Transcript 09_IO - pantherFILE

Input and Output
Computer Organization and Assembly Language: Module 9
Motivation
 Input
and output devices (I/O devices) allow
computers to perceive and effect their environment
 Input devices are often much slower than the
computer: the computer can process data faster than
the device can provide data
 Some output devices have are also slow relative to
the speed of memory access or CPU computation;
they cannot accept data at the rate provided
General Categories of I/O
 User
interface devices
 Input devices: mouse, keyboard
 Output devices: terminal display, printer
 Mass storage devices
 Disks,
tape drives
 Gateways
and networks
Hardware: I/O instructions
 The
hardware implements special instructions to
read from/write to I/O devices.
 Perhaps
lwio (load word I/O) and swio
 The
devices have a separate address space from
memory
 lwio
$s0, 16 and lw $s0, 16 access different data
 lwio
$s0, 16 reads the next word from device 16
 lw $s0, 16 reads the word stored in memory location 16.
 MIPS
does NOT use special I/O instructions
Hardware: Memory-Mapped I/O
 The
hardware can be designed in such a way that
some memory addresses are not really memory at all
but a collection of communication channels to I/O
devices.
 In memory-mapped I/O, load and store from/to the
communication channels provide a means of
performing I/O, i.e., load and store instructions with
an I/O address are treated as I/O operations.
lw $a0, KeyboardData
communication
channels
sw $t0, DisplayData
Hardware: Memory-Mapped I/O
 MIPS
RISC architecture uses memory-mapped I/O.
 I/O devices, unlike memory, may be unavailable. If
a device is not ready to accept or transmit data then
it is busy.
 The
system must provide a mechanism for allowing a
program to detect if a device is ready or busy
 The
processor communicates with an I/O device
using two addresses: one for data exchange and the
other to obtain the status of the I/O device.
Memory mapped addresses in SPIM
keyboard_control = 0xffff0000 display_control = 0xffff0008
keyboard_data = 0xffff0004
display_data = 0xffff000c
Data
7 6 5 4 3 2 1 0
Control
1 0
Interrupt enable (1 = enabled)
Device busy/ready (1 = ready)
Memory mapped addresses in SPIM
 Define
the memory mapped constants like this:
.data 0xffff0000
keyboard_control:
keyboard_data:
display_control:
display_data:
.space
.space
.space
.space
4
4
4
4
Software: Managing I/O
 Programmed
I/O (Polling)
 Interrupt driven I/O
Software: Programmed I/O
In programmed I/O, the CPU stays in a loop until
the I/O unit indicates that is ready for data transfer or
if the CPU has issued a command to the I/O module
it must wait until the operation is complete.
wait: lw $s0,keyboard_control
and $s0, $s0, 1
beq $s0, 0, wait
lw $v0,keyboard_data
1 = ready
0 = busy
least significant bit
wait: lw $s0,display_control
and $s0, $s0, 1
beq $s0, 0, wait
sw $v0,display_data
Software: Interrupt Driven I/O
 Instead
of spin-waiting or doing a regular check whether
an I/O device is ready, the I/O device can just inform the
CPU that it is ready to receive or transmit data. The I/O
device sends an interrupt signal to the system.
 In this mechanism, control is transferred to the exception
handler (part of the operating system) which saves the
current state of the interrupted program. The requests are
then serviced and control is given back to the interrupted
program. This mechanism is called an exception.
Software: Interrupt Driven I/O
 Exceptions
 Interrupts
come in two varieties
are generated by hardware
 I/O
device
 Clock
 Power down
 Traps
are generated by code execution
 Division
by zero
 Illegal memory address
 System call
How interrupt driven I/O works
User code/data
.text
.
la $a0, A
li $v0, 4
syscall
.
.
.data
A:
.asciiz “cat”
Kernel data
Output buffer
Display
How interrupt driven I/O works
User code/data
.text
.
la $a0, A
li $v0, 4
syscall
.
.
.data
A:
.asciiz “cat”
Kernel data
Output buffer
c a t
Display
How interrupt driven I/O works
User code/data
.text
.
la $a0, A
li $v0, 4
syscall
.
.
.data
A:
.asciiz “cat”
Kernel data
Display
c
Output buffer
c a t
How interrupt driven I/O works
User code/data
.text
.
la $a0, A
li $v0, 4
syscall
.
.
.data
A:
.asciiz “cat”
Kernel data
Display
ca
Output buffer
c a t
How interrupt driven I/O works
User code/data
.text
.
la $a0, A
li $v0, 4
syscall
.
.
.data
A:
.asciiz “cat”
Kernel data
Display
cat
Output buffer
c a t
Exception Mechanism
 The
exception handler determines which event has
caused the exception and decides what should be
done based on it.
 Since
an exception handler can be invoked anytime, an
exception handler can not have parameters nor it can return
values.
 It must also save register values being used by the
interrupted program and restore them before returning
control to the interrupted program.
Role of the Operating System
 The
operating system is a program that allocates and
controls the use of all system resources: the processor,
memory, and I/O devices.
 Since there are many processes that can run concurrently,
the operating system uses interrupt to allocate the
processor to different processes periodically -- allowing
processes to share processing time with each other.
 The exception handler plus other codes used to decide
what process should be executed next is called the kernel.