Power ISA™ Version 2.03 - Power.org

Version 2.03
Programming Note
Since the update frequency of the Time Base is implementation-dependent,
the algorithm for converting the
current value in the Time Base to time of day is also
implementation-dependent.
As an example, assume that the Time Base is incremented
at a constant rate of once for every 32 cycles of
a 1 GHz CPU instruction clock. What is wanted is the
pair of 32-bit values comprising a POSIX standard
clock: 1 the number of whole seconds that have passed
since 00:00:00 January 1, 1970, UTC, and the remaining
fraction of a second expressed as a number of
nanoseconds.
Assume that:
The value 0 in the Time Base represents the start
time of the POSIX clock (if this is not true, a simple
64-bit subtraction will make it so).
The integer constant ticks_per_sec contains the
value
1 GHz
------------------
32
= 31,250,000
which is the number of times the Time Base is
updated each second.
The integer constant ns_adj contains the value
1,000,000,000
------------------------------------- = 32
31,250,000
which is the number of nanoseconds per tick of the
Time Base.
When the processor is in 64-bit mode, the POSIX clock
can be computed with an instruction sequence such as
this:
mfspr Ry,268 # Ry = Time Base
lwz Rx,ticks_per_sec
divd Rz,Ry,Rx# Rz = whole seconds
stw
mulld
sub
lwz
mulld
stw
Rz,posix_sec
Rz,Rz,Rx# Rz = quotient × divisor
Rz,Ry,Rz# Rz = excess ticks
Rx,ns_adj
Rz,Rz,Rx# Rz = excess nanoseconds
Rz,posix_ns
For the Embedded environment when the processor is
in 32-bit mode, it is not possible to read the Time Base
using a single instruction. Instead, two instructions
must be used, one of which reads TBL and the other of
which reads TBU. Because of the possibility of a carry
from TBL to TBU occurring between the two reads, a
sequence such as the following must be used to read
the Time Base.
loop:
mfspr Rx,TBU # load from TBU
mfspr Ry,TB # load from TB
mfspr Rz,TBU # load from TBU
cmp cr0,0,Rx,Rz# check if ‘old’=’new’
bne loop #branch if carry occurred
Non-constant update frequency
In a system in which the update frequency of the Time
Base may change over time, it is not possible to convert
an isolated Time Base value into time of day.
Instead, a Time Base value has meaning only with
respect to the current update frequency and the time of
day that the update frequency was last changed. Each
time the update frequency changes, either the system
software is notified of the change via an interrupt (see
Book III), or the change was instigated by the system
software itself. At each such change, the system software
must compute the current time of day using the
old update frequency, compute a new value of
ticks_per_sec for the new frequency, and save the time
of day, Time Base value, and tick rate. Subsequent
calls to compute Time of Day use the current Time
Base Value and the saved value.
1. Described in POSIX Draft Standard P1003.4/D12, Draft Standard for Information Technology -- Portable Operating System Interface (POSIX) --
Part 1: System Application Program Interface (API) - Amendment 1: Real-time Extension [C Language]. Institute of Electrical and Electronics Engineers,
Inc., Feb. 1992.
Chapter 4. Time Base
371