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User’s Manual
IBM PowerPC 750GX and GL RISC Microprocessor
gx_04.fm.(1.2)
March 27, 2006
Exceptions
Page 177 of 377
4.5.21 Timer Facilities
At power-on reset (POR), the 750GX initializes the Time Base and Decrementer Registers to the following
values:
• Time Base Upper Register (TBU) = 0x00000000
• Time Base Lower Register (TBL) = 0x00000000
• Decrementer Register (DEC) = 0xFFFFFFFF
4.5.22 External Access Instructions
The 750GX implements the eciwx and ecowx instructions. Executing these instructions while MSR[DR] = 0
is considered a programming error, and the physical address on the bus is undefined. Executing these
instructions to a direct-store (T = 1) segment causes a data-storage exception (DSI).
The 750GX implements the External Access Register (EAR) to support the external access instructions. Bit 0
implements the Enable bit. Bits 1 to 25 are reserved. Bits 26 and 27 are not implemented and are reserved.
Bits 28 to 31 are the implemented bits of the Resource ID (RID).
Instruction Fetch ISI
Once this type of exception is detected, dispatch is halted and
the current instruction stream is allowed to drain out of the
machine. If completing any of the instructions in this stream
causes an exception, that exception is taken and the instruc-
tion fetch exception is forgotten. Otherwise, once the machine
is empty and a recoverable state is reached, the instruction
fetch exception is taken.
Instruction Dispatch/Execution
Program, DSI, Alignment, FPA,
SC, IABR, DABR
This type of exception is determined at dispatch or execution of
an instruction. The exception remains pending until all instruc-
tions in program order before the exception-causing instruction
are completed. The exception is then taken without completing
the exception-causing instruction. If any other exception condi-
tion is created in completing these previous instructions in the
machine, that exception takes priority over the pending Instruc-
tion Dispatch/Execution exception, which is then forgotten.
Post Instruction Execution Trace
This type of exception is generated following execution and
completion of an instruction while a trace mode is enabled. If
executing the instruction produces conditions for another type
of exception, that exception is taken and the Post Instruction
Execution exception is forgotten for that instruction.
Table 4-14. Front-End Exception Handling Summary (Page 2 of 2)
Exception Type Specific Exception Description