Motorola MC68020 User Manual

MOTOROLA M68020 USER’S MANUAL 1- 7
The SR (see Figure 1-4) stores the processor status. It contains the condition codes that
reflect the results of a previous operation and can be used for conditional instruction
execution in a program. The condition codes are extend (X), negative (N), zero (Z),
overflow (V), and carry (C). The user byte, which contains the condition codes, is the only
portion of the SR information available in the user privilege level, and it is referenced as
the CCR in user programs. In the supervisor privilege level, software can access the entire
SR, including the interrupt priority mask (three bits) and control bits that indicate whether
the processor is in:
1. One of two trace modes (T1, T0)
2. Supervisor or user privilege level (S)
3. Master or interrupt mode (M)
015
C
1
V
2
Z
3
N
4
X
5
0
6
0
7
0
8
I0
9
I1
10
I2
11
0
12
M
13
S
14
T0T1
SYSTEM BYTE
USER BYTE
(
CONDITION CODE REGISTER
)
TRACE
E
NABL
E
INTERRUPT
PRIORITY MASK
SUPERVISOR/USER LEVE
L
MASTER/INTERRUPT MOD
E
C
ARR
Y
O
VERFLOW
Z
ER
O
N
EGATIV
E
E
XTEN
D
Figure 1-4. Status Register (SR)
The VBR contains the base address of the exception vector table in memory. The
displacement of an exception vector is added to the value in this register to access the
vector table.
The alternate function code registers, SFC and DFC, contain 3-bit function codes. For the
MC68020, function codes can be considered extensions of the 32-bit linear address that
optionally provide as many as eight 4-Gbyte address spaces; for the MC68EC020,
function codes can be considered extensions of the 24-bit linear address that optionally
provide as many as eight 16-Mbyte address spaces. Function codes are automatically
generated by the processor to select address spaces for data and program at the user
and supervisor privilege levels and to select a CPU address space for processor functions
(e.g., coprocessor communications). Registers SFC and DFC are used by certain
instructions to explicitly specify the function codes for operations.
The CACR controls the on-chip instruction cache of the MC68020/EC020. The CAAR
stores an address for cache control functions.

Motorola MC68020 Specifications

General

Brand	Motorola
Model	MC68020
Category	Computer Hardware
Language	English