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User’s Manual
IBM PowerPC 750GX and GL RISC Microprocessor
gx_04.fm.(1.2)
March 27, 2006
Exceptions
Page 153 of 377
4.2 Exception Recognition and Priorities
Exceptions are roughly prioritized by exception class, as follows.
1. Nonmaskable, asynchronous exceptions have priority over all other exceptions. These are system reset
and machine-check exceptions (although the machine-check exception condition can be disabled so the
condition causes the processor to go directly into the checkstop state). These exceptions cannot be
delayed and do not wait for completion of any precise-exception handling.
2. Synchronous, precise exceptions are caused by instructions and are taken in strict program order.
3. Imprecise exceptions (imprecise mode floating-point enabled exceptions) are caused by instructions, and
they are delayed until higher-priority exceptions are taken. Note that the 750GX does not implement an
exception of this type.
4. Maskable asynchronous exceptions (external, decrementer, thermal-management, system-management,
performance-monitor, and interrupt exceptions) are delayed if higher-priority exceptions are taken.
The following list of exception categories describes how the 750GX handles exceptions up to the point of
signaling the appropriate interrupt to occur. Note that a recoverable state is reached if the completed store
queue is empty (drained, not cancelled), and the instruction that is next in program order has been signaled to
complete and has completed. If MSR[RI] = 0, the 750GX is in a nonrecoverable state. Also, instruction
completion is defined as updating all architectural registers associated with that instruction, and then
removing that instruction from the completion buffer.
Program 00700 As defined by the PowerPC Architecture (for example, an instruction opcode error).
Floating-point unavail-
able
00800
As defined by the PowerPC Architecture. MSR[FP] = 0, and a floating-point instruction is
executed.
Decrementer 00900
As defined by the PowerPC Architecture, when the most-significant bit of the Decre-
menter Register (DEC) changes from 0 to 1, and MSR[EE] = 1.
Reserved 00A00–00BFF —
System call 00C00 Execution of the System Call (sc) instruction.
Trace 00D00
MSR[SE] = 1, or a branch instruction is completing and MSR[BE] = 1. The 750GX differs
from the OEA by not taking this exception on an Instruction Synchronize (isync) instruc-
tion.
Reserved 00E00
The
750GX does not generate an exception to this vector. Other PowerPC processors
might use this vector for floating-point assist exceptions.
Reserved 00E10–00EFF —
Performance monitor 00F00 The limit specified in PMCn is met and MMCR0[ENINT] = 1 (750GX-specific).
Instruction address
breakpoint
01300
IABR[0–29] matches EA[0–29] of the next instruction to complete, IABR[TE] matches
MSR[IR], and IABR[BE] = 1 (750GX-specific).
System management
exception
01400
A system management exception is enabled if MSR[EE] = 1, and is signaled to the
750GX by the assertion of an input signal pin, the system management interrupt (SMI
).
Reserved 01500–016FF —
Thermal-management
interrupt
01700
Thermal management is enabled, junction temperature exceeds the threshold specified in
THRM1 or THRM2, and MSR[EE] = 1 (750GX-specific).
Reserved 01800–02FFF —
Table 4-2. Exceptions and Conditions (Page 2 of 2)
Exception Type
Vector Offset
(hex)
Causing Conditions