MOTOROLA
MC68302 USER’S MANUAL
2-1
SECTION 2
MC68000/MC68008 CORE
The MC68302 integrates a high-speed M68000 processor with multiple communications pe-
ripherals. The provision of direct memory access (DMA) control and link layer management
with the serial ports allows high throughput of data for communications-intensive applica-
tions, such as basic rate Integrated Services Digital Network (ISDN).
The MC68302 can operate either in the full MC68000 mode with a 16-bit data bus or in the
MC68008 mode with an 8-bit data bus by tying the bus width (BUSW) pin low. UDS/A0 func-
tions as A0 and LDS/DS functions as DS in the MC68008 mode.
NOTE
The BUSW pin is static and is not intended to be used for dy-
namic bus sizing. If the state of BUSW is changed during oper-
ation of the MC68302, erratic operation may occur.
Refer to the MC68000UM/AD,
complete details of the on-chip microprocessor. Throughout this manual, references may
use the notation M68000, meaning all devices belonging to this family of microprocessors,
or the notation MC68000, MC68008, meaning the specific microprocessor products.
M68000 8-/16-/32-Bit Microprocessors User's Manual
, for
2.1 PROGRAMMING MODEL
The M68000 microprocessor executes instructions in one of two modes: user or supervisor.
The user mode provides the execution environment for most of the application programs.
The supervisor mode, which allows some additional instructions and privileges, is intended
for use by the operating system and other system software.
Shown in Figure 2-1, the M68000 core programming model offers 16, 32-bit, general-pur-
pose registers (D7–D0, A7–A0), a 32-bit program counter (PC), and an 8-bit condition code
register (CCR) when running in user space. The first eight registers (D7–D0) are used as
data registers for byte (8-bit), word (16-bit), and long-word (32-bit) operations. The second
set of seven registers (A6–A0) and the stack pointer (USP in user space) may be used as
software stack pointers and base address registers. In addition, the address registers may
be used for word and long-word operations. All 16 registers may be used as index registers.