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
|
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.
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 |
bNXs oof
jmp foo
oof: |
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: |
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.