Typewriter User Manual

5- 50 MC68340 USER’S MANUAL MOTOROLA
If an instruction is executed and an interrupt is pending on completion, the trace exception
is processed before the interrupt exception.
If an instruction forces an exception, the forced exception is processed before the trace
exception.
If an instruction is executed and a breakpoint is pending upon completion of the
instruction, the trace exception is processed before the breakpoint.
If an attempt is made to execute an illegal, unimplemented, or privileged instruction while
tracing is enabled, no trace exception will occur because the instruction is not executed.
This is particularly important to an emulation routine that performs an instruction function,
adjusts the stacked PC to beyond the unimplemented instruction, and then returns. The
SR on the stack must be checked to determine if tracing is on before the return is
executed. If tracing is on, trace exception processing must be emulated so that the trace
exception handler can account for the emulated instruction.
Tracing also affects normal operation of the STOP and LPSTOP instructions. If either
instruction begins execution with T1 set, a trace exception will be taken after the
instruction loads the SR. Upon return from the trace handler routine, execution will
continue with the instruction following STOP (LPSTOP), and the processor will not enter
the stopped condition.
5.5.2.11 INTERRUPTS. There are seven levels of interrupt priority and 192 assignable
interrupt vectors within each exception vector table. Careful use of multiple vector tables
and hardware chaining will permit a virtually unlimited number of peripherals to interrupt
the processor.
Interrupt recognition and subsequent processing are based on internal interrupt request
signals (
IRQ7–IRQ1) and the current priority set in SR priority mask I2–I0. Interrupt
request level zero (
IRQ7–IRQ1 negated) indicates that no service is requested. When an
interrupt of level one through six is requested via
IRQ6–IRQ1, the processor compares
the request level with the interrupt mask to determine whether the interrupt should be
processed. Interrupt requests are inhibited for all priority levels less than or equal to the
current priority. Level seven interrupts are nonmaskable.
IRQ7–IRQ1 are synchronized and debounced by input circuitry on consecutive rising
edges of the processor clock. To be valid, an interrupt request must be held constant for
at least two consecutive clock periods.
Interrupt requests do not force immediate exception processing, but are left pending. A
pending interrupt is detected between instructions or at the end of exception processing—
all interrupt requests must be held asserted until they are acknowledged by the CPU. If
the priority of the interrupt is greater than the current priority level, exception processing
begins.
Exception processing occurs as follows. First, the processor makes an internal copy of the
SR. After the copy is made, the processor state bits in the SR are changed—the S-bit is
set, establishing supervisor access level, and bits T1 and T0 are cleared, disabling
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