input and output how things get into and out of the cpu

Input and Output

How things get into and out of the CPU

2CMPE12c Cyrus Bazeghi

Computer System


I/O Devices

Keyboard

• User presses ‘A’ key -> ‘a’• ASCII code is 0x61• Keyboard sends this on wires• 1 for start, 8-bits of data, 0 for stop• ‘a’ is: 1011000010• Buffer at computer catches these bits


Displays

• Character display works with the reverse process (sort of)

• Most displays are “bit mapped”

I/O Devices

Printers

• Just like a display but now being “printed” to paper, not a screen.

• Again, most printers are now “bit mapped” verses character.


I/O Devices

Hard Disk

•A spinning disk (4600, 5200, 7200, 10000+ RPM)•2 – 240 GB and growing FAST•Magnetic and read/write (like tape)•Both sides •Usually a stack of platters•Disk access

•Electronic speeds are in the nanoseconds (10-9 sec)•Disk speeds are in the milliseconds (10-3 sec)•Why use a disk?

Queuing Seek Rotation Transfer

Depends

10ms 10ms 1ms


I/O Devices

Questions

• How does CPU ask for a char to be printed?• Which printer?• Which display? Who’s?• When is printer ready for the next char?• When does keyboard have the next char?• What about the million times slower?


MAL I/O

putc $s0 is

# address of char is in $s0lb $4, ($s0) # $4 char to be printedaddi $2, $0, 11 # this syscall is like:syscall # jal operating_system_function

getc $s0 is

addi $2, $0, 12 # this syscall is like:syscall # jal operating_system_function

# returns with char read in $2


MAL I/O

•Don’t use “jal” because•OS doesn’t trust user to provide the correct address•Want to switch into OS mode, where more thingsare allowed

•Allowed by what?•OS catches “syscall” and uses value in $2 to determine what to do•OS will not allow (or should not)

•Users to read each other’s keyboards•Users to send infinite jobs to printers (well, actually…)


MAL I/O

How does the OS do I/O?

What instructions cause an I/O?•Could have special instructions•Hard to anticipate all possibilities

Solutions•overload load and store•Memory-mapped I/O


Memory Mapped IO

• Idea is to place devices other than RAM chips at physical address locations.

• This way to access IO devices you use the same load and store instructions.


Memory Mapped I/O

Design hardware and software to recognize certain addresses

Real Memory - RAM

0x00000000

0xffff0000

0xffff0008

0xffff0010

From keyboard

To display

Set some labels and use those to access devices

keyboardData equ 0xffff0008displayData equ 0xffff0010


Memory Mapped I/O

CPU MEM

Keyboard Buffer0xffff0008

Display Buffer0xffff0010

•Devices on bus watch for their address•getc

operating_system_function_12: # getc char and put it in $2lw $2, KeyboardData“return from syscall”

•putcoperating_system_function_11: # putc char, where char is in $4

sw $2, DisplayData“return from syscall”

•But is there a new char to read?•But is the display done with the last char?

System bus


Device Status

Need I/O device status to coordinate

Set up some more labelsKeyboardStatusequ 0xffff000cDisplayStatus equ 0xffff0014

Assume Status is word where MSB==1 means ready.

0x00000000

0xffff0000

0xffff00080xffff000c

DATA from keyboardSTATUS from keyboard

0xffff00100xffff0014

DATA to Display

STATUS from Display

Real Memory - RAM


Device Status

GETC

Operating_system_function_12: # getc char and put it in $2WaitLoop12:

lw $14, KeybaordStatusbgez $14, WaitLoop12 # keep waiting if $14 non-negativelw $2, KeyboardData # same as before“return from syscall”

PUTCOperating_system_function_11: # putc char, where char is in $4WaitLoop12:

lw $14, DisplayStatusbgez $14, WaitLoop11 # keep waiting if $14 non-negativesw $4, DisplayData # same as before“return from syscall”

MAL OS calls


Device Status

Polling (non-interrupt) I/O

GETCHARGETCHAR:

LDAA SCSR ; status registerANDA #$20 ; rdrf bit maskBEQ GETCHAR ; loop if rdrf = 0LDAA SCDR ; read dataRTS

OUTCHAROUTCHAR:

LDAB SCSR ; load sci status registerBITB #$80 ; tdre bitBEQ OUTCHAR ; loop intil tdre = 0STAA SCDR ; write character to portRTS

HC11


Device Status

•How much time is spent spinning?•A putc or getc is less than 10 instructions, or 10ns on a modern processor

•Mechanical devices take milliseconds•Almost all time is spent spinning

•Must do useful work while waiting•Periodically poll devices and send

characters when ready


Polling I/O

•The OS must check regularly (poll) for ready devices•Perhaps once a millisecond

•If ready, then OS services device•Keyboard: transfer character and put on queue

•Display: transmit character to the graphics HW

•Problems:•How often to poll?•How does the OS code get run?•What happens to the user program?•Is there a better solution?

input and output how things get into and out of the cpu

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