Page 382 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

12.4 The 68300 Series 359

Figure 12.8. Register Set of 68300 Series Microcomputers

allows X to be set specifically for multiple-precision arithmetic and lets C be set for
more instructions (such as MOVE) to facilitate testing using instructions (such as BLS).
The high byte of the status register contains a bit, S, that distinguishes the mode as user
or system. When it is set, the program uses the system stack pointer whenever it uses
address register 7; and when it is clear, the program uses the user stack pointer whenever
it uses address register 7. Further, several instructions can only be executed when the
program is in the system mode (S = 1), and hardware can be built so that some memory
or I/O devices may be accessed only when the program is in the system mode. This
permits the writing of secure operating systems that can have multiple users in a time-
sharing system, so that the users cannot accidentally or maliciously damage each other.
The 68300 series memory is organized as shown in Figure 12.9a. The 16-bit-wide
memory is actually addressed as an 8-bit memory, so that a 16-bit word (the unit of
memory read or written as a whole) is logically two consecutive locations. Instructions