HCS12 datasheet(320/452 Pages) FREESCALE

320 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

320

Freescale Semiconductor

7.8.2 Reset Exception Processing

If reset caused the exception, processing continues to cycle 2.0. This cycle

sets the S, X, and I bits in the CCR. Cycles 3.0 through 5.0 are program

word fetches that refill the instruction queue. Fetches start at the address

pointed to by the reset vector. When the fetches are completed, exception

processing ends, and the CPU starts executing the instruction at the head

of the instruction queue.

7.8.3 Interrupt and Unimplemented Opcode Trap Exception Processing

If an exception was not caused by a reset, a return address is calculated.

• Cycles 2.1and 2.2 are both S cycles (stack a 16-bit word), but the

CPU12 performs different return address calculations for each type of

exception.

–

When an X- or I-related interrupt causes the exception, the return

address points to the next instruction that would have been

executed had processing not been interrupted.

–

When an exception is caused by an SWI opcode or by an

unimplemented opcode (see 7.6 Unimplemented Opcode Trap),

the return address points to the next address after the opcode.

• Once calculated, the return address is pushed onto the stack.

• Cycles 3.1 through 9.1 are identical to cycles 3.2 through 9.2 for the

rest of the sequence, except for optional setting of the X mask bit

performed in cycle 8.1 (see below).

• Cycle 3.1/3.2 is the first of three program word fetches that refill the

instruction queue.

• Cycle 4.1/4.2 pushes Y onto the stack.

• Cycle 5.1/5.2 pushes X onto the stack.

• Cycle 6.1/6.2 is the second of three program word fetches that refill

the instruction queue. During this cycle, the contents of the A and B

accumulators are concatenated into a 16-bit word in the order B:A.

This makes register order in the stack frame the same as that of the

M68HC11, M6801, and the M6800.

• Cycle 7.1/7.2 pushes the 16-bit word containing B:A onto the stack.

HCS12 Datasheet(PDF) 320 Page - Freescale Semiconductor, Inc