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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
gx_01.fm.(1.2)
March 27,2006
PowerPC 750GX Overview
Page 43 of 377
The OEA defines numerous Special-Purpose Registers that serve a variety of functions, such as providing
controls, indicating status, configuring the processor, and performing special operations. During normal
execution, a program can access the registers shown in Figure 2-1 on page 58, depending on the program’s
access privilege (supervisor or user, determined by the privilege-level (PR) bit in the MSR). GPRs and FPRs
are accessed through operands that are defined in the instructions. Access to registers can be explicit (that
is, through the use of specific instructions for that purpose such as Move-to Special-Purpose Register
(mtspr) and Move-from Special-Purpose Register (mfspr) instructions) or implicit, as the part of the execu-
tion of an instruction. Some registers can be accessed both explicitly and implicitly.
In the 750GX, all SPRs are 32 bits wide. Table 1-2 describes the architecture-defined SPRs implemented by
the 750GX. In the PowerPC Microprocessor Family: The Programming Environments Manual, these registers
are described in detail, including bit descriptions. Section 2.1.1, Register Set, on page 57 describes how
these registers are implemented in the 750GX. In particular, that section describes those features defined as
optional in the PowerPC Architecture that are implemented on the 750GX.
Table 1-2. Architecture-Defined SPRs Implemented
(Page 1 of 2)
Register Level Function
LR User
The Link Register (LR) can be used to provide the branch target address and to hold
the return address after branch and link instructions.
BATs Supervisor
The architecture defines eight Block Address Translation Registers (BATs), each imple-
mented as a pair of 32-bit SPRs. In the 750GX, the BAT facility has been extended to
include 16 BATs (32 total SPRs), eight for instruction translation and eight for data
translation. BATs are used to define and configure blocks of memory.
CTR User
The Count Register (CTR) is decremented and tested by branch-and-count instruc-
tions.
DABR Supervisor
The optional
Data Address Breakpoint Register (DABR) supports the data address
breakpoint facility.
DAR User
The Data Address Register (DAR) holds the address of an access after an alignment or
data-storage interrupt (DSI) exception.
DEC Supervisor
The Decrementer Register (DEC) is a 32-bit decrementing counter that provides a way
to schedule time-delayed exceptions.
DSISR User
The Data Storage Interrupt Status Register (DSISR) defines the cause of data access
and alignment exceptions.
EAR Supervisor
The External Access Register (EAR) controls access to the external access facility
through the External Control In Word Indexed (eciwx) and External Control Out Word
Indexed (ecowx) instructions.
PVR Supervisor
The Processor Version Register (PVR) is a read-only register that identifies the proces-
sor version and revision level.
SDR1 Supervisor
Storage Description Register 1 (SDR1) specifies the page table address and size used
in virtual-to-physical page-address translation.
SRR0 Supervisor
The Machine Status Save/Restore Register 0 (SRR0) saves the address used for
restarting an interrupted program when an rfi instruction executes (also known as
exceptions).
SRR1 Supervisor
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.