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
Pseudo-Op |BYTE    WORD    LONG    LONG      non-PC relative
          +-------------------------------------------------
     jbsr |bsrs    bsr     bsrl    jsr       jsr
      jra |bras    bra     bral    jmp       jmp
*     jXX |bXXs    bXX     bXXl    bNXs;jmpl bNXs;jmp
*    dbXX |dbXX    dbXX        dbXX; bra; jmpl
*    fjXX |fbXXw   fbXXw   fbXXl             fbNXw;jmp

XX: condition
NX: negative of condition XX

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.

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

For the cases of non-PC relative displacements and long displacements on the 68000 or 68010, as issues a longer code fragment in terms of NX, the opposite condition to XX. For example, for the non-PC relative case:

    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

Other than for word and byte displacements, when the source reads `dbXX foo', as emits

     dbXX oo1
     bra oo2
 oo1:jmpl 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

For branch targets that are not PC relative, as emits

     fbNX oof
     jmp foo
 oof:

when it encounters `fjXX foo'.