Motorola MC68020 User Manual

MOTOROLA M68020 USER’S MANUAL 9- 15
Another way to optimize the CPU-to-memory access times in a system is to use a clock
frequency less than the rated maximum of the specific MC68020/EC020 device. Table 9-5
provides calculated t
AVDV
(see Equation 9-7 of Table 9-4) results for a 16 MHz
MC68020/EC020 and a 25 MHz MC68020/EC020 operating at various clock frequencies.
If the system uses other clock frequencies, the above equations can be used to calculate
the exact access times.
Table 9-5. Calculated t
AVDV
Values for Operation at Frequencies
Less Than or Equal to the CPU Maximum Frequency Rating
Equation 9-7 t
AVDV
16-MHz MC68020/EC020 25-MHz MC68020/EC020
Clocks Per (N) and Type
Bus Cycle
Wait
States
Clock at
12.5 MHz
Clock at
16.67 MHz
Clock at
16.67 MHz
Clock at
20 MHz
Clock at
25 MHz
3 Clock Asynchronous 0 181 121 131 101 71
4 Clock Asynchronous 1 261 181 191 151 111
5 Clock Asynchronous 2 341 241 251 201 151
6 Clock Asynchronous 3 421 301 311 251 191
9.7 MODULE SUPPORT
The MC68020/EC020 includes support for modules with the CALLM and RTM
instructions. The CALLM instruction references a module descriptor. This descriptor
contains control information for entry into the called module. The CALLM instruction
creates a module stack frame and stores the current module state in that frame and loads
a new module state from the referenced descriptor. The RTM instruction recovers the
previous module state from the stack frame and returns to the calling module.
The module interface facilitates finer resolution of access control by external hardware.
Although the MC68020/EC020 does not interpret the access control information, it
communicates with external hardware when the access control is to be changed and
relies on the external hardware to verify that the changes are legal.
9.7.1 Module Descriptor
Figure 9-10 illustrates the format of the module descriptor. The first long word contains
control information used during execution of the CALLM instruction. The remaining
locations contain data that can be loaded into processor registers by the CALLM
instruction.

Motorola MC68020 Specifications

General

Brand	Motorola
Model	MC68020
Category	Computer Hardware
Language	English