Instruction forms

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The instructions are four byte long and are word-aligned.

All instructions are four bytes long and are word-aligned. Therefore, when the processor fetches instructions (for example, branch instructions), the two low-order bits are ignored. Similarly, when the processor develops an instruction address, the two low-order bits of the address are 0.

Bits 0-5 always specify the op code. Many instructions also have an extended op code (for example, XO-form instructions). The remaining bits of the instruction contain one or more fields. The alternative fields for the various instruction forms are shown in the following:

  • I Form
    Bits Value
    0-5 OPCD
    6-29 LI
    30 AA
    31 LK
  • B Form
    Bits Value
    0-5 OPCD
    6-10 BO
    11-15 BI
    16-29 BD
    30 AA
    31 LK
  • SC Form
    Bits Value
    0-5 OPCD
    6-10 ///
    11-15 ///
    16-29 ///
    30 XO
    31 /
  • D Form
    Bits Value
    0-5 OPCD
    6-10 RT, RS, FRT, FRS, TO, or BF, /, and L
    11-15 RA
    16-31 D, SI, or UI
  • DS Form
    Bits Value
    0-5 OPCD
    6-10 RT or RS
    11-15 RA
    16-29 DS
    30-31 XO
  • X Instruction Format
    Bits Value
    0-5 OPCD
    6-10 RT, FRT, RS, FRS, TO, BT, or BF, /, and L
    11-15 RA, FRA, SR, SPR, or BFA and //
    16-20 RB, FRB, SH, NB, or U and /
    21-30 XO or EO
    31 Rc
    • XL Instruction Format
      Bits Value
      0-5 OPCD
      6-10 RT or RS
      11-20 spr or /, FXM and /
      21-30 XO or EO
      31 Rc
    • XFX Instruction Format
      Bits Value
      0-5 OPCD
      6-10 RT or RS
      11-20 spr or /, FXM and /
      21-30 XO or EO
      31 Rc
    • XFL Instruction Format
      Bits Value
      0-5 OPCD
      6 /
      7-14 FLM
      15 /
      16-20 FRB
      21-30 XO or EO
      31 Rc
    • XO Instruction Format
      Bits Value
      0-5 OPCD
      6-10 RT
      11-15 RA
      16-20 RB
      21 OE
      22-30 XO or EO
      31 Rc
  • A Form
    Bits Value
    0-5 OPCD
    6-10 FRT
    11-15 FRA
    16-20 FRB
    21-25 FRC
    26-30 XO
    31 Rc
  • M Form
    Bits Value
    0-5 OPCD
    6-10 RS
    `11-15 RA
    16-20 RB or SH
    21-25 MB
    26-30 ME
    31 Rc

For some instructions, an instruction field is reserved or must contain a particular value. This is not indicated in the previous figures, but is shown in the syntax for instructions in which these conditions are required. If a reserved field does not have all bits set to 0, or if a field that must contain a particular value does not contain that value, the instruction form is invalid.