Thorne : Section 20.1, 20.2 (Irvine Edition IV : Section 17.1) SYSC3006 1

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1 Parallel Input/Output Thorne : Section 20.1, 20.2 (Irvine Edition IV : Section 17.1) SYSC3006 1

2 Basic Concepts of I/O Input/Output is the information exchange between CPU and (external) connected devices Block Diagram of a Simple Computer System Bus Processor Memory I/O connected devices keyboard mouse display printer disk drives comm n links, etc. SYSC3006 2

3 Basic Concepts of I/O Interfacing and programming I/O devices are different from memory programming : 1. Electrical characteristics may be different from CPUs Analog devices, power, current drive 2. I/O devices operate asynchronously from the CPU (and the program being run) To transfer any data, the processor and I/O device must synchronize or handshake to exchange information SYSC3006 3

4 Basic Concepts of I/O The I/O box in our simple computer represents the set of independent I/O components associated with each connected device. bus I/O keyboard component(s) comm n component(s) connected devices keyboard mouse display printer disk drives comm n links, etc. I/O components are interfaces that electrically connect an external device to computer s internal bus. The bus connection allows the CPU to read and/or write the device SYSC3006 4

5 I/O Programming g on a PC Access to the Hardware is routed through several layers User Programs (Applications) Operating System BIOS Hardware Operating System : General device-independent access to hardware BIOS : low level routines in ROM for device support (I/O, configuration, diagnostics) SYSC3006 5

6 I/O Ports A port allows exchange of information i between bus (connected to CPU and memory) and I/O components (that in turn are connected to devices) An I/O component typically has 3 kinds of Ports: Control ports: write values to these control behaviour of component/device Status ports: read values from these find out about current state of component/device Data port: read and/or write values of these exchange application information Some ports are read-only, write-only or read&write. SYSC3006 6

7 I/O Addresses When connected to a computer system, each device port is assigned an I/O address A device (port) is identified by its I/O addresses Computer will read/write from/to this I/O address to receive/send data from/to device In microprocessor architectures, there are two kinds of I/O address connections 1. Isolated I/O Concept! 2. Memory-Mapped I/O SYSC3006 7

8 I/O Addressing Schemes : Isolated I/O (Intel) - The microprocessor has dedicated instructions for I/O operations. - The microprocessor has a separate address space for I/O devices. Control (RD/WR) Processor Data Address Memory Memory Map FFFFF FFFFh 0000 I/O Map I/O Control (IOR/IOW) I/O Device 0000 SYSC3006 8

9 I/O Addressing Scheme : Memory Mapped I/O (Motorola) The microprocessor uses the same instruction set to perform memory accesses and I/O operations. The I/O devices and memory components are resident in the same memory space. Control (RD/WR) Memory Map Processor FFFFF Data Address Memory I/O Map I/O Device 0000 SYSC3006 9

10 Intel Uses Isolated I/O Implementation of a Concept Within 80x86 family, I/O addresses range 0-FFFFh For the PC, devices are assigned standard I/O addresses (used by all manufacturers of PCs) Keyboard 60h (data port) Speaker 61h (data port) Parallel Printer (LPT1) 3BCh (data port) 3BDh (status port) 3BEh (control port) I/O ports memory cells Just because a port might allow a value to be written to it DOES NOT mean that the port can have a value read from it (or vice versa!) SYSC

11 I/O Instructions Inteluses the Isolated I/O address scheme I/O ports and memory address can have same value but represent ese locations o in two separate e address spaces There are separate instructions that transfer data to/from I/O ports For memory transfers : MOV AL, [61h] Offset address of memory For I/O transfers : IN AL, 61h OUT 61h, AL I/O port address SYSC

12 Intel 8086 IN Instruction Mnemonic : Semantics : IN Read from I/O port Syntax : Accumulator reg! Data IN AL, imm8 8-bit read IN AX, imm8 16-bit read Legacy of 8085 which had an 8-bit I/O space imm8 specifies an 8-bit I/O address in the range 00h-FFh IN IN des src AL, DX AX, DX 16-bit data reg! Addressing Modes are different! DX specifies a 16-bit I/O address in the range 0000h-FFFFh SYSC

13 Intel 8086 OUT Instruction Mnemonic OUT Semantics : Write to I/O port Syntax : des src OUT imm8, AL 8-bit write OUT imm8, AX 16-bit write Destination looks like immediate! OUT OUT des src DX, AL DX, AX SYSC

14 I/O Example Suppose we have a display device for ASCII characters The programmer s model is a single write-only data port at I/O Address = 04E9H The display is cursor driven : The ASCII character written to the data port is display at the current cursor position Cursor position is maintained by the display device When a character is written, cursor position is advanced Advancement handles new lines and scrolling too. Write a code fragment showing the display of the character A SYSC

15 I/O Example Solution : Write a code fragment showing the display of the character A MOV DX, 04E9H MOV AL, 41h OUT DX, AL A character is a byte. Question : IN AL, DX Will AL contain 41H? This port address is 16 bits, so must load it into DX first (Immediate is only for 8-bit port addresses) SYSC

16 Lab PC s LED/Switch Box Our labs have attached an I/O Box to the PCs 5 LEDs (Light Emitting Diodes) each may be either ON or OFF 5 switches each may be either ON or OFF LEDs are connected to bits of an 8-bit output parallel port Each LED is driven by a particular bit in the port 8-bit port LE D D l fbit value of bit i determines whether LED is ON or OFF bit i SYSC

17 Programmer s Model for the Lab LEDs LED data port address: E010 H Bit configuration: LEDS are labelled [ bit 7 = most significant ; bit 0 = least significant ] bit LED x x x through 5 indicate bits for LEDs 1 through 5 x indicates unused (don t care what value is written) To turn LED ON: set bit associated with the LED ie. write an 8-bit value to the port in which the bit associated with the LED is a "1" To turn LED OFF: clear bit associated with LED SYSC

18 Programming the LEDs Challenges : The LED s interface is a 8-bit port. Although h we want to set/clear a particular bit, we must write an entire byte to the LED port. Writing any value to the LED port affects all LEDs! To modify the state of one LED, we must know the state of all LEDS. but Reading port is meaningless (write-only port) In particular, we cannot read the LED port to get the current state of all LEDs. SYSC

19 Programming the LEDs To manipulate the LEDs individually, the program must maintain the state of the LEDs as a variable Update the variable each time a new value is written to the LED port so that it always represents the current LED state. LED_State t DB? ; current state t of LEDs ; To turn on LED x ; set appropriate bit in LED_State ; write LED_State to LED port ; To turn off LED x ; Clear the appropriate bit in LED_State ; Write LED_State to the LED port Question : How do you initialise iti this program? SYSC

20 Lab Switches The 5 switches on the I/O box are connected to 5 bits of an 8-bit input parallel port There is one bit (in port) per switch It is a read-only port used to get current setting of all switches A write to the port has no effect If the switch is ON, its bit is set (i.e. "1") If the switch is OFF, its bit is clear (i.e. "0") The switches labelled "A" through "E" Switch data port address: E011 H Bit config: [bit7 = mostsignif signif ; bit 0 = least signif ] bit Switch E D C B A x x x x indicates unused d( (undefined) d) SYSC

21 Switch De-bouncing A switch is a mechanical device Moving the switch position, opens or closes a circuit Switches have metal contacts that will complete the circuit when joined The switches in the lab are spring-loaded to hold open/closed position When the position is changed, the contacts can bounce Analogy : Diving i board For a program reading the switch port, the value of switch output will appear to oscillate (open/closed) until bouncing stops The program must filter out the oscillations so that the program only sees one switch state change per position change. This is called dde-bouncing. SYSC

22 Simple De-Bouncing Write a loop that polls the switch until first change is seen Waste enough time (do-nothing-loop) until sure switch stopped bouncing Questions : How much is enough time? What if the program waits longer than necessary? What if the program does not wait long enough? SYSC

23 Adaptive De-Bouncing In a loop, poll the switch until first change is seen Set a loop counter to an init_value Repeat { Poll switch Explain why this approach If the switch has change state again is adaptive? { loop counter = init_value } else Remaining Issue : Decide { on the init_value decrement loop counter } } until loop counter == 0 SYSC

16-Bit Intel Processor Architecture

IBM-PC Organization 16-Bit Intel Processor Architecture A-16 bit microprocessor can operate on 16 bits of data at a time. 8086/8088 have the simplest structure 8086/8088 have the same instruction set,