MC68020 Motorola, MC68020 Datasheet - Page 130

• Manufacturer Part Number: MC68020
• Description: (MC68020 / MC68EC020) MICROPROCESSORS USERS MANUAL
• Manufacturer: Motorola

The processor begins exception processing for a bus error by making an internal copy of
the current SR. The processor then enters the supervisor privilege level (by setting the S-
bit in the SR) and clears the T1 and T0 bits in the SR. The processor generates exception
vector number 2 for the bus error vector. It saves the vector offset, PC, and the internal
copy of the SR on the active supervisor stack. The saved PC value is the logical address
of the instruction that was executing at the time the fault was detected. This is not
necessarily the instruction that initiated the bus cycle since the processor overlaps
MOTOROLA
Figure 6-1. Reset Operation Flowchart
M68020 USER’S MANUAL
T1, T0 (SR)
I2–I0 (SR)
ENTRIES INVALIDATED
INSTRUCTION CACHE
PREFETCH 3 WORDS
BEGIN INSTRUCTION
FETCH VECTOR #0
FETCH VECTOR #1
M (SR)
SP (VECTOR #0)
PC (VECTOR #1)
S (SR)
CACR
VBR
OTHERWISE
OTHERWISE
OTHERWISE
EXECUTION
ENTRY
EXIT
1
0
0
$7
$00000000
$00000000
ADDRESS ERROR
BUS ERROR
BUS ERROR OR
BUS ERROR
EXIT
EXIT
EXIT
(DOUBLE BUS FAULT)
(DOUBLE BUS FAULT)
(DOUBLE BUS FAULT)
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