Filter
Top Products
Programming Model 2-13
Adding the largest 32-bit twos-complement negative number, 0x8000 0000, to itself results in an
arithmetic overflow and 0x0000 0000 is recorded in the target register. CR[CR0]
EQ, SO
is set,
indicating a result of 0, but the infinitely precise result is negative.
The CR[CR0]
SO
subfield is a copy of XER[SO]. Instructions that do not alter the XER[SO] bit cannot
cause an overflow, but even for these instructions CR[CR0]
SO
is a copy of XER[SO].
Some instructions set CR[CR0] differently or do not specifically set any of the subfields. These
instructions include:
• Compare instructions
cmp, cmpi, cmpl, cmpli
• CR logical instructions
crand, crandc, creqv, crnand, crnor, cror, crorc, crxor, mcrf
• Move CR instructions
mtcrf, mcrxr
• stwcx.
The instruction descriptions provide detailed information about how the listed instructions alter
CR[CR0].
2.3.4 The Time Base
The PowerPC Architecture provides a 64-bit time base. “Time Base” on page 6-1 describes the
architected time base. Access to the time base is through two 32-bit time base registers (TBRs). The
least-significant 32 bits of the time base are read from the Time Base Lower (TBL) register and the
most-significant 32 bits are read from the Time Base Upper (TBU) register.
User-mode access to the time base is read-only, and there is no explicitly privileged read access to
the time base.
The mftb instruction reads from TBL and TBU. Writing the time base is accomplished by moving the
contents of a GPR to a pair of SPRs, which are also called TBL and TBU, using mtspr.
Table 2-4 shows the mnemonics and names of the TBRs.
2.3.5 Machine State Register (MSR)
The Machine State Register (MSR) controls processor core functions, such as the enabling or
disabling of interrupts and address translation.
The MSR is written from a GPR using the mtmsr instruction. The contents of the MSR can be read
into a GPR using the mfmsr instruction. MSR[EE] is set or cleared using the wrtee or wrteei
instructions.
Table 2-4. Time Base Registers
Mnemonic Register Name Access
TBL Time Base Lower (Read-only) Read-only
TBU Time Base Upper (Read-only) Read-only