MC68EC000 Motorola, MC68EC000 Datasheet - Page 96

• Manufacturer Part Number: MC68EC000
• Description: Core Processor (SCM 68000)
• Manufacturer: Motorola

4.2 EXCEPTION PROCESSING DESCRIPTION
The processing of an exception occurs in four steps, with variations for different exception
causes:
4.2.1 Exception Vectors
An exception vector is a memory location from which the SCM68000 fetches the address of
a routine to handle an exception. Each exception type requires a handler routine and a
unique vector. All exception vectors are two words in length (see Figure 4-6) and reside in
the supervisor data space, except for the reset vector, which is four words long and resides
in the supervisor program space.
A vector number is an 8-bit number that is multiplied by four to obtain the address of an
exception vector. The SCM68000 forms the vector address by left-shifting the vector num-
ber two bit positions and zero-filling the upper-order bits to obtain a 32-bit long-word vector
address (see Figure 4-7).
The vector numbers can be found in the vector table (see Table 4-2) which is 512 words long
(1024 bytes), starting at address 0 and proceeding through address 1023 (decimal). The
vector table provides 255 unique vectors, some of which are reserved for trap and other sys-
tem function vectors. Of the 255 vectors, 192 are reserved for user interrupt vectors. How-
ever, the first 64 entries are not protected, so user interrupt vectors may overlap at the
discretion of the system designer.
MOTOROLA
A31
1. Make a temporary copy of the status register and set the status register for exception
2. Obtain the exception vector.
3. Save the current SCM68000 context.
4. Obtain a new context and resume instruction processing.
processing.
WORD 0
WORD 1
ALL ZEROES
Figure 4-7. Address Translated from 8-Bit Vector Number
Freescale Semiconductor, Inc.
EC000 CORE PROCESSOR USER’S MANUAL
Figure 4-6. Exception Vector Format
For More Information On This Product,
EVEN BYTE (A0 = 0)
A10
Go to: www.freescale.com
NEW PROGRAM COUNTER (HIGH)
NEW PROGRAM COUNTER (LOW)
A9
V7
A8
V6
A7
V5
A6
V4
ODD BYTE (A0 = 1)
A5
V3
A4
V2
A3
V1
Exception Processing
A1 = 0
A1 = 1
A2
V0
A1
0
4-11
A0
0