MPC555
/
MPC556
CENTRAL PROCESSING UNIT
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
3-23
3.9.4 Time Base Facility (TB) — OEA
As described in
3.8 PowerPC VEA Register Set — Time Base
, the time base (TB)
provides a 64-bit incrementing counter. The VEA defines user-level, read-only access
to the TB. Writing to the TB is reserved for supervisor-level applications such as oper-
ating systems and bootstrap routines. The OEA defines supervisor-level, write access
to the TB.
The TB can be written to at the supervisor privilege level only. The
mttbl
and
mttbu
simplified mnemonics write the lower and upper halves of the TB, respectively. The
mtspr
,
mttbl
, and
mttbu
instructions treat TBL and TBU as separate 32-bit registers;
setting one leaves the other unchanged. It is not possible to write the entire 64-bit time
base in a single instruction.
For information about reading the time base, refer to
.
3.9.5 Decrementer Register (DEC)
The decrementer (DEC, SPR 22) is a 32-bit decrementing counter defined by the
MPC555 / MPC556 to provide a decrementer exception after a programmable delay.
The DEC satisfies the following requirements:
• Loading a GPR from the DEC has no effect on the DEC.
• Storing a GPR to the DEC replaces the value in the DEC with the value in the
GPR.
• Whenever bit 0 of the DEC changes from zero to one, a decrementer exception
request (unless masked) is signaled. Multiple DEC exception requests may be re-
ceived before the first exception occurs; however, any additional requests are
canceled when the exception occurs for the first request.
• If the DEC is altered by software and the content of bit 0 is changed from zero to
one, an exception request is signaled.
• PORESET resets and stops the decrementer, HRESET/SRESET do not.
The decrementer frequency is based on a subdivision of the processor clock. A bit in
the system clock control register (SCCR) in the SIU determines the clock source of
both the decrementer and the time base. For details on the decrementer and time base
TB
— Time Base (Write Only)
SPR 284, 285
0
31 32
63
TBU
TBL
RESET: UNCHANGED
Table 3-14 Time Base Field Definitions (Write Only)
Bits
Name
Description
0:31
TBU
Time Base (Upper) — The high-order 32 bits of the time base
32:63
TBL
Time Base (Lower) — The low-order 32 bits of the time base
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