(@cindex @samp{--pcrel}): Rewrite option description.
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Nick Clifton
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2000-08-01 Nick Clifton <nickc@redhat.com>
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* doc/c-m68k.texi (section M680x0 Options): Turn into a table
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index by command line option.
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2000-08-01 Michael Sokolov <msokolov@ivan.Harhan.ORG>
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* doc/c-m68k.texi (@cindex @samp{--pcrel}): Rewrite option description.
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(@node M68K-Branch): Rewrite to match the reality.
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2000-07-31 Jason Eckhardt <jle@cygnus.com>
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* doc/c-i860.texi: New file.
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@ -27,9 +27,12 @@
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@cindex options, M680x0
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@cindex M680x0 options
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The Motorola 680x0 version of @code{@value{AS}} has a few machine
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dependent options.
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dependent options:
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@table @samp
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@cindex @samp{-l} option, M680x0
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@item -l
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You can use the @samp{-l} option to shorten the size of references to undefined
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symbols. If you do not use the @samp{-l} option, references to undefined
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symbols are wide enough for a full @code{long} (32 bits). (Since
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@ -41,6 +44,7 @@ This may be useful if you want the object file to be as small as possible, and
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you know that the relevant symbols are always less than 17 bits away.
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@cindex @samp{--register-prefix-optional} option, M680x0
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@item --register-prefix-optional
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For some configurations, especially those where the compiler normally
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does not prepend an underscore to the names of user variables, the
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assembler requires a @samp{%} before any use of a register name. This
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@ -54,6 +58,7 @@ refer to C variables and functions with the same names as register
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names.
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@cindex @samp{--bitwise-or} option, M680x0
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@item --bitwise-or
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Normally the character @samp{|} is treated as a comment character, which
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means that it can not be used in expressions. The @samp{--bitwise-or}
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option turns @samp{|} into a normal character. In this mode, you must
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@ -62,6 +67,7 @@ at the beginning of a line.
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@cindex @samp{--base-size-default-16}
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@cindex @samp{--base-size-default-32}
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@item --base-size-default-16 --base-size-default-32
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If you use an addressing mode with a base register without specifying
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the size, @code{@value{AS}} will normally use the full 32 bit value.
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For example, the addressing mode @samp{%a0@@(%d0)} is equivalent to
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@ -73,6 +79,7 @@ default behaviour.
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@cindex @samp{--disp-size-default-16}
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@cindex @samp{--disp-size-default-32}
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@item --disp-size-default-16 --disp-size-default-32
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If you use an addressing mode with a displacement, and the value of the
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displacement is not known, @code{@value{AS}} will normally assume that
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the value is 32 bits. For example, if the symbol @samp{disp} has not
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@ -85,15 +92,21 @@ to instead assume that the displacement is 16 bits. In this case,
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@samp{--disp-size-default-32} option to restore the default behaviour.
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@cindex @samp{--pcrel}
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Always generate PC relative branches. Actually what this option really
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does is to prevent PC relative branches from being turned into absolute
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jumps. If this cannot be done (because the specific architecture does
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not have a suitable PC relative branch instruction), the assembler will
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generate an error message.
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@item --pcrel
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Always keep branches PC-relative. In the M680x0 architecture all branches
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are defined as PC-relative. However, on some processors they are limited
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to word displacements maximum. When @code{@value{AS}} needs a long branch
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that is not available, it normally emits an absolute jump instead. This
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option disables this substitution. When this option is given and no long
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branches are available, only word branches will be emitted. An error
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message will be generated if a word branch cannot reach its target. This
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option has no effect on 68020 and other processors that have long branches.
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@pxref{M68K-Branch,,Branch Improvement}.
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@cindex @samp{-m68000} and related options
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@cindex architecture options, M680x0
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@cindex M680x0 architecture options
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@item -m68000
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@code{@value{AS}} can assemble code for several different members of the
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Motorola 680x0 family. The default depends upon how @code{@value{AS}}
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was configured when it was built; normally, the default is to assemble
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@ -175,6 +188,7 @@ Do not assemble 68851 MMU instructions. This is the default for the
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68000, 68010, and the CPU32. The 68040 accepts a somewhat different set
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of MMU instructions.
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@end table
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@end table
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@node M68K-Syntax
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@section Syntax
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@ -410,28 +424,39 @@ cases that are more fully described after the table:
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@smallexample
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Displacement
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+-------------------------------------------------
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| 68020 68000/10
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Pseudo-Op |BYTE WORD LONG LONG non-PC relative
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+-------------------------------------------------
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jbsr |bsrs bsr bsrl jsr jsr
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jra |bras bra bral jmp jmp
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* jXX |bXXs bXX bXXl bNXs;jmpl bNXs;jmp
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* dbXX |dbXX dbXX dbXX; bra; jmpl
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* fjXX |fbXXw fbXXw fbXXl fbNXw;jmp
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+------------------------------------------------------------
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| 68020 68000/10, not PC-relative OK
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Pseudo-Op |BYTE WORD LONG ABSOLUTE LONG JUMP **
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+------------------------------------------------------------
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jbsr |bsrs bsrw bsrl jsr
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jra |bras braw bral jmp
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* jXX |bXXs bXXw bXXl bNXs;jmp
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* dbXX | N/A dbXXw dbXX;bras;bral dbXX;bras;jmp
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fjXX | N/A fbXXw fbXXl N/A
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XX: condition
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NX: negative of condition XX
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@end smallexample
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@center @code{*}---see full description below
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@center @code{**}---this expansion mode is disallowed by @samp{--pcrel}
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@table @code
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@item jbsr
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@itemx jra
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These are the simplest jump pseudo-operations; they always map to one
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particular machine instruction, depending on the displacement to the
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branch target.
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branch target. This instruction will be a byte or word branch is that
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is sufficient. Otherwise, a long branch will be emitted if available.
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If no long branches are available and the @samp{--pcrel} option is not
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given, an absolute long jump will be emitted instead. If no long
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branches are available, the @samp{--pcrel} option is given, and a word
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branch cannot reach the target, an error message is generated.
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In addition to standard branch operands, @code{@value{AS}} allows these
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pseudo-operations to have all operands that are allowed for jsr and jmp,
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substituting these instructions if the operand given is not valid for a
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branch instruction.
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@item j@var{XX}
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Here, @samp{j@var{XX}} stands for an entire family of pseudo-operations,
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@ -442,10 +467,11 @@ list of pseudo-ops in this family is:
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jvs jpl jmi jge jlt jgt jle
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@end smallexample
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For the cases of non-PC relative displacements and long displacements on
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the 68000 or 68010, @code{@value{AS}} issues a longer code fragment in terms of
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@var{NX}, the opposite condition to @var{XX}. For example, for the
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non-PC relative case:
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Usually, each of these pseudo-operations expands to a single branch
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instruction. However, if a word branch is not sufficient, no long branches
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are available, and the @samp{--pcrel} option is not given, @code{@value{AS}}
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issues a longer code fragment in terms of @var{NX}, the opposite condition
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to @var{XX}. For example, under these conditions:
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@smallexample
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j@var{XX} foo
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@end smallexample
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@ -464,12 +490,24 @@ The full family of pseudo-operations covered here is
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dbf dbra dbt
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@end smallexample
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Other than for word and byte displacements, when the source reads
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Motorola @samp{db@var{XX}} instructions allow word displacements only. When
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a word displacement is sufficient, each of these pseudo-operations expands
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to the corresponding Motorola instruction. When a word displacement is not
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sufficient and long branches are available, when the source reads
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@samp{db@var{XX} foo}, @code{@value{AS}} emits
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@smallexample
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db@var{XX} oo1
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bra oo2
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oo1:jmpl foo
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bras oo2
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oo1:bral foo
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oo2:
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@end smallexample
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If, however, long branches are not available and the @samp{--pcrel} option is
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not given, @code{@value{AS}} emits
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@smallexample
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db@var{XX} oo1
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bras oo2
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oo1:jmp foo
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oo2:
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@end smallexample
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@ -483,13 +521,9 @@ This family includes
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fjugt fjule fjult fjun
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@end smallexample
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For branch targets that are not PC relative, @code{@value{AS}} emits
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@smallexample
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fb@var{NX} oof
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jmp foo
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oof:
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@end smallexample
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when it encounters @samp{fj@var{XX} foo}.
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Each of these pseudo-operations always expands to a single Motorola
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coprocessor branch instruction, word or long. All Motorola coprocessor
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branch instructions allow both word and long displacements.
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@end table
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