Page 305 - Embedded Microprocessor Systems Real World Design

Levels 1 through 3 have fewer privileges. The essential point is that the MMU
will not permit any memory access that is off-limits to a program at a given privi-
lege level. A memory segment can be set so that it is read-only for levels 1 and
below. A program at privilege level 0 can write to that segment, but a program at
level 1,2, or 3 can only read it. Other registers in the MMU control things like what
privilege level is permitted to disable interrupts or to modify the MMU registers.

Motorola
The Motorola memory management scheme on the 68060 is different from Intel’s,
but the result is the same-a table is used to translate a logical address to a physical
address. The 68060 has seven key MMU registers. One register points to a descrip
tor table for the supervisor level and one register points to the user descriptor table.
One register controls various functions like page size (4K or 8K) , and four registers
provide translation information for code and data (two registers each).

Exception Handling

What happens when a program tries to write to read-only memory or disable inter-
rupts when its privilege level is not high enough? When this happens, an exception
is generated by the MMU. An exception is similar to an interrupt and handled
much the same way. Exceptions are not disabled by disabling interrupts, although
the MMU can be programmed not to generate exceptions. The exception handler,
part of the operating system, decides what to do if an illegal operation is attempted.

Application-Specific Microcontrollers

Traditionally, microcontrollers have been general-purpose devices, with port pins,
timers, and other features that the designer could program for a specific applica-
tion. Some newer microcontrollers are targeted at specific markets with specialized
interfaces or other 1/0 features. A few brief examples follow:
The Microchip rfPIC12C509AG/509AF is a PIGfamily microcontroller with a 310
to 480MHz RF transmitter on-chip.
Most microcontrollers have primarily digital I/O. Some devices provide limited
analog 1/0 capability with ADCs or onchip comparators, but most of the 1/0
pins are still digital. The Microchip PIC16C781/’782 microcontrollers turn this
around; these devices are designed as programmable analog controllers and
include an %channel, &bit ADC, an &bit DAC, a programmable opamp, two
programmable analog comparators, and a PWM output module.
Other microcontrollers include onchip USB interfaces, LCD controllers, and
in-CAN bus interfaces. All of these devices are intended to provide a low-cost
solution to a specific class of design problems.

286 EmWd Micr@rocessw Systems