9.23.6.1 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 prepending ‘j’ to the start of Motorola mnemonic. These pseudo opcodes are not affected by the ‘--short-branches’ or ‘--force-long-branches’ options.

The following table summarizes the pseudo-operations.

                             Displacement Width
          +-------------------------------------------------------------+
          |                     Options                                 |
          |    --short-branches           --force-long-branches         |
          +--------------------------+----------------------------------+
       Op |BYTE             WORD     | BYTE          WORD               |
          +--------------------------+----------------------------------+
      bsr | bsr <pc-rel>    <error>  |               jsr <abs>          |
      bra | bra <pc-rel>    <error>  |               jmp <abs>          |
     jbsr | bsr <pc-rel>   jsr <abs> | bsr <pc-rel>  jsr <abs>          |
     jbra | bra <pc-rel>   jmp <abs> | bra <pc-rel>  jmp <abs>          |
      bXX | bXX <pc-rel>    <error>  |               bNX +3; jmp <abs>  |
     jbXX | bXX <pc-rel>   bNX +3;   | bXX <pc-rel>  bNX +3; jmp <abs>  |
          |                jmp <abs> |                                  |
          +--------------------------+----------------------------------+
     XX: condition
     NX: negative of condition XX
     

jbsr

jbra

These are the simplest jump pseudo-operations; they always map to one particular machine instruction, depending on the displacement to the branch target.

jbXX

Here, ‘jbXX’ 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:

           jbcc   jbeq   jbge   jbgt   jbhi   jbvs   jbpl  jblo
           jbcs   jbne   jblt   jble   jbls   jbvc   jbmi

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

              jbXX foo

gives

               bNXs oof
               jmp foo
           oof: