Input and Output How things get into and out of the CPU.
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Transcript of 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
3CMPE12c Cyrus Bazeghi
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
4CMPE12c Cyrus Bazeghi
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.
5CMPE12c Cyrus Bazeghi
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
6CMPE12c Cyrus Bazeghi
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?
7CMPE12c Cyrus Bazeghi
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
8CMPE12c Cyrus Bazeghi
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…)
9CMPE12c Cyrus Bazeghi
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
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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.
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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
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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
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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
14CMPE12c Cyrus Bazeghi
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
15CMPE12c Cyrus Bazeghi
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
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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
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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?