MPC555
/
MPC556
CENTRAL PROCESSING UNIT
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
3-19
3.7.7 Count Register (CTR)
The count register (CTR) is a 32-bit register for holding a loop count that can be dec-
remented during execution of branch instructions that contain an appropriately coded
BO field. If the value in CTR is 0 before being decremented, it is –1 afterward. The
count register provides the branch target address for the Branch Conditional to Count
Register (
bcctr
x
) instruction.
3.8 PowerPC VEA Register Set — Time Base
The PowerPC virtual environment architecture (VEA) defines registers in addition to
those in the UISA register set. The PowerPC VEA register set can be accessed by all
software with either user- or supervisor-level privileges.
The PowerPC VEA includes the time base facility (TB), a 64-bit structure that contains
a 64-bit unsigned integer that is incremented periodically. The frequency at which the
counter is updated is implementation-dependent. For details on the time base clock in
the MPC555 / MPC556, refer to
6.7 MPC555 / MPC556 Time Base (TB)
/ MPC556 Internal Clock Signals
, and
8.12.1 System Clock Control Register (SC-
.
The TB consists of two 32-bit registers: time base upper (TBU) and time base lower
(TBL). In the context of the VEA, user-level applications are permitted read-only ac-
cess to the TB. The OEA defines supervisor-level access to the TB for writing values
to the TB. Different SPR encodings are provided for reading and writing the time base.
In 32-bit PowerPC implementations such as the RCPU, it is not possible to read the
entire 64-bit time base in a single instruction. The
mftb
simplified mnemonic copies
the lower half of the time base register (TBL) to a GPR, and the
mftbu
simplified mne-
monic copies the upper half of the time base (TBU) to a GPR.
CTR
— Count Register
SPR 9
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
Loop Count
RESET: UNCHANGED
TB
— Time Base (Read Only)
SPR 268, 269
0
31 32
63
TBU
TBL
RESET: UNCHANGED
Table 3-11 Time Base Field Definitions (Read 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
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
.
..