
MOTOROLA
Chapter 4. Exceptions
4-31
4.5.8 Floating-Point Unavailable Exception (0x00800)
A floating-point unavailable exception occurs when no higher priority exception exists, an
attempt is made to execute a floating-point instruction (including floating-point load, store,
and move instructions), and the floating-point available bit in the MSR is disabled
(MSR[FP] = 0); note that on the EC603e microprocessor, the MSR[FP] is always cleared
to 0. Register settings for this exception are described in Chapter 6, “Exceptions,” in
The
Programming Environments Manual
When a floating-point unavailable exception is taken, instruction execution for the handler
begins at offset 0x00800 from the physical base address indicated by MSR[IP].
4.5.9 Decrementer Exception (0x00900)
The 603e implements the decrementer interrupt exception as it is defined in the PowerPC
architecture. A decrementer exception request is made when the decrementer counts down
through zero. The request is held until there are no higher priority exceptions and
MSR[EE] = 1. At this point the decrementer exception is taken. If multiple decrementer
exception requests are received before the first can be reported, only one exception is
reported. The occurrence of a decrementer exception cancels the request. Register settings
for this exception are described in Chapter 6, “Exceptions,” in
The Programming
Environments Manual.
When a decrementer exception is taken, instruction execution for the handler begins at
offset 0x00900 from the physical base address indicated by MSR[IP].
4.5.10 System Call Exception (0x00C00)
The 603e implements the system call exception as it is defined by the PowerPC
architecture. A system call exception request is made when a system call (
sc
) instruction is
completed. If no higher priority exception exists, the system call exception is taken, with
SRR0 being set to the EA of the instruction following the
sc
instruction. Register settings
for this exception are described in Chapter 6, “Exceptions,” in
The Programming
Environments Manual.
When a system call exception is taken, instruction execution for the handler begins at offset
0x00C00 from the physical base address indicated by MSR[IP].