
MOTOROLA
MPC823 USER’S MANUAL
B-1
INSTRUCTION
SET
B
APPENDIX B
MPC823 INSTRUCTION SET
This section lists the MPC823 instruction set in alphabetical order by mnemonic. Each entry
includes the instruction formats and a quick reference legend that provides information like
the level(s) of the PowerPC architecture in which the instruction can be found, user- or
supervisor-level (an instruction is user-level unless the legend specifies otherwise), and the
instruction formats. The format diagrams show all valid combinations of the instruction
fields. The architecture specification refers to user- and supervisor-level as problem state
and privileged state, respectively.
B.1 INSTRUCTION FORMATS
Instructions are four bytes long and word-aligned, so when instruction addresses are
presented to the processor (as in branch instructions) the two low-order bits are ignored.
Similarly, whenever the processor develops an instruction address, its two low-order bits are
zero. Bits 0–5 always specify the primary opcode and many instructions also have an
extended opcode. The remaining bits of the instruction contains one or more fields for the
different instruction formats. Some instruction fields are reserved or must contain a
predefined value as shown in the individual instruction layouts. If a reserved field does not
have all bits cleared, or if a field that must contain a particular value does not contain that
value, the instruction form is invalid.
B.2 SPLIT-FIELD NOTATION
Some instruction fields occupy more than one contiguous sequence of bits or a contiguous
sequence of bits used in permuted order. These fields are called split fields. Those split
fields that represent the concatenation of the sequences from left to right are shown in
lowercase letters. For instance, spr and tbr are shown in the following table.
Split fields that represent the concatenation of the sequences in some order, which need not
be left to right are shown in uppercase letters. For instance, MB, ME, and SH are shown in
the following table.
FIELD
DESCRIPTION
spr (11–20)
This field is used to specify a special-purpose register for the mtspr and mfspr instructions.
tbr (11–20)
This field is used to specify either the time base lower (TBL) or time base upper (TBU).