Input/Output Interfaces: Ch 8.1-8.3

Problems Algorithms

software Prog. Lang & Interfaces H/w – s/w interface Instruction Set Architecture Microarchitecture (Organization) Circuits Devices (Transistors) hardware Bits

MEMORY

MAR MDR

INPUT OUTPUT Keyboard Monitor Mouse PROCESSING UNIT Printer Scanner LED Disk ALU TEMP Disk

CONTROL UNIT

PC IR

Processing unit • Computation Instructions Memory unit • Accessed in Instruction fetch • And load/store instructions Control unit • Controls phases of instruction cycle (fetch, decode .. etc) • Control transfer instructions

How does program interact with input/output (I/O) components?

I/O devices allow data to be input/output between computer and outside world Like memory, we need to read from and write to I/O devices For memory, we provide address and read or write Similarly for I/O, we need to identify which device and what operation There is only one memory, so there is no need to specify “which” memory (but only which address) But there are many I/O devices so we need to say which device To make it manageable, there is a uniform interface for all I/O devices: each I/O device is identified by a bunch of device registers

Each I/O device has some number of device registers • Control register: for CPU to specify what operation to do – Read or write or some other (for certain I/O devices) – For disk (permanent memory with storage of large amounts of data), we specify address but keyboard handles only one character at a time so no address • Data register: for holding data to/from CPU • Status register: for showing status of the I/O device – Is the device still busy with the previous operation? – Is there some input for CPU? – Is it safe for CPU to send the next output data or is the previous output data not done yet?

Control and Data registers are analogous to memory where we specified read or write and the data to be transferred But what is this status business? We did not have this for memory • I/O devices are millions of times slower than CPU • And the I/O device speed varies (keyboard typing) • So it is best to have I/O device go on its own speed and not be lock-stepped with the CPU clock – Lock-stepping with CPU clock would mean waiting for millions of cycles which is silly, cumbersome, and wasteful (to use extra hardware to wait for millions of cycles )

So we use asynchronous interaction between CPU and I/O • Using CPU clock would be synchronous Asynchronous means CPU and I/O work independently When data needs to be sent between CPU and I/O they do handshaking (aka protocol) • Needed because not lock-stepped with CPU • Handshaking works by I/O device making its status known to the CPU (status as in “I have a new input for CPU”, or “I am done with outputting previous data”, etc) • This status is shown in the status register (1-bit keyboard status to show if there is a new character and 1-bit monitor status to show if monitor is done displaying last character)

Example: LC3 keyboard (input) and monitor (output) • KBSR – keyboard status register: is there a key ready? • KBDR – keyboard data register: what is the key value? • CRTSR – CRT status register: ready to accept output • CRTDR – CRT data register: write data here for display Device registers are different from “General Purpose Registers” • Not in processing unit • Not used for ordinary instructions So, how do we get access to these device registers?

Synchronous • Device always becomes ready after a fixed number of processor clock cycles; processor should read/write periodically Asynchronous • No guarantees of device timing • Instead, device has status registers to indicate readiness • Requires processor to do some “synchronization” task to determine when device is ready

Asynchronous is far more common

I / O

© 2009 Vijaykumar ECE495K Lecture Notes: Chapter 8 10 Accessing Device Registers Option 2: Memory-mapped I/O • Device registers accessed through regular load and store operations to some specific memory addresses • These addresses are set aside and are not part of memory • In LC3, xFE00 to xFFFF are for I/O device registers • For read/write to these addresses, memory does not do anything and the specific I/O device register is read/written

Memory-mapped I/O far more common

Location I/O Register Function

Bit [15] is one when device Display Status ready to display another xFE04 Register (DSR) char on screen. ASCII character written to Display Data Register bits [7:0] will be displayed xFE06 (DDR) on screen. Bit [15] is one when Keyboard Status Reg keyboard has received a xFE00 (KBSR) new character.

Keyboard Data Reg xFE02 (KBDR)

© 2009 Vijaykumar ECE495K Lecture Notes: Chapter 8 12 Example: Input from Keyboard When user types a character: • Keyboard places ASCII code in KBDR[7:0] (high bits 0) • Keyboard sets KBSR[15] to 1 (no guarantee on other bits) – How to test if highest bit is 1? • KBSR[15] being 1 disables keyboard – for a very short time where further key strokes are highly unlikely but if they keyboard data occur are ignored15 8 7 0 KBDR 1514 0 ready bit KBSR When program loads KBDR: • Program sets KBSR[15] to 0 •which automatically re-enables Keyboard

Polling • Keep checking status register until new data available or device ready to accept new data • “Are we there yet? Are we there yet? Are we there yet?” – Bart Simpson Interrupts • Device sends a special signal called interrupt to CPU which drops what it is doing, handles the interrupt, goes back to what it was doing • “Wake me when we get there.” – Sleep-loving teenager • interrupt disrupts ordinary flow of instruction cycle

Polling • Because most I/O devices are much slower than CPU, polling checks result in “not ready yet” • CPU wastes many clock cycles checking unnecessarily when it could be doing other useful work Interrupts • CPU pays attention to I/O device only when there is something – so no wastage of CPU • But interrupts are not free – they do incur some overhead but much less than polling especially for slow I/O devices • Interrupts occur any time in the middle of an instruction, CPU hardware to handle interrupts is complex (ECE437)

INPOLL LDI R0, KBSRlabel • Why LDI/STI? BRzp INPOLL • Why BRzp? LDI R0, KBDRlabel • What does this OUTPOLL LDI R1, DSRlabel program do? BRzp OUTPOLL • What could happen if STI R0, DDRlabel we read KBDR without ... checking KBSR? KBSRlabel .FILL xFE00 • What could happen if KBDRlabel .FILL xFE02 we write DDR without DSRlabel .FILL xFE04 checking DSR? DDRlabel .FILL xFE06 • Recall Ch.6 character count eg where input came from keyboard

Two parts: • Generate the interrupt signal – In hardware – A little bit of details now • Perform service for the interrupt – In software – Details in later courses

In 495k, we cover 8.1-8.3 and a little bit of 8.5 Read pages 230-235 (pages 219-229 will be covered later)

Program running on CPU may be doing “higher priority” work and not want to be interrupted So each I/O device has a “interrupt enable” (IE) bit in device status register – program clears this bit if it wants I/O device to shut up • If IE is clear device ready bit does not matter • When ready bit and IE bit set, device interrupts CPU

interrupt enable bit 151413 0 ready bit KBSR interrupt signal to processor

I/O devices and their interrupts are prioritized by their urgency • Internet Explorer vs. missile control • Details can be complex ( we won't go into page 213)

CPU must test whether interrupt signal has been set, and respond accordingly • Modify instruction cycle

1. Fetch instruction from memory 2. Decode instruction 3. Evaluate memory address if needed 4. Fetch operands for computation 5. Execute instruction using operands 6. Store result into specified destination 7. Check if interrupt signal set – If not, go to step #1 for next PC – If so, save aside PC and CCRs; transfer to “interrupt service routine.” More detail in Chapter 10.5

For small data, load/store from CPU, for large data 2 options Programmed I/O (PIO) • Computer uses load/store instructions to transfer data between memory-mapped device data registers and memory via GPRs • Wastes CPU to do lowly work of transferring data Direct Memory Access (DMA) • Device directly transfers data from/to specified regions in memory without CPU intervention, so saves CPU time • adds complexity to memory system design • Typically used for bulk data transfers from devices like disks, network interfaces, etc. © 2009 Vijaykumar ECE495K Lecture Notes: Chapter 8 21 Where are we headed? Ch 9.1-9.2

Problems Algorithms

software Prog. Lang & Interfaces H/w – s/w interface Instruction Set Architecture Microarchitecture (Organization) Circuits Devices (Transistors) hardware Bits