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MPC555
/
MPC556
CENTRAL PROCESSING UNIT
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
3-24
clock in the MPC555 / MPC556, refer to
6.6 MPC555 / MPC556 Decrementer
MPC555 / MPC556 Internal Clock Signals
, and
8.12.1 System Clock Control Reg-
The DEC does not run after power-up and must be enabled by setting the TBE bit in
the TBSCR register, see
. Power-on reset stops its counting and clears the
register. A decrementer exception may be signaled to software prior to initialization.
3.9.6 Machine Status Save/Restore Register 0 (SRR0)
The machine status save/restore register 0 (SRR0) is a 32-bit register that identifies
where instruction execution should resume when an
rfi
instruction is executed follow-
ing an exception. It also holds the effective address of the instruction that follows the
System Call (
sc
) instruction.
When an exception occurs, SRR0 is set to point to an instruction such that all prior in-
structions have completed execution and no subsequent instruction has begun execu-
tion. The instruction addressed by SRR0 may not have completed execution,
depending on the exception type. SRR0 addresses either the instruction causing the
exception or the immediately following instruction. The instruction addressed can be
determined from the exception type and status bits.
When an exception occurs, SRR0 is set to point to an instruction such that all prior in-
structions have completed execution and no subsequent instruction has begun execu-
tion. The instruction addressed by SRR0 may not have completed execution,
depending on the exception type. SRR0 addresses either the instruction causing the
exception or the immediately following instruction. The instruction addressed can be
determined from the exception type and status bits.
3.9.7 Machine Status Save/Restore Register 1 (SRR1)
SRR1 is a 32-bit register used to save machine status on exceptions and to restore
machine status when an
rfi
instruction is executed.
DEC
— Decrementer Register
SPR 22
MSB
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
LSB
31
Decrementing Counter
RESET: UNCHANGED
SRR0
— Machine Status Save/Restore Register 0
SPR 26
MSB
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
LSB
31
SRR0
RESET: UNDEFINED
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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