Programming Model - Motorola MC68020 User Manual

1.2 PROGRAMMING MODEL

The programming model of the MC68020/EC020 consists of two groups of registers, the
user model and the supervisor model, that correspond to the user and supervisor privilege
levels, respectively. User programs executing at the user privilege level use the registers
of the user model. System software executing at the supervisor level uses the control
registers of the supervisor level to perform supervisor functions.
As shown in the programming models (see Figures 1-2 and 1-3), the MC68020/EC020
has 16 32-bit general-purpose registers, a 32-bit PC two 32-bit SSPs, a 16-bit SR, a 32-bit
VBR, two 3-bit alternate function code registers, and two 32-bit cache handling (address
and control) registers.
The user programming model remains unchanged from earlier M68000 family
microprocessors. The supervisor programming model supplements the user programming
model and is used exclusively by MC68020/EC020 system programmers who utilize the
supervisor privilege level to implement sensitive operating system functions. The
supervisor programming model contains all the controls to access and enable the special
features of the MC68020/EC020. All application software, written to run at the
nonprivileged user level, migrates to the MC68020/EC020 from any M68000 platform
without modification.
Registers D7–D0 are data registers used for bit and bit field (1 to 32 bits), byte (8 bit),
word (16 bit), long-word (32 bit), and quad-word (64 bit) operations. Registers A6–A0 and
the USP, ISP, and MSP are address registers that may be used as software stack
pointers or base address registers. Register A7 (shown as A7 in Figure 1-2 and as A7'
and A7" in Figure 1-3) is a register designation that applies to the USP in the user
privilege level and to either the ISP or MSP in the supervisor privilege level. In the
supervisor privilege level, the active stack pointer (interrupt or master) is called the SSP.
In addition, the address registers may be used for word and long-word operations. All of
the 16 general-purpose registers (D7–D0, A7–A0) may be used as index registers.
The PC contains the address of the next instruction to be executed by the
MC68020/EC020. During instruction execution and exception processing, the processor
automatically increments the contents of the PC or places a new value in the PC, as
appropriate.
1- 4 M68020 USER'S MANUAL MOTOROLA