Input and Output How things get into and out of the CPU

CMPE12cCyrus Bazeghi 2 Computer System

CMPE12cCyrus Bazeghi 3 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: Buffer at computer catches these bits

CMPE12cCyrus Bazeghi 4 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.

CMPE12cCyrus Bazeghi 5 I/O Devices Hard Disk A spinning disk (4600, 5200, 7200, 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? QueuingSeekRotationTransfer Depends10ms 1ms

CMPE12cCyrus Bazeghi 6 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?

CMPE12cCyrus Bazeghi 7 MAL I/O putc $s0 is # address of char is in $s0 lb $4, ($s0)# $4 char to be printed addi $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

CMPE12cCyrus Bazeghi 8 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 things are 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…)

CMPE12cCyrus Bazeghi 9 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

CMPE12cCyrus Bazeghi 10 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.

CMPE12cCyrus Bazeghi 11 Memory Mapped I/O Design hardware and software to recognize certain addresses Real Memory - RAM 0x xffff0000 0xffff0008 0xffff0010 From keyboard To display Set some labels and use those to access devices keyboardDataequ0xffff0008 displayDataequ0xffff0010

CMPE12cCyrus Bazeghi 12 Memory Mapped I/O CPUMEM Keyboard Buffer 0xffff0008 Display Buffer 0xffff0010 Devices on bus watch for their address getc operating_system_function_12:# getc char and put it in $2 lw$2, KeyboardData “return from syscall” putc operating_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

CMPE12cCyrus Bazeghi 13 Device Status Need I/O device status to coordinate Set up some more labels KeyboardStatusequ0xffff000c DisplayStatusequ0xffff0014 Assume Status is word where MSB==1 means ready. 0x xffff0000 0xffff0008 0xffff000c DATA from keyboard STATUS from keyboard 0xffff0010 0xffff0014 DATA to Display STATUS from Display Real Memory - RAM

CMPE12cCyrus Bazeghi 14 Device Status GETC Operating_system_function_12:# getc char and put it in $2 WaitLoop12: lw $14, KeybaordStatus bgez $14, WaitLoop12# keep waiting if $14 non-negative lw $2, KeyboardData# same as before “return from syscall” PUTC Operating_system_function_11:# putc char, where char is in $4 WaitLoop12: lw $14, DisplayStatus bgez $14, WaitLoop11# keep waiting if $14 non-negative sw $4, DisplayData# same as before “return from syscall” MAL OS calls

CMPE12cCyrus Bazeghi 15 Device Status Polling (non-interrupt) I/O GETCHAR GETCHAR: LDAASCSR; status register ANDA#$20; rdrf bit mask BEQGETCHAR; loop if rdrf = 0 LDAASCDR; read data RTS OUTCHAR OUTCHAR: LDABSCSR; load sci status register BITB#$80; tdre bit BEQOUTCHAR; loop intil tdre = 0 STAASCDR; write character to port RTS HC11

CMPE12cCyrus Bazeghi 16 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

CMPE12cCyrus Bazeghi 17 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?