User’s Manual
Preliminary PPC440x5 CPU Core
timers.fm.
September 12, 2002 Page 211 of 589
lwz Ry, lower
li Rz, 0 # set GPR Rz to 0
mtspr TBL,Rz # force TBL to 0 (thereby preventing wrap into TBU)
mtspr TBU,Rx # set TBU to initial value
mtspr TBL,Ry # set TBL to initial value
7.2 Decrementer (DEC)
The DEC is a 32-bit privileged SPR that decrements at the same rate that the time base increments. The
DEC is read and written using mfspr and mtspr, respectively. When a non-zero value is written to the DEC,
itbegins to decrement with the next time base clock. A Decrementer exception is signalled when a decrement
occurs on a DEC count of 1, and the Decrementer Interrupt Status field of the Timer Status Register
(TSR[DIS]; see Page 216) is set. A Decrementer interrupt will occur if it is enabled by both the Decrementer
Interrupt Enable field of the Timer Control Register (TCR[DIE]; see Page 215) and by the External Interrupt
Enable field of the Machine State Register (MSR[EE]; see Machine State Register (MSR) on page165).
Interrupts and Exceptions on page 159 provides more information on the handling of Decrementer interrupts.
The Decrementer interrupt handler software should clear TSR[DIS] before re-enabling MSR[EE], in order to
avoid another Decrementer interrupt due to the same exception (unless TCR[DIE] is cleared instead).
The behavior of the DEC itself upon a decrement from a DEC value of 1 depends on which of two modes it is
operating in -- normal, or auto-reload. The mode is controlled by the Auto-Reload Enable (ARE) field of the
TCR. When operating in normal mode (TCR[ARE]=0), the DEC simply decrements to the value 0 and then
stops decrementing until it is re-initialized by software.
When operating in auto-reload mode (TCR[ARE]=1), however, instead of decrementing to the value 0, the
DEC is reloaded with the value in the Decrementer Auto-Reload (DECAR) register (see Figure7-5 on
page 212), and continues to decrement with the next time base clock (assuming the DECAR value was non-
zero). The DECAR register is a 32-bit privileged, write-only SPR, and is written using mtspr.
The auto-reload feature of the DEC is disabled upon reset, and must be enabled by software.
Figure 7-4 illustrates the DEC.
Figure 7-5 illustrates the DECAR.
Figure 7-4.Decrementer (DEC)
0:31 Decrement value
0 31