Datasheet

Central Processing Unit (CPU)
Opcodes and Operands
MC9S12DP256 — Revision 1.1
Central Processing Unit (CPU)
Opcodes and Operands
The STAR12 CPU uses 8-bit opcodes. Each opcode identifies a
particular instruction and associated addressing mode to the CPU.
Several opcodes are required to provide each instruction with a range of
addressing capabilities.
Only 256 opcodes would be available if the range of values were
restricted to the number that can be represented by 8-bit binary
numbers. To expand the number of opcodes, a second page is added to
the opcode map. Opcodes on the second page are preceded by an
additional byte with the value $18.
To provide additional addressing flexibility, opcodes can also be
followed by a postbyte or extension bytes. Postbytes implement certain
forms of indexed addressing, transfers, exchanges, and loop primitives.
Extension bytes contain additional program information such as
addresses, offsets, and immediate data.
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