MC68030 Datasheet, PDF (397/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Coprocessor Interface Description
The busy primitive should only be used in response to a write to the command or condition
CIR. It should be the first primitive returned after the main processor attempts to initiate a
general or conditional category instruction. In particular, the busy primitive should not be
issued after program-visible resources have been altered by the instruction. (Program-
visible resources include coprocessor and main processor program-visible registers and
operands in memory, but not the scanPC.) The restart of an instruction after it has altered
program-visible resources causes those resources to have inconsistent values when the
processor reinitiates the instruction.
The MC68030 responds to the busy primitive differently in a special case that can occur
during a breakpoint operation (refer to 8.1.12 Multiple Exceptions). This special case
occurs when a breakpoint acknowledge cycle initiates a coprocessor F-line instruction, the
coprocessor returns the busy primitive in response to the instruction initiation, and an
interrupt is pending. When these three conditions are met, the processor re-executes the
breakpoint acknowledge cycle after the interrupt exception processing has been completed.
A design that uses a breakpoint to monitor the number of passes through a loop by
incrementing or decrementing a counter may not work correctly under these conditions. This
special case may cause several breakpoint acknowledge cycles to be executed during a
single pass through a loop.
10.4.4 Null Primitive
The null coprocessor response primitive communicates coprocessor status information to
the main processor. This primitive applies to instructions in the general and conditional
categories. Figure 10-24 shows the format of the null primitive.
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
`
CA PC 0
0
1
0
0
IA
0
0
0
0
0
0 PF TF
Figure 10-24. Null Primitive Format
This primitive uses the CA and PC bits as previously described.
Bit [8], the IA bit, specifies the interrupts allowed optional operation. This bit determines
whether the MC68030 services pending interrupts prior to rereading the response CIR after
receiving a null primitive. Interrupts are allowed when the IA bit is set.
MOTOROLA
MC68030 USER’S MANUAL
10-37