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1 .The Basics of Exception Handling
Interrupts
Initiated outside the instruction stream
Arrive asynchronously (at no specific time),
Examples:
o I/O device status change
o I/O device error condition
Traps occur due to something in instruction stream.
Examples:
o Unaligned address error
o Arithmetic overflow
o System call
2 .An exception takes away control from the user and
transfers it to the supervisor (i.e. the operating system)
Think of the various reasons an exception can occur.
An exception triggers an unscheduled procedure call.
3 .Coprocessor C0 has a cause register (Register 13) that
contains a 4-bit code to identify the cause of exception
Cause register
pending exception code
interrupt
Bits 15-10 Bits 5-2
[Exception Code = 0 means I/O interrupt
= 12 means arithmetic overflow etc]
MIPS instructions that cause overflow (or some other
violation) lead to an exception, which sets the exception
code. It then switches to the kernel mode (designated by
a bit in the status register of C0, register 12) and
transfers control to a predefined address to invoke a
routine (exception handler) for handling the exception.
Interrupt
Status register Enable
Interrupt Mask
15-8 1 0
Kernel/User
4 .(EPC = Exception Program Counter, Reg 14 of C0)
Memory
L: add $t0, $t1, $t2 overflow!
Return address (L+4) Exception
handler routine
is saved in EPC
Next instruction
Overflow
ra ← EPC; jr ra
Invalid instruction
ra ← EPC; jr ra
System Call
ra ← EPC; jr ra
The Exception Handler determines the cause of the
exception by looking at the exception code bits. Then it
jumps to the appropriate exception handling routine.
Finally, it returns to the main program.
5 .Visualizing Exception Handling
main
interrupt / exception
Analyze the cause of exception
arithmetic interrupt from keyboard?
overflow?
Exceptions cause mostly unscheduled procedure calls.
6 .Example:
Read
one
input
from
a
Keyboard
Consider
reading
a
value
from
the
keyboard.
Assume
that
the
interrupt
enable
bit
is
set
to
1.
The
first
line,
".text
0x80000080"
places
the
code
explicitly
at
the
memory
location
where
the
interrupt
service
routine
is
called.
.text
0x80000080
mfc0
$k0,
$13
#
$k0
=
$Cause;
mfc0
$k1,
$14
#
$k1
=
$EPC;
andi
$k0,
$k0,
0x003c
#
$k0
&=
0x003c
(hex);
#
Filter
the
Exception
Code;
bne
$k0,
$zero,
NotIO
#
if
($k0
≠
0)
go
to
NotIO
#
Exception
Code
0
=>
I/O
instr.
sw
$ra,
save0($0)
#
save0
=
$ra;
jal
ReadByte
#
ReadByte();
(Get
the
byte).
lw
$ra,
save0($0)
#
$ra
=
save0;
jr
$k1
#
return;
NotIO:
Other
routines
here
Note that procedure ReadByte must save all registers
that it plans to use, and restore them later.
7 .Understanding Logic Design
Appendix C of your Textbook on the CD
When you write add $t0, $t1, $t2, you imagine something
like this:
$t1
Adder
$t0
$t2
What kind of hardware can ADD two binary integers?
We need to learn about GATES and BOOLEAN ALGEBRA
that are foundations of logic design.
8 .AND gate
X Y X.Y
0 0 0 X X.Y
0 1 0 Y
1 0 0
1 1 1
OR gate
X Y X+Y
0 0 0 X
0 1 1 X+Y
1 0 1 Y
1 1 1
NOT gate
X X
0 1 X X
1 0
Typically, logical 1 = +3.5 volt, and logical 0 = 0 volt. Other
representations are possible.
9 .Analysis of logical circuits
X X.Y
F
Y X.Y
What is the value of F when X=0 and Y=1?
Draw a truth table.
X Y F
0 0 0
0 1 1
1 0 1
1 1 0
This is the exclusive or (XOR) function. In algebraic
form F= X.Y + X.Y
10 .More practice
1. Let A.B + A.C = 0. What are the values of A, B, C?
2. Let (A + B + C).(A + B + C) = 0. What are the
possible values of A, B, C?
• Draw truth tables.
• Draw the logic circuits for the above two functions.
A
B
C
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