
MOTOROLA
Chapter 1. Overview
1-7
Overview
1.1.2.1
True Little-Endian Mode
True little-endian mode is supported in the G2_LE core to minimize the impact on software
porting from true little-endian systems. The true little-endian mode applies for all
instruction fetches and data load and store operations to and from memory. The G2_LE
powers up in one of two endian modes, big-endian mode or true little-endian mode, selected
by the core_tle signal at the negation of core_hreset. Like all the mode control signals, the
state of core_tle is captured at the negation of core_hreset. The state of MSR[ILE],
MSR[LE], and HID2[LET] are set to the value that is dictated by core_tle. The endian mode
should be set at the negation of core_hreset and should remain unchanged by software for
the duration of the system operation.
Bit 4 of HID2, (HID2[LET]) is used in conjunction with MSR[LE] to indicate the endian
mode of operation of the G2_LE core as shown in Table 1-1.
1.1.2.2
Critical Interrupt
A new input interrupt signal, core_cint, is provided to trigger the critical interrupt exception
on the G2_LE core. This asynchronous exception uses vector offset 0x00A00. MSR[CE] is
allocated for enabling the critical interrupt, and a new instruction, Return from Critical
Interrupt (
rfci
), is implemented to return from these exception handlers. Also, two new
registers, CSRR0 and CSRR1, are used to save and restore the processor state for critical
interrupts.
Table 1-2 shows the bit allocation of MSR[CE], which enables and disables the critical
interrupt.
1.1.2.3
Other New Signals
There are four additional signals that support the breakpoint state outputs (core_iabr,
core_iabr2, core_dabr, and core_dabr2) and one additional watchpoint debug feature
(core_tdo_oe).
Table 1-1. Endian Mode Indication
MSR[LE]
HID2[LET]
Endian Mode
0
x
Big-endian
1
0
Modified (PowerPC) little-endian
1
1
True little-endian
Table 1-2. Critical Interrupt Enabling Bit
MSR[CE]
Mode
24
0 Disables critical interrupt
1 Enables critical interrupt
F
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