Decrementer Register (Dec); Machine Status Save/Restore Register 0 (Srr0) - Motorola MPC533 Reference Manual

OEA Register Set
3.9.5

Decrementer Register (DEC)

The decrementer register (DEC), SPR 22, is decrementing counter that provides a
decrementer exception after a programmable delay. The DEC satisfies the following
requirements:
• Remains unaffected when a GPR is loaded from the DEC
• Replaces the value in the DEC with the value in the GPR when a GPR is stored in
the DEC
• Signals a decrementer exception request (unless masked) whenever bit 0 of the DEC
changes from zero to one. Multiple DEC exception requests may be received before
the first exception occurs; however, any additional requests are canceled when the
exception occurs for the first request
• Signals an exception request if the DEC is altered by software and the content of bit
0 is changed from zero to one
• Stops counting and clears the register with PORESET (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 clock, refer to
Section 6.1.6, "Decrementer (DEC)," Section 8.5, "Internal Clock Signals," and
Section 8.11.1, "System Clock Control Register (SCCR)."
The DEC does not run after power-up and must be enabled by setting the TBE bit in the
TBSCR register, see Table 6-18. Power-on reset stops its counting and clears the register.
A decrementer exception may be signaled to software prior to initialization.
MSB 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
0
Field
PORESET
Reset
Addr
3.9.6

Machine Status Save/Restore Register 0 (SRR0)

The machine status save/restore register 0 (SRR0), SPR 26, identifies where instruction
execution should resume when an rfi instruction is executed following an exception. It also
holds the effective address of the instruction that follows the system call (sc) instruction.
3-26
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
Decrementing Counter
0000_0000_0000_0000_0000_0000_0000_0000
Figure 3-16. DEC — Decrementer Register
MPC533 Reference Manual
Unchanged
SPR 22
31
MOTOROLA