HCS12 datasheet(99/452 Pages) FREESCALE

99 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

6.7 Glossary

This subsection contains an entry for each assembler mnemonic, in

alphabetic order.

Special Notation for Branch Taken/Not Taken Cases

PPP/P — Short branches require three cycles if taken, one cycle

if not taken. Since the instruction consists of a single

word containing both an opcode and an 8-bit offset,

the not-taken case is simple — the queue advances,

another program word fetch is made, and execution

continues with the next instruction. The taken case

requires that the queue be refilled so that execution

can continue at a new address. First, the effective

address of the destination is determined, then the

CPU performs three program word fetches from that

address.

OPPP/OPO — Long branches require four cycles if taken, three

cycles if not taken. Optional cycles are required

because all long branches are page two opcodes, and

thus include the $18 prebyte. The CPU12 treats the

prebyte as a special 1-byte instruction. If the prebyte

is misaligned, the optional cycle is used to perform a

program word access; if the prebyte is aligned, the

optional cycle is used to perform a free cycle. As a

result, both the taken and not-taken cases use one

optional cycle for the prebyte. In the not-taken case,

the queue must advance so that execution can

continue with the next instruction, and another

optional cycle is required to maintain the queue. The

taken case requires that the queue be refilled so that

execution can continue at a new address. First, the

effective address of the destination is determined,

then the CPU performs three program word fetches

from that address.

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