
MOTOROLA
Chapter 2. Register Model
2-9
Register Set
SPRG0–SPRG3 defined in Section 2.1.2.11, “SPRG4–SPRG7 (G2_LE
Only).”
– DSISR. The DSISR defines the cause of data access and alignment
exceptions.
– Machine status save/restore register [0–1] (SRR0, SRR1). The SRR0 and
SRR1 are used to save machine status on exceptions and to restore machine
status when an
rfi
instruction is executed.
Implementation Note
—The G2 core implements the KEY bit (bit 12) in the
SRR1 register to simplify the table search software. For more information
refer to Chapter 6, “Memory Management.”
Note that to support critical interrupts, two new registers, CSRR0 and CSRR1,
are implemented on the G2_LE core, which are not defined by the PowerPC
architecture. These registers have same bit assignments as SRR0 and SRR1,
and are described in Section 2.1.2, “Implementation-Specific Registers.”
— Miscellaneous registers
– The time base facility (TB) for writing. The TB is a 64-bit register pair that
can be used to provide time-of-day or interval timing. It consists of two 32-bit
registers—time base upper (TBU) and time base lower (TBL). The TB is
incremented once every four clock cycles on the core.
– Decrementer (DEC). The DEC register is a 32-bit decrementing counter that
provides a mechanism for causing a decrementer exception after a
programmable delay. The DEC is decremented once every four bus clock
cycles.
– External access register (EAR). The EAR is a 32-bit register used in
conjunction with the
eciwx
and
ecowx
instructions. Although the PowerPC
architecture specifies that EAR26–EAR31 are used to select a device, the G2
core implements only bits 28–31. Note that EAR and the
eciwx
and
ecowx
instructions are optional in the PowerPC architecture and may not be
supported in all processors that implement the OEA.
2.1.2
Implementation-Specific Registers
The G2 core defines the DMISS, IMISS, DCMP, ICMP, HASH1, HASH2, and RPA
registers for software table search operations. These registers should be accessed only when
address translation is disabled (MSR[IR] and MSR[DR] are both zero). For a complete
discussion, refer to Section 6.5.2, “Implementation-Specific Table Search Operation.”
Also, HID0, HID1, and IABR SPRs are defined and described in this section. These
registers can be accessed by supervisor-level instructions only using the SPR numbers
shown in Figure 2-1.
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