What allows processor to communicate with peripheral devices?

Our focus will be the common bus. As you might imagine, we must ensure that every byte sent reaches the correct device and any received must be correctly labeled as to where it came from.

One way to do this is to reserve a set of memory locations for this purpose. Then a set of memory addresses represent the data byte, others represent the control registers in the device. This is a simple approach but in today's multi-processing systems, there is a lack of control that is not acceptable.

Another approach is to use ports. On the PC, there are 65,536 ports, each one separately addressable. Input/output operations are then performed by the instructions IN and OUT, which read and write directly to the ports. They have two operands, the accumulator and a port specified.

It is possible to use an accumulator of any size, AL, AX, EAX, and a port address in the range of 0 to 65535 (0FFFFh). However, yoou can not specify a port of greater than 255 as an immediate operand. To exceed that, you must use a register:

mov dx, 3D8h ; store port number in DX out al, dx ; send byte contained in AL to port 3D8h;

Many of the ports are already reserved for IBM-Compatible PCs.

Port AddressDevice Connected020h - 023hProgrammable interrupt controller 8259030h - 03FhInterrupt controller 1 (AT and PS/2)040h - 043hTimer060h - 06FhPPI on PC and XT, keyboard on AT and PS/21F0h - 1F8hFixed disk controller (AT)278h - 27BhParallel printer (LPT2)2E8h - 2EFhSerial Port 42F8h - 2FFhSerial Port 2378h - 37BhParallel printer (LPT1)3B0h - 3BBhMDA or VGA video adapter3BCh - 3BFhParallel printer (LPT3)3C0h - 3CfhEGA/VGA video adapter3D0h - 3DFhCGA/VGA video adapter3E8h - 3EFhSerial port 33F0h - 3F7hFloppy disk control3F8h - 3FFhSerial port 1

The Basics of Controlling the Peripherals

I/O ports are physically located in different chips belonging either to the motherboard (the timer) or to other cards (disk or video controllers). The chips that used to be most commonly used are:Peripheral interfaceIntel 8255Interrupt controllerIntel 8259TimerIntel 8253Serial PortUART 8250

It must be noted that not all systems use the same chips and some of the chips have been upgraded. Sometimes it is helpful to have your program determine which system you have:

; ; Program DetPC ; Copyright 1993, Vitaly Maljugin, Jacov Izrailevich, Semyon Lavon ; Aleksandr Sopin ; .model large HiBios segment at 0F000h org 0FFFEh PcType db ? ; Computer idenifier HiBios ends .code ASSUME es:HiBios ; ES wil be used for accessing ROM BIOS area Begin: mov ax,@data ; load segment address of DATA segement mov ds,ax ; DS points to DATA segment mov ax,HiBios ; load segment address or ROM BIOS data mov es,ax ; ES points to ROM BIOS segment mov cx,Ltable ; Load length of table to be searched mov dl,PcType ; Extract the type from BIOS area Search: mov bx,cx ; Address of current table element cmp dl,TypeTbl[bx-1] ;Compare type and element of table je EndSear ; If found, stop searching loop Search ; Test next element of table EndSear:mov al,cl ; number element found passed as return code mov ah,4Ch ; DOS service 4H - terminate process int 21h ; DOS service call .data ; ; Table of microprocessors' types db 0 ; 0 - Uknown type TypeTbl db 0F8h ; 1 - IBM PS/2 model 80 db 0F9h ; 2 - IBM PC Convertible db 0FAh ; 3 - IBM PS/2 Model 30 db 0FBh ; 4 - PC XT Ext keyboard, 3.5" drives db 0FCh ; 5 - PC-AT or PS/2 Models 50,60 db 0FDh ; 6 - IBM PC-JR db 0FEh ; 7 - PC-XT db 0FFh ; 8 - IBM PC db 09Ah ; 9 - Compaq XT / Compaq Plus db 030h ; 10 - Sperry PC db 02Dh ; 11 - Compaq PC / Compaq Deskpro Ltable equ $-TypeTbl end begin

Methods of Controlling the Hardware

There are two types of interrupts, hardware and software, that are in use here. This allows us to have two different approaches for I/O, polling and event-driven (where the event is an interrupt).

If it is polling, the computer must continual, even constantly, check to see if there is any event. If the program is constantly checking, it can do nothing else. This makes the program very easy to debug, but it prevents any other useful work from being perform, which lowers the overall speed and program efficiency.

The other approach is to use hardware interrupts. The interrupt handle is activated when there is a key pressed on the keyboard. The handler preserves the state of the CPU, gets the input and deals with it.

; ; Program ScanCodD ; Copyright 1993, Vitaly Maljugin, Jacov Izrailevich, Semyon Lavon ; Aleksandr Sopin ; .model small .stack .data CR equ 00Dh ; Carriage return code LF equ 00Ah ; Line feed code EscScan equ 001h ; Scan code for ESC key KbdPort equ 060h ; PPI 9225 port A EndMsg equ 024h ; Dollar sign - end of message for DOS service NewHand dd NewInt9 ; Reference to the new handler for int 09 ScanCod db 0 BegMsg db CR,LF,LF db ' SCAN CODES BROWSER 3.0 ', CR,CR,LF db 'Interrupt 09h handler totally replaced',CR,LF db EndMsg Kbd83 db CR,LF,'You have a standard 83-key keyboard',CR,LF,EndMsg Kbd101 db CR,LF,'You have an enhanced 101/102 keyboard',CR,LF,EndMsg InsMsg db CR,LF,'Press ESC to exit else',CR,LF db 'Press and relese any other key',CR,LF,LF,EndMsg FinMsg db CR,LF,' Job terminated',CR,LF db ' Standard value of interrupt vector 09h ' db 'has been restored.', CR,LF, EndMsg .code OutB PROC NEAR USES AX cmp al,0Ah ; Is it decimal digit ( 0 - 9)? jb NoCorr ; No correction - first 10 hex digit are dec cmp al,10h ; Is it hexadecimal digit ( A - F )? jb HexCyph ; If so correct for character representation mov al,' ' ; If not, replace it with blank jmp OutScr ; and output HexCyph:add al,07h ; This transform hex digits A - F NoCorr: add al,30h ; Convert integer to character OutScr: mov ah,0Eh ; Function 0Eh - write character int 10h ; BIOS video service inc NumL cmp NumL,60 jl NoNewL mov al,CR int 10h mov al,LF int 10h mov NumL,0 NoNewL: ret NumL dw 0 OutB ENDP PrtByte PROC near uses AX DX mov ah,0 ; Clear high part of AX mov dx,0010h ; Divider into BX div dl ; AL - result, AH - remainder mov dx,ax ; Save results call OutB mov al,dh call OutB mov al,' ' ; Character to be printed is a blank call OutB ret ; Return to the caller PrtByte ENDP EndInt PROC NEAR in al,61h ; read Port B or al,80h ; set bit 7 to 1 out 61h,al ; output to port B jmp $+2 ; delay (needed for fast PC) and al, not 80h ; clear bit 7 out 61h,al ; output to port B mov al,20h ; EOI code into AL out 20h,al ; signal EOI ret EndInt ENDP NewInt9 PROC near mov SaveAX,ax ContKey:pushf ; save original flags in al,60h ; read scan code cmp al,EscScan ; ESC pressed? je Fin ; if so, return call PrtByte ; output scan code Call EndInt sti ; allow interrupts popf ; restore original flags iret Fin: Call EndInt mov ax,SaveAX popf Jmp9 db 0EAh ; opcode for JMP FAR OldInt9 dw ?,? ; Address of old handler for interrupt 09h SaveAX dw ? NewInt9 ENDP .startup lea dx,BegMsg ; Addres of start message into DX mov ah,09 ; Function 09h - output text string int 21h ; Dos service call mov ax,40h ; 40h - segment address for BIOS data area mov es,ax ; Place this address into ES test byte ptr es:[96h],10h ; Bit 4 - 101-key keyboard indicator jnz Pres101 lea dx,Kbd83 ; Addres of start message into DX mov ah,09 ; Function 09h - output text string int 21h ; Dos service call jmp PrtInstr Pres101: lea dx,Kbd101 ; Addres of start message into DX mov ah,09 ; Function 09h - output text string int 21h ; Dos service call PrtInstr: lea dx,InsMsg ; Addres of start message into DX mov ah,09 ; Function 09h - output text string int 21h ; Dos service call mov ah,35h ; Function 35h - Get interrupt vector mov al,09h ; Interrupt number is 09h int 21h ; DOS service call mov OldInt9[0],bx ; Save offset addres of old handler mov OldInt9[2],es ; Save segment address of old handler push ds ; DS will contain the segment of new handler lds dx,NewHand ; Full addres of new handler into DS:DX mov ah,25h ; Function 25h - set interrupt vector int 21h ; DOS service call pop ds ; Restore the data segment register NextKey:mov ah,0Ch ; Function 0Ch - clear the keyboard buffer int 21h ; Dos service call mov ah,0 ; Function 00h - read character from keyboard int 16h ; BIOS keyboard service cmp ah,EscScan ; Is the ESC key pressed? jne NextKey ; If not - process the next key Finis: lea dx,FinMsg ; Addres of start message into DX mov ah,09 ; Function 09h - output text string int 21h ; Dos service call mov dx,OldInt9[0] ; Offset address for old handler into DX mov ds,OldInt9[2] ; Segment address for old handler into DS mov ax,2509h ; Set interrupt vector for INT 9 int 21h ; DOS service call mov ax,4C00h ; Function 4Ch - terminate process int 21h ; DOS service call END

Parallel Ports LPT1 - LPT3 (Printer Output)

There can be three printers on a PC (four on a PS/2), however the number on a given system can be different from system to system. There is a reference to a parallel port, which refers to the plug where the printer cable attaches, and there are PC ports which are used to talk to a device. LPT1 uses PC ports 378h to 37Bh. Every printers port has its own adapter, which is controlled by by three registers, available through the corresponding I/O ports. The I/O ports belonging to a parallel port have sequentially increasing address, the first of swhich is referred to as the base address of a corresponding parallel port. The base address for LPT1 is stored in the BIOS data area at locatiion 0040h:0008h. If the value of the base address is zero, then the port is not installed. There is also a Configuration Word at 0040h:0010h, whose bits 14-15 holds the number of parallel ports installed.

The three parallel port registers are:

• Output Data Register
  Receives the byte which is being printed. The address of the this register for LPT1 is stored in the word 0040h:0008h.
• Status register
  Describes the most recent I/O operation. To get the address of the port connected to this register, simply add 1 to the base address, eg. for LPT1 it is 379h. It bits are:The Printer Status RegisterBitMeaning if set0Timeout1-2Not used3I/O error4Printer is On-Line5Out of Paper6Acknowledge (if zero, normal setting)7Printer is not busy
• Control Register
  Initializes the adapter and controls the output of data to the printer. Its address is equal to the addres of the output data register (base address) + 2, eg. for LPT1 it is 37Ah, the meanings of the bits are:The Printer Control RegisterBitMeaning if set0sending byte1CR treated as CR + LF2normal setting (reset printer when 0)3select printer4enable printe IRQ (IRQ 7 for LPT1)5 - 7not used

Levels of Access to the Printer

There are three levels of mastering the hardware:Logical levelMS-DOS service available through functions of interrupt 21hBasic levelBIOS service available through interrupt 17hLow levelControlling the printer ports directlyThe logical level is what high-level languages use to implement I/O. This allows the programmer to output strings and not worry about the details.

The basic level allows you to programmically check on the status of things and to provide a different type of control, instead of relying on the standard DOS functions.

The low level is the most powerful and most difficult. In the case of the printer, there is not too much different in low level and the basic level, except that there can be a slight improvement in speed.

Mouse Input

When the mouse is moved or when its keys are presed, signals are generated. They are accepted by the mouse driver and used for controlling a special screen pointer called the mouse cursor. Key presses must also include the information about which key was pressed.

Programs which support the mouse must be capable of receiving and interpreting the signals generated by the mouse. There are different types of mice, and each requires a unique driver.

All signals generated by a mouse could be processed in a user program, however very few applicaitons actually do this. Normally, the program uses the servcies of the mouse driver.

The Mouse Driver

Interrupt 33h is used for as the handler for the mouse service functions. Old versions of the operating system did not have an interrupt handler for this and the interrupt table pointed to 0000h:0000h.

Make sure that if you are programming with a mouse, that you don't call the null address!

An application program that uses a mouse gets information about its state and location and performs a relevant action. One of the most common uses of the mouse in applications is for selecting an item in a menu. This process involves the following steps:

• Output the menu to the screen.
• Check the mouse state - has a button be pressed? If not, repeat this step.
• Get the coordinates of the mouse when a button was pressed. It is was within a certain area of the screen, do that selection.

The mouse cursor is generated by the mouse driver that changes its location as you move the mouse. "When it is moved, the mouse generates short impulses called mickeys (christened by Bill Gates). The number of mickeys per inches is dependent on how the mouse was built and typically ranges from 200 to 400 mickeys per inch. Usually, the drivers moves the cursor by 1 pixel per mickey horizontally and 2 pixes per mickey vertically.

;*********************************************************************** ; Program PMouse2 ( Chapter 10 ) ; ; The demo program for mouse (the menu selection, Text Mode) ; ; Author: A.I.Sopin, Voronezh University. 1993 ; ; The interrupt 33h (mouse service) is used ; ; Mouse driver must be installed ; ;*********************************************************************** NAME PMOUSE2 .DOSSEG .MODEL SMALL .STACK 100h ;---------------------------------------------------------- .DATA BELL EQU 07 ; sound signal LF EQU 10 ; Line Feed CR EQU 13 ; Carriage Return TEXT0 DB " The MOUSE demo program (INT 33h). " DB " Press any key to continue...", BELL, CR, LF, "$" TEXT1 DB " The mouse driver is not installed !!!." DB " Press any key...", BELL, CR, LF, "$" TEXT2 DB " An active mouse driver found." DB " Press any key...", BELL, CR, LF, "$" TEXT3 DB 'The menu command selection using the mouse (text mode).' Ltxt3 EQU $-TEXT3 TEXT8 DB "Select Command and press Left Button:" Ltxt8 EQU $-TEXT8 TEXT10 DB "1 - Command one " Ltxt10 EQU $-TEXT10 TEXT11 DB "2 - Command two " Ltxt11 EQU $-TEXT11 TEXT12 DB "3 - Command three" Ltxt12 EQU $-TEXT12 TEXT13 DB "4 - Command four " Ltxt13 EQU $-TEXT13 TEXT14 DB "5 - Command five " Ltxt14 EQU $-TEXT14 TEXT15 DB "6 - Exit " Ltxt15 EQU $-TEXT15 TXT3L DB "Left button pressed. Command " NumSel DB 20h DB " selected." DB BELL, "$" VMODE DB 0 ; video mode saved ATTR DB 0 ; ROW0 DB 0 COL0 DB 0 CX0 DW 0 DX0 DW 0 ;---------------------------------------------------------- .CODE OutMsg MACRO Txt ;======= output text message lea dx,Txt ; adders of message mov ah,09h ; function 09h - output text string int 21h ; DOS service call ENDM WaitKey MACRO ;======= Wait for a key pressed xor ah,ah ; function 0 - wait for key pressed int 16h ; BIOS keyboard service ENDM SetCurs MACRO Row,Column ;======= Move the cursor mov ah,2 ; function 02h - set cursor position xor bh,bh ; video page 0 is used mov dh,&Row ; cursor row mov dl,&Column ; cursor column int 10h ; BIOS vide service call ENDM PutStr MACRO Row,Column,Text,Leng,Attrib Local M0 push si mov cx,Leng ; string length lea si,Text ; DS:SI - address of text string mov dl,Column ; initial position (column) cld ; process strings from left to right ; Outputting one character M0: SetCurs Row,dl ; lodsb ; AL - character to be output mov bl,Attrib ; BL - attribute mov ah,9 ; function 09 - output char+attr xor bh,bh ; video page 0 is used push cx ; save cycle counter mov cx,1 ; number of characters output int 10h ; BIOS video service call pop cx ; restore cycle counter inc dl ; next position for output loop M0 ; next cycle step pop si ; ENDM ;------------------------------------------------------------------- .STARTUP mov ah,0Fh ; function 0Fh - get video mode int 10h ; BIOS video service call mov VMODE,al ; save current video mode mov ah,0 ; function 0 - set video mode mov al,3 ; 80x25 Text int 10h ; BIOS video service call ; Output initial message OutMsg TEXT0 ; output initial message WaitKey ; check for mouse driver present mov ax, 03533h ; function 35h - get interrupt vector int 21h ; DOS service call mov ax,es ; segment address of handler or ax,bx ; AX - segment .OR. offset of int 33 jz Nomouse ; if full adders is 0 - no mouse mov bl,es:[bx] ; get first instruction of handler cmp bl,0CFh ; is this IRET instruction? jne Begin ; if not - driver installed Nomouse: OutMsg TEXT1 ; output message "driver not found" WaitKey ; wait for key pressed jmp Exit ; Exit program ;------------------------------------------------------------------- Begin: OutMsg TEXT2 ; output message "driver installed" WaitKey ; wait for key pressed ;------------------------------------------------------------------- ; Initialize mouse and report status (function 0 of INT 33h) Func0: xor ax,ax ; Initialize mouse int 33h ; mouse service call cmp ax,0 ; is mouse installed? jnz Clear25 ; if so, pass to function 10 jmp Exit ; if not, exit program ; Fill the screen (yellow character on blue background) Clear25:SetCurs 0,0 ; cursor to left upper corner mov ah,9 ; function 09h - output char+attr xor bh,bh ; video page 0 is used mov al,20h ; character to be output mov bl,1Eh ; attribute - yellow on blue mov cx,2000 ; number of characters to be output int 10h ; BIOS video service call ;------------------------------------------------------------------- ; Output the header and the menu text onto the screen PutStr 2,16,TEXT3, Ltxt3, 1Eh PutStr 8,20,TEXT8, Ltxt8, 1Eh PutStr 10,20,TEXT10,Ltxt10,1Fh PutStr 11,20,TEXT11,Ltxt11,1Fh PutStr 12,20,TEXT12,Ltxt12,1Fh PutStr 13,20,TEXT13,Ltxt13,1Fh PutStr 14,20,TEXT14,Ltxt14,1Fh PutStr 15,20,TEXT15,Ltxt15,1Fh SetCurs 25,80 ; move cursor out of screen ;------------------------------------------------------------------- ; Function 10 - define text cursor Func10: mov ax,10 ; define text cursor xor bx,bx ; software cursor is used mov cx,0FFFFh ; screen Mask mov dx,4700h ; cursor Mask int 33h ; mouse service call ;------------------------------------------------------------------- ; Function 1 - show the mouse cursor Func1: mov ax,1 ; function 01 - show mouse cursor int 33h ; mouse service call ;------------------------------------------------------------------- ; Determining mouse keys pressed Func3: mov ah,1 ; function 01h - check keyboard buffer int 16h ; BIOS keyboard service jz ContF3 ; if no key pressed, continue jmp Exit ; exit if key pressed ContF3: mov ax,3 ; func. 03 - button status and location int 33h ; mouse service call mov CX0,cx ; save X coordinate (column) mov DX0,dx ; save Y coordinate (row) test bx,1 ; left button pressed? jnz X_Range ; OK ! jmp short Func3 ; no button pressed - check again ; Check horizontal cursor location X_Range:mov ax,CX0 ; X coordinate (Column) mov cl,3 ; number bits to shift shr ax,cl ; shift by 3 - divide by 8 cmp ax,20 ; cursor on the left ? jb Func3 ; not - continue check cmp ax,36 ; cursor on the right? ja Func3 ; not - continue check ; Check vertical cursor location Y_Range:mov ax,DX0 ; X coordinate (Column) mov cl,3 ; number bits to shift shr ax,cl ; shift by 3 - divide by 8 cmp ax,10 ; cursor on the top ? jb Func3 ; not - continue check cmp ax,15 ; cursor on the bottom? ja Func3 ; not - continue check ; report the number of the command selected mov ax,DX0 ; Y coordinate (Row) mov cl,3 ; number bits to shift shr ax,cl ; shift by 3 - divide by 8 cmp ax,15 ; line 15 (Exit) ? je Exit ; if so - finish sub ax,9 ; number of command selected or al,30h ; convert to ASCII character mov NumSel,al ; put number to output message SetCurs 17,20 ; move cursor OutMsg TXT3L ; output message "command selected" jmp short Func3 ; check again ;------------------------------------------------------------------- ; Terminate program and exit to DOS Exit: mov al,VMODE ; remember video mode on entry mov ah,0 ; function 0 - set video mode int 10h ; BIOS video service Call CLRKEY ; clear keyboard buffer mov ax,4C00h ; function 4Ch - terminate process int 21h ; DOS service call ;------------------------------------------------------------------- ; ; This procedure clears the keyboard buffer ; ;------------------------------------------------------------------- CLRKEY PROC NEAR uses ax es mov ax,40h ; address of BIOS data segment mov ES,ax ; ES points to BIOS data cli ; no interrupts - system data modified mov ax,ES:[1Ah] ; buffer head printer mov ES:[1Ch],ax ; clear buffer (head ptr = tail ptr) sti ; buffer cleared - allow interrupts ret CLRKEY ENDP END

The Serial Ports - COM X

Parallel transfers are done with all bits being transmitted at the same time, with each bit being transferred down a separate wire in the cable. Serial transfers are done by sending one bit at a time down the same wire.

Sending or receiving one byte of data through the serial channel actuall involves transferring the following sequence of bits:

• Header (one start bit)
• Information bits (8, 7, 6, 5, or 4, depending on the work mode)
• Parity bit (optional)
• Stop bits (one or two)

This transfer takes place at a speed (measured in bauds).

A special chip, the Intel UART8250 (Universal Asynchronous Receiver Transmitter) was designed to support serial data transfers. Access to this chip is thorough the I/O ports.

MS-DOS support two basic communications ports - COM1 and COM2. Their base addresses are stored in the BIOS data area at 0040:0000h (COM1) and 0040:0002h (COM2). The ports used by COM1 and COM2 are not fixed, but 3F8h and 2F8h are common, but so is the reverse. Alway get the port numbers from the BIOS data area. Many different devices can be connected via the serial ports.

The 8250 chip is programmed by 10 one-byte registers, available through the corresponding I/O ports. Some registers are write-only, some are read only and the remainder can be both.

The Asynchronous Adapter RegistersPortMode valueDLABMeaning3F8hOUT0Transmitter holding register3F8hIN0Receiver holding register3F8hOUT1Divisor latch (low byte)3F9hOUT1Divisor latch (high byte)3F9hOUT0Interrupt enable register3FAhINInterrupt identification register3FBhOUTLine control register3FChOUTModem control register3FDhINLine status register3FEhINModem status register

This range covers only 7 addresses, you can increase the number of registers actually available to 10 by setting the 7th bit of the line control register (3FBh). This bit is called DLAB - Divisor Latch Access Bit).

Levels of controlling the serial ports

The three ways of controlling ports are:

• Using the MS-DOS services
• Using the BIOS service
• Direct Control

The MS-DOS service provides function 40h (Write File with Handle) of interrupt 21h. This method treats the communications port as a standard file named AUX with a logical number (descriptor) equal to 3. (This is only for COM1.) it is set to 2400 baud with no parity control, one stop bit and 8 bits per symbol.

You can use the BIOS service through interrupt 14h. These functions allow you to initialize the port, set parameters for transferring data such as number of stop bits, the type of parity control and the speed of transfer. however, it will only allow you to set the speed to 9600 baud.

Low-level programming lets you use all the UART 8250 chip facilities, including working with the transfer speed to 115K and over, but requires that the hardware interrupt related to communications ports be processed by your program.

Peripheral Interface - PPI

The PPI was originally based on the 8255A chip, and is used for controlling the keyboard, the internal sound generator, and for getting information about the system configuration.

The 8255A chip is controlled by the ports that are attached to it.

The Programmable Peripheral InterfaceDenotationAddressTypePurposePort A60hR/WKeyboard inputPort B61hR/WConfiguratin info, speaker and keyboard controlPort C62hRGet system information63hRMode control for ports A-Ch4hRKeyboard status (AT or PS/2)Port B of PPI - The Port 61hBitMeaning0Timer 2 gate (speaker)1Timer 2 data2Must be 031 = read high switches; 0 = read low swithces40 = enable RAM parity checking; 1 = disable50 = enable I/O channel check60 = hold keyboard clock low70 = enable keyboard; 1 = disable keyboardContents of PPI Port A (Bit 7 of Port B is set)BitContents00 - no floppy drives1Not used2-3The number of memory banks on the system board4-5Display mode
  11 = monochrome
  10 - color 80x25
  01 - color 40x256-7PC: The number of floppy disk drivesThe Contents of Port CBitMeaning0Values of DIP switches as in Equipment List1"2"3"4Must be 05If set - Time channel 2 out6if set - I/O channel check7if set RAM parity check error occurred

The Interrupt Controller - Intel 8259

The 8259 is also known as the PIC (Programmable Interrupt Controller) and it ensures that hardware interrupts are processed according to their priorities. The interrupt singal is generated by the hardware and sent to the microprocessor to inform it that the hardware state has changed and certain operations need to be performed.

In order to handle multiple interrupts occurring at the same time, there are 8 or 16 interrupt levels (using one or two 8259's). We know them as IRQ0 - IRQ7 and IRQ8 - IRQ15. The smaller the number, the higher the priority is for that interrupt. The highest priority IRQ0 is for the system timer. IRQ1 is for the keyboard. There is an interrupt vector in the BIOS for each IRQ. You can disable some or all of the interrupts. You disable all of them with the CLI (Clear Interrupt) and enable all of them with STI (Set Interrupt). NOTE: IRQ2 (I/O channel) can not can not be disabled and is called a non-maskable interrupt.

There is an Interrupt Mask Register (IMR) that allows you to disable only certain interrupts. The least significant bit is IRQ0 and if it is set to 1, the interrupt is disabled. The first 8 interrupts are controlled through port 21h and the second 8 are controlled through port 0A1h.

The Hardware InterruptsLevelVectorEnable MaskDisable MaskMeaningIRQ008hXXXX XXX0XXXX XXX1TimerIRQ109hXXXX XX0XXXXX XX1XKeyboardIRQ20AhXXXX X0XXXXXX X1XXI/O channelIRQ30BhXXXX 0XXXXXXX 1XXXCOM2 (AT), COM1 (XT)IRQ40ChXXX0 XXXXXXX1 XXXXCOM1 (AT), COM2 (XT)IRQ50DhXX0X XXXXXX1X XXXXHDD (LPT2 for AT)IRQ60EhX0XX XXXXX1XX XXXXFDD controllerIRQ70fh0XXX XXXX1XXX XXXXLPT1IRQ870hXXXX XXX0XXXX XXX1Real-time ClockIRQ971hXXXX XX0XXXXX XX1XTranslated into IRQ2IRQ1072hXXXX X0XXXXXX X1XXReservedIRQ1173hXXXX 0XXXXXXX 1XXXReservedIRQ1274hXXX0 XXXXXXX1 XXXXReservedIRQ1375hXX0X XXXXXX1X XXXXMath co-processorIRQ1476hX0XX XXXXX1XX XXXXHDD controllerIRQ1577h0XXX XXXX1XXX XXXXreserved

The Programmable Timer

The timer has three channels and operate at 1.193Mhz under control of the 8284A generator. The channels are available through ports 40h, 41h, and 42. The command register is available through port 43h.The Programmable Timer Command RegisterBitMeaning00 - binary data, 1 - BCD1-3Number of mode used (000-101)4-5Operation Type
  00 - send the value of counter
  01 - read/write high byte
  10 - read/write low/byte
  11 - read/write both high and low bytes6-7Number of channel to be programmed

Channel 0 is used by teh system clock to calculate the time of day. It is programmed for 18.2 pulses per second (called ticks) and stored in the BIOS data area at 0040h:006Ch. Every pulse initiates a timer interrupt 8h (IRQ0). Don't mess with this one!

Channel 1 is responsible for refreshing RAM and counts the pulses during disk operations, so that it can reset the timer counter upon completion of an operation. This is also not a good one to mess with.

Channel 2 (port 42h) is connected to the computer's speaker and issues square wave pulses used to make sounds. You can change the sound frequency with this channel.The 8255 chip, (PPI) is also involved in generating sound and that bits 0 and 1 of port 61h also control the speaker.

Which computer bus which allows the processor to communicate with the peripheral devices?

Which technique is used to communicate with peripheral devices?