1. Input/Output Interface
Address bus
Data bus
Control bus
address
decoder
address
decoder
fffffffc Addr DE
I/O Interface
Mem 1
...
Mem n
Keyboard

2. How to do I/O By “Polling” + Very simple and fast
li $t1 0xfffffffc ;Interface address
waitkey: lb $t0 0($t1) ;Poll Keybard
nop
beq $t0 $r0 waitkey ;no key strike
+ Very simple and fast
- Can’t do any useful work while waiting
- Only OK in a single process system, might miss data if other process takes over

3. External Interrupt
Let the Interface generate an External Interrupt when data is available
+ Access I/O Interface only when needed
+ I/O handling is done by OS (Operating System)
I/O addresses can be “privileged” for SAFETY
+ Data is never lost (if processor fast enough)
- Slower than “Polling”
We will address that problem later

4. Input/Output Interface
Address bus
Data bus
Control bus
CTRL[INT2]=keyboard strike
fffffffc Addr DE
I/O Interface
Moore about:
Buffered I/O
“Intelligent Devices”
etc. later.
Keyboard

5. An External Interrupt Occurs
The “context” of the User program must be restored
User Program
Kernel Program . ..
bne $t0 $r0 upp
mfc0 $k0 $14
jr $k0
rfe
add $t0 $t1 $t2 .. .

6. Multiple Processes User 1 User 2 User 3 Kernel .text .text .text
.ktext
Instructions
.data
.data
.data
.kdata
Data

7. Time Slicing, Context Switch
On Each Timer Interrupt
Store the User program “Context”
Choose next User program (process)
Round robin process scheduling
Restore its “Context”
Pass control to User program

8. Process Control Block (PCB)
Resume Address (next PC for this process)
Global Data pointer ($gp)
Stack pointer ($sp)
All registers but $k0, $k1, $gp, $sp
$k0, $k1 might be trashed by the Kernel code
$gp, $sp stored in the PCB

9. Process Control Block PCB for Process p1 next_pc $gp $sp .text
resume addr
base for global
data area for p1
.data
top of
user p1 stack
.p1 stack
$pc,$sp,$gp are initiated by the OS at “run” time

10. Where are the 28 registers?
PCB p1
PCB p1
next_pc
$gp
$sp
Let’s put them on
the user program’s
stack!
next_pc
$gp
$sp
And adjust the $sp
$t0
$t1 … ..
$t0
$t1 … ..
.p1 stack
.p1 stack