
Development Support
MOTOROLA
MPC801 USER’S MANUAL
18-3
18
NOTE
To keep the pin count of the chip as low as possible, the VSYNC
signal is not implemented as one of the chip’s external pins.
Instead, it is asserted and negated using the serial interface
implemented in the development port. For more information on
this interface, refer to
Section 18.3.3 The Development Port
Forcing the core to show all fetch cycles marked with the
program trace cycle attribute can be accomplished by either
asserting the VSYNC signal (as mentioned above) or by
programming the fetch show cycle bits in the instruction support
control register (ICTRL). For more information refer to
18.1.2 Controlling the Instruction Fetch Show Cycle
.
Section
.
When the VSYNC signal is asserted, all fetch cycles marked with the program trace cycle
attribute become visible on the external bus. These cycles generate regular bus cycles when
the instructions reside in one of the external devices or generate address-only cycles when
the instructions are in one of the internal devices. When VSYNC is asserted, some
performance degradation occurs because of the additional external bus cycles. Since this
performance degradation is expected to be very small, it is possible to program the machine
to show all indirect flow changes, perform these additional external bus cycles, and maintain
the same behavior when VSYNC is asserted and negated. For more information see
Table 18-18.
The status pins are divided into two groups:
VF [0 . . 2]
Instruction Queue Status—Denotes the type of the last fetched instruction or how many
instructions were flushed from the instruction queue. These status pins are used for
both functions because queue flushes only happen in clocks where there is no fetch
type information to be reported. See Table 18-1 for the definition of possible instruction
types.
— Possible instruction queue flushes:
000–None
001–1 instruction was flushed from the instruction queue
010–2 instructions were flushed from the instruction queue
011–3 instructions were flushed from the instruction queue
100–4 instructions were flushed from the instruction queue
101–5 instructions were flushed from the instruction queue
110–Reserved
111–Like VF = ‘111’