ZiLOG System 8000 Hardware Reference Manual page 178

HRM Zilog
HRM
breakpoint suspends execution of the user's program and
saves all registers, program counters (PC), and the Flag
Control Word (FCW) in the memory area provided. It then
displays a message reporting the break and the address where
it occurred.
Any number of breakpoints can be set manually by setting the
instruction at the desired breakpoint address to %7FOO
(%
indicates the address is in hexadecimal notation). This
interrupts the executing program and jumps (traps) to the
breakpoint procedure. When the breakpoint is no longer
required, the original instruction must be manually
restored.
The BREAK command saves the address where the breakpoint is
being set and the instruction that it is replacing. When
the breakpoint is cleared, the instruction is automatically
restored. The BREAK command also stores a repetition
counter, n. Execution is not suspended until the nth time
this breakpoint is encountered unless another breakpoint is
encountered first.
The following restrictions on the user program are necessary
to set breakpoints:
1. This program must be able to execute with interrupts
enabled after encountering
~he
breakpoint.
2. The program should not be timing-dependent because
there will be some timing distortion each time the
breakpoint is encountered.
3.
The user program must not use Channel
3
of the Z80A
Counter Timer Circuit (CTC #0), because it is used to
implement the multiple execution feature.
4. The breakpoint cannot be within an interrupt procedure
entered by an interrupt from Channels 0 through 2 of
the Z80B CTC #0.
The BREAK and the NEXT commands us~ instruction modification
and the interrupt system. Therefore, the program being
debugged cannot be in the PROM area and cannot involve
modification of the interrupt status.
Any set breakpoints must be cleared
by
hitting RESET before
a new program is loaded from the System 8000; otherwise,
previously set breakpoints continue to operate on the new
program during debugging.
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Zilog
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