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G2 PowerPC Core Reference Manual
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MOTOROLA
Implementation-Specific Information
breakpoint control (DBCR), a new instruction breakpoint register (IABR2), and two data
address breakpoint registers (DABR and DABR2) are added to the G2_LE core.
In the G2 core, all SPRs are 32 bits wide.
1.3.1.7.1
User-Level SPRs
The following SPRs are accessible by user-level software:
Link register (LR)—The LR can be used to provide the branch target address and to
hold the return address after branch and link instructions. The LR is 32 bits wide in
32-bit implementations.
Count register (CTR)—The CTR is decremented and tested automatically as a result
of branch-and-count instructions. The CTR is 32 bits wide in 32-bit
implementations.
XER register—The 32-bit XER contains the summary overflow bit, integer carry
bit, overflow bit, and a field specifying the number of bytes to be transferred by a
Load String Word Indexed (
lswx
) or Store String Word Indexed (
stswx
) instruction.
1.3.1.7.2
Supervisor-Level SPRs
The core also contains SPRs that can be accessed only by supervisor-level software. These
registers consist of the following:
The DSISR defines the cause of data access and alignment exceptions. The cause of
a DSI exception for a data breakpoint (match with DABR and DABR2) can be
determined by the value of the DSISR[DABR] bit (bit 9).
The data address register (DAR) holds the address of an access after an alignment
or DSI exception. For example, it contains the address of the breakpoint match
condition.
The decrementer register (DEC) is a 32-bit decrementing counter that provides a
mechanism for causing a decrementer exception after a programmable delay.
SDR1 specifies the page table format used in virtual-to-physical address translation
for pages. (Note that physical address is referred to as real address in the architecture
specification.)
The machine status save/restore register 0 (SRR0) is used for saving the address of
the instruction that caused the exception, and the address to return to when a Return
from Interrupt (
rfi
) instruction is executed.
The machine status save/restore register 1 (SRR1) is used to save machine status on
exceptions and to restore machine status when an
rfi
instruction is executed.
The SPRG0–SPRG3 registers are provided for operating system use, which reduce
the latency that may be incurred because of saving registers to memory while in a
handler. Note that G2_LE implements four additional SPRGs.
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