Interfacing with I/O Devices

CEG 320/520Interfacing with I/O devices2 Overview Memory mapped vs programmatic I/O Organization of a memory mapped I/O device Polled I/O Interrupt-driven I/O –ISRs –Shared IRQs –Vectored interrupts

CEG 320/520Interfacing with I/O devices3 I/O Device Models Programmed I/O –Special instructions to read and write from I/O devices Memory-mapped I/O –I/O devices live at memory addresses –For example: MOVE.B $8000,D0 might get a byte from an I/O device and not memory. CPU Memory I/O

CEG 320/520Interfacing with I/O devices4 I/O Models for Common Processors MC68000 – Memory mapped. Intel CPUs – Programmed I/O –Separate instructions for I/O read/writes –Can also memory-map some devices PowerPC – Memory mapped, but uses a separate address space –A special control register controls which address space is being accessed.

CEG 320/520Interfacing with I/O devices5 Organization of an I/O device Base memory location –Jumpers, PnP, etc. Status register –Sin – A word is waiting in the input register –Sout – The device is ready for output The device knows when a register is written to or read from $8002 $8000 Sout $8004 Sin Input register Output register Status register 1 0 Device Device Controller

CEG 320/520Interfacing with I/O devices6 Polled I/O New instruction: BTST.L #bit,Dn –Set Z according to bit of register Dn –BEQ will branch if bit = 0, BNE if bit = 1 GETCHLEABUFFER,A0 POLLIMOVE.B$8004,D0 BTST.B#0,D0 BEQPOLLI MOVE.B$8000,(A0)+ PUTCHMOVE.B$8004,D1 BTST.B#1,D1 BEQPUTCH MOVE.BD0,$8002 $8002 $8000 Sout $8004 Sin Input register Output register Status reg. 1 0 Device Device Controller

CEG 320/520Interfacing with I/O devices7 Drawbacks of Polled I/O The CPU cant do anything else while it is polling. You must explicitly check every device in the system. –How often should you check the modem to see if there is a ring? No way for devices to get the attention of the CPU until it is their turn. –No way to assure fast response to external events.

CEG 320/520Interfacing with I/O devices8 Interrupt-driven I/O When IRQ goes active, jump to a special memory location: the ISR, or interrupt service routine. Activate IACK to tell the device that the interrupt is being serviced, and it can stop activating the IRQ line. CPU Memory I/O IRQ IACK

CEG 320/520Interfacing with I/O devices9 An ISR for a keyboard driver CPU MEM I/O IRQ IACK KBINITMOVE.L#KBUF,KBPTR RTS KBISRMOVEM.LD0/A0-A1,-(SP) MOVEA.LKBPTR,A0 MOVE.B$8000,D0 CMPI.B#$0D,D0; CR? BEQPROC_INPUT MOVEA.LA0,A1 SUBA.L#KBUF,A1 CMPA.L#KBSIZE,A1 BGEKB_OVERFLOW MOVE.BD0,(A0)+ MOVEA.LA0,KBPTR DONEMOVEM.L(SP)+,D0/A0-A1 RTE; a new instr KBSIZEEQU132 KBUFDS.BKBSIZE KBPTRDS.L1 $8002 $8000 Sout $8004 Sin Input register Output register Status reg 1 0 Device Controller

CEG 320/520Interfacing with I/O devices10 ISRs: Even more transparent May be called at any time. Must be completely invisible to the current running program. Must save all register values & restore them. Some systems do this in hardware, some rely on the ISR to take care of it. The splits up the work… –The SR and the PC are saved when an INTR is triggered –The ISR must save and restore any other registers used –The RTE instruction restores the SR and the PC

CEG 320/520Interfacing with I/O devices11 What happens on an interrupt SR: When an interrupt is activated, the CPU: 1.Pushes the PC (L) and the SR (W) on the stack 2.Switches to supervisor mode (S=1) 3.Jumps to the ISR In supervisor mode, we can write to the status register and use the supervisor stack! TSIIIZVCXN Interrupt priority Condition codes Supervisor Trace

CEG 320/520Interfacing with I/O devices12 The RTE instruction When the ISR is done servicing the interrupt, it returns using the RTE (return from exception) instruction. This instruction is similar to RTS, but it does more. RTE does the following: 1.SR [[SP]] 2.SP [SP] PC [[SP]] 4.SP [SP] + 4 This will restore the S bit along with the rest of the SR. If we were in user mode, before the interrupt, we are back in user mode when we return from the ISR.

CEG 320/520Interfacing with I/O devices13 Interrupts from multiple devices When IRQ is activated, the CPU checks bit I in the status register of each device to see which one (or more) need an IRQ serviced. –The order is fixed, so the first device polled has higher de facto priority CPU Memory I/O 1I/O 2I/O 3I/O 4 IRQ SoutSin Input register Output register 1 0 Device Controller 1 I Bus

CEG 320/520Interfacing with I/O devices14 Vectored Interrupts If we have multiple devices, we need a very large ISR that knows how to deal with all of them! Using vectored interrupts, we can have a different ISR for each device. Each I/O device has a special register where it keeps a special number called the interrupt vector. –The vector tells the CPU where to look for the ISR.

CEG 320/520Interfacing with I/O devices15 A vectored-interrupt device $8002 $8000 $8004 Input register Output register Device Controller Status register $8006 Interrupt Vector Register 67 When I trigger an interrupt, look up address number 67 in the vector table, and jump to that address.When I trigger an interrupt, look up address number 67 in the vector table, and jump to that address. … S in S out IE SIN IE SOUT

CEG 320/520Interfacing with I/O devices16 Getting the interrupt vector INTA tells a device to put the interrupt vector on the bus INTA is daisy chained so only one device will respond CPU Memory I/O 1I/O 2I/O 3I/O 4 IRQ INTA

CEG 320/520Interfacing with I/O devices17 The Vector Table The interrupt vector table is stored in memory addresses $0000 through $03FC. The contents of the vector table are addresses to jump to when each vectored interrupt occurs. (Absolute long) … $0100 … $0014 $0010 $000C $0008 $0004 $0000 $03FC 64 – 255 … 5 – Div by zero 4 – Illegal Instr 3 – Address Err 2 – Access Fault 1 – Reset PC 0 – Reset SP user defined…

CEG 320/520Interfacing with I/O devices18 The Vector table When we do TRAP #14 we jump to the address stored here! Each entry in the vector table is the address of an ISR. Each entry takes 2 words, so the location of vector n is address (n × 4)

CEG 320/520Interfacing with I/O devices19 Installing a vectored interrupt device Step 1: Write an ISR and store it in memory. –The ISR must know the addresses of the registers in the device Step 2: Put the address of the ISR somewhere in the vector table Step 3: Put the vector number in the Vector Register of the device KBISRLEABUFFER,A0 MOVE.B$8000,D0 CMPI.B#0D,D0 ; CR? BEQPROCESS MOVE.BD0,(A0)+ … RTE MOVE.L#KBISR, $100 ;$100 = 256 = 64 * 4 MOVE.L#64, $8006 ;$8006 = Vect. Reg.

CEG 320/520Interfacing with I/O devices20 Memory map for a vectored interrupt device $1000 … (256) $0100 $8000 ISR code … RTE $8002 $8004 Output register Status register $ $ Input register

CEG 320/520Interfacing with I/O devices21 A simple keyboard driver $8002 $8000 $8004 Input register Ouptut register Status register $8006 Ivec = 26 DATAIN EQU $8000 DATAOUT EQU $8002 STATUS EQU $8004 VECTOR EQU $8006 LINE DS.B 81 PNTR DS.L 1 MOVE.L #26, VECTOR ; 26 x 4 = 104 = $68

CEG 320/520Interfacing with I/O devices22 Priority Interrupts Only! What do we do if we are processing an interrupt and another device signals an interrupt? –We can disable (mask) all interrupts while processing an interrupt (IE bit in the SR) Some devices require very low interrupt latency (e.g. system clock) while some can tolerate long latency (e.g. keyboard). The has 8 interrupt priorities. –While processing an interrupt of level n, interrupts from other devices of level n and lower are ignored. Only higher priority interrupts are allowed.

CEG 320/520Interfacing with I/O devices23 Interrupt Priorities TSIIIZVCXN Interrupt priority Condition codes Supervisor Trace An ISR can set the interrupt mask (or interrupt priority bits) by directly changing the contents of the status register. MOVE.W xx,SR is a privileged instruction. Which means it is only allowed in supervisor mode : An interrupt at level 7 is always accepted A level 7 interrupt is a nonmaskable interrupt.

CEG 320/520Interfacing with I/O devices24 Software interrupts Interrupts can be triggered by more than just I/O devices: –Program errors, like division by zero, can trigger interrupts. –The OS uses interrupts for multitasking and other purposes : The TRAP instruction triggers a software interrupt –You can assign any ISR you wish to the vector entries for TRAP #0 through TRAP #15

CEG 320/520Interfacing with I/O devices25 Direct Memory Access (DMA) Even with interrupts, a lot of CPU effort is expended just moving bytes around from I/O devices to/from memory –Modem – interrupt every time a byte arrives? DMA allows devices to access memory directly –A large amount of data can be stored to a memory location, and then an interrupt can be triggered to process all the data at once.

CEG 320/520Interfacing with I/O devices26 A DMA Controller A DMA controller can transfer a large amount of data between a device and memory –Example – send a stream of print data to a printer, and notify the CPU when done The controller needs to know: –Where in memory should the data be found/put? –How many bytes of data should be transferred? Starting address Byte count Status and control register: IRQIE DoneR/W CPUMemory Disk Printer Bus

CEG 320/520Interfacing with I/O devices27 Bus Arbitration Both the CPU and the DMA controller can use the bus There are often multiple DMA controllers No two devices can use the bus at the same time Memory Disk Printer Bus DMA Controller CPU

CEG 320/520Interfacing with I/O devices28 Bus Arbitration Details Signal Bus Request (BR) to request the bus CPU asserts Bus Granted (BG) –If DMA controller didnt request the bus, pass it on to BG2, BG3, etc. Controller asserts Bus Busy (BBSY) until done CPU DMA Controller 1 DMA Controller 2 BBSY BR BG1BG2

CEG 320/520Interfacing with I/O devices29 Bus Arbitration Policies No one can use the bus (not even the CPU) while BBSY is asserted, except for the DMA controller that is the current bus master. Cycle stealing – When you get the bus, perform a few transfers and then release it –High-speed peripherals, like disks, get bus priority Block mode – Perform an entire transfer before releasing the bus.

CEG 320/520Interfacing with I/O devices30 You should know… General concepts for I/O devices: –Programmatic vs. memory-mapped I/O –Polling vs. Interrupt-driven I/O –Vectored interrupts, interrupt priorities –DMA, Bus Arbitration Details for the –Exactly what happens in the CPU on an interrupt What does the RTE instruction do? –How to write a simple ISR –How to initialize the I/O device and the vector table

CEG 320/520Interfacing with I/O devices Pinouts