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Address Wrap-Around
The 80186/80188 microprocessor represents a memory address as a selector:offset pair. The
selector and the offset are each 16 bits long. A memory address in the break/trace board is a
20-bit address.
As shown in Table 4-2, the difference in memory address lengths causes discrepancies when
wrap-arounds occur.
Wrap-arounds do not affect bus information, but they can make break and trace information
hard to follow. Avoid wrap-arounds by not executing instructions near segment boundaries.
Break Information
A break normally occurs immediately after the target instruction executes. The following sec
tions describe the three cases where a break is not recognized until one or more instructions
after the breakpoint.
Slipping Past Instruction Breakpoints
A break sometimes slips past the specified breakpoint because the probe emulates at full proc
essor speed and PICE probe hardware cannot always break on the exact instruction specified.
An extra instruction is executed when the number of bytes in the target instruction and the
number of cycles required to execute that instruction match. For instance, a two-byte instruc
tion that executes in two bus cycles causes the PICE probe to slip. Other combinations of
instruction bytes and bus cycles can also cause a breakpoint to slip. In general, the greater the
number of cycles required to execute an instruction, the lower the chances of slipping.
The newest trace frame contains the last instruction executed. The break message contains the
address of the next instruction to be executed.
Slipping Past Breakpoints on Combined Instructions
Although you can specify a breakpoint between parts of combined instructions, the PICE
hardware never detects it. The following combined instructions cause slipping:
•
Repeat prefixes
•
LOCK prefixes
•
Segment override prefixes
•
MOV to a segment register
•
POP a segment register
Breaking in the Middle of an Instruction
In two cases the PICE probe can break in the middle of an instruction: with the WAIT com
mand and with repeated string command instructions. These commands contain wait test cycles
or primitive operations that can be recognized, incorrectly, as a breakpoint.
The PICE™ System Personality Modules (Probes)
4 -9
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