March 2002 Release
377
Virtex-II Pro™ Platform FPGA Documentation
1-800-255-7778
Branch and Flow-Control Instructions
R
System Trap
lists the PowerPC
system-trap
instructions. System-trap instructions are
normally used by software-debug applications to set breakpoints. These instructions test
for a specified set of conditions and cause a program exception to occur if any of the
conditions are met. If the tested conditions are not met, instruction execution continues
normally with the instruction following the system-trap instruction (a program exception
does not occur). The system-trap handler can be called from the program-interrupt handler
when it is determined that a system-trap instruction caused the exception. See
for more information on program exceptions caused by the
system-trap instructions.
Trap instructions can also be used to cause a debug exception. See
for more information.
Simplified mnemonics are defined for the system-trap instructions. See
The TO operand field in the system-trap instructions specifies the test conditions
performed on the remaining two operands. Multiple test conditions can be set
simultaneously, expanding the number of possible conditions that can cause the trap
(program exception). If all bits in the TO operand field are set, the trap always occurs
because one of the trap conditions is always met. The bits within the TO field are defined
as shown in
.
Table 3-12:
System-Trap Instructions
Mnemonic
Name
Operation
Operand
Syntax
tw
Trap Word
The contents of
r
A are compared with
r
B. A
program exception occurs if the comparison meets
any test condition enabled by the TO operand.
TO,
r
A,
r
B
twi
Trap Word Immediate
The contents of
r
A are compared with the sign-
extended SIMM operand. A program exception
occurs if the comparison meets any test condition
enabled by the TO operand.
TO,
r
A,SIMM
Table 3-13:
TO Field Bit Definitions
TO Bit
Description
TO[0]
Less-than arithmetic comparison.
0—Ignore trap condition.
1—Trap if first operand is arithmetically less-than second operand.
TO[1]
Greater-than arithmetic comparison.
0—Ignore trap condition.
1—Trap if first operand is arithmetically greater-than second operand.
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