as: M68K-Branch
9.21.6.1 Branch Improvement
...........................
Certain pseudo opcodes are permitted for branch instructions. They
expand to the shortest branch instruction that reach the target.
Generally these mnemonics are made by substituting 'j' for 'b' at the
start of a Motorola mnemonic.
The following table summarizes the pseudo-operations. A '*' flags
cases that are more fully described after the table:
Displacement
+------------------------------------------------------------
| 68020 68000/10, not PC-relative OK
Pseudo-Op |BYTE WORD LONG ABSOLUTE LONG JUMP **
+------------------------------------------------------------
jbsr |bsrs bsrw bsrl jsr
jra |bras braw bral jmp
* jXX |bXXs bXXw bXXl bNXs;jmp
* dbXX | N/A dbXXw dbXX;bras;bral dbXX;bras;jmp
fjXX | N/A fbXXw fbXXl N/A
XX: condition
NX: negative of condition XX
'*'--see full description below
'**'--this expansion mode is disallowed by '--pcrel'
'jbsr'
'jra'
These are the simplest jump pseudo-operations; they always map to
one particular machine instruction, depending on the displacement
to the branch target. This instruction will be a byte or word
branch is that is sufficient. Otherwise, a long branch will be
emitted if available. If no long branches are available and the
'--pcrel' option is not given, an absolute long jump will be
emitted instead. If no long branches are available, the '--pcrel'
option is given, and a word branch cannot reach the target, an
error message is generated.
In addition to standard branch operands, 'as' allows these
pseudo-operations to have all operands that are allowed for jsr and
jmp, substituting these instructions if the operand given is not
valid for a branch instruction.
'jXX'
Here, 'jXX' stands for an entire family of pseudo-operations, where
XX is a conditional branch or condition-code test. The full list
of pseudo-ops in this family is:
jhi jls jcc jcs jne jeq jvc
jvs jpl jmi jge jlt jgt jle
Usually, each of these pseudo-operations expands to a single branch
instruction. However, if a word branch is not sufficient, no long
branches are available, and the '--pcrel' option is not given, 'as'
issues a longer code fragment in terms of NX, the opposite
condition to XX. For example, under these conditions:
jXX foo
gives
bNXs oof
jmp foo
oof:
'dbXX'
The full family of pseudo-operations covered here is
dbhi dbls dbcc dbcs dbne dbeq dbvc
dbvs dbpl dbmi dbge dblt dbgt dble
dbf dbra dbt
Motorola 'dbXX' instructions allow word displacements only. When a
word displacement is sufficient, each of these pseudo-operations
expands to the corresponding Motorola instruction. When a word
displacement is not sufficient and long branches are available,
when the source reads 'dbXX foo', 'as' emits
dbXX oo1
bras oo2
oo1:bral foo
oo2:
If, however, long branches are not available and the '--pcrel'
option is not given, 'as' emits
dbXX oo1
bras oo2
oo1:jmp foo
oo2:
'fjXX'
This family includes
fjne fjeq fjge fjlt fjgt fjle fjf
fjt fjgl fjgle fjnge fjngl fjngle fjngt
fjnle fjnlt fjoge fjogl fjogt fjole fjolt
fjor fjseq fjsf fjsne fjst fjueq fjuge
fjugt fjule fjult fjun
Each of these pseudo-operations always expands to a single Motorola
coprocessor branch instruction, word or long. All Motorola
coprocessor branch instructions allow both word and long
displacements.
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