREF="INS.htm">INSD</A>, the processor transfers 32 bits from the port
 108 to the memory location indicated by ES:EDI. The destination segment register
 109 choice (ES) cannot be changed for the
 110 <A HREF="INS.htm">INS</A> instruction. Combined with the
 111 <A HREF="REP.htm">REP</A> prefix,
 112 <A HREF="INS.htm">INS</A> moves a block of information from an input
 113 port to a series of consecutive memory locations.
 114 <P>
 115 <A HREF="OUTS.htm">OUTS</A> (Output String to Port)
 116 transfers a byte, word, or doubleword string
 117 element to an output port from memory. The mnemonics
 118 <A HREF="OUTS.htm">OUTSB</A>,
 119 <A HREF="OUTS.htm">OUTSW</A>, and
 120 <A HREF="OUTS.htm">OUTSD</A>
 121 are variants that explicitly specify the size of the operand. If a program
 122 specifies
 123 <A HREF="OUTS.htm">OUTSB</A>, the processor transfers 8 bits
 124 from the memory location
 125 indicated by ES:EDI to the the selected port. If a program specifies
 126 <A HREF="OUTS.htm">OUTSW</A>,
 127 the processor transfers 16 bits from the memory location indicated by ES:EDI
 128 to the the selected port. If a program specifies
 129 <A HREF="OUTS.htm">OUTSD</A>, the processor
 130 transfers 32 bits from the memory location indicated by ES:EDI to the the
 131 selected port. Combined with the
 132 <A HREF="REP.htm">REP</A> prefix,
 133 <A HREF="OUTS.htm">OUTS</A> moves a block of
 134 information from a series of consecutive memory locations indicated by
 135 DS:ESI to an output port.
 136 <P>
 137 <HR>
 138 <P>
 139 <B>up:</B> <A HREF="c08.htm">
 140 Chapter 8 -- Input/Output</A><BR>
 141 <B>prev:</B> <A HREF="s08_01.htm">8.1  I/O Addressing</A><BR>
 142 <B>next:</B> <A HREF="s08_03.htm">8.3  Protection and I/O</A>
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