Device Drivers: Programmed IO and Printers

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Device DriversProgrammed I/O and Printers
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I/O Ports

• Communication with External Devices requires an
  interface.
• The interface is an electronic chip that is
  connected to the mother board of the computer.
• Peripheral interface chips have registers called
  PORTS
• Kinds of Ports
• Data Port To hold the data to be transmitted or
  received from the device
• Control Port To set send or receive modes
• Status Port To provide information about the
  device.

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I/O Instructions

• The various ports attached to the 8086-family
  based microprocessors are referred to by a number
  (0-0FFFFH)
• Setting a Port
• (1) OUT DX, Accumulator (AL/AX)
• (2) OUT port (0-0FFH), Accumulator
• Getting Data from a Port
• (1) IN Accumulator, DX
• (2) IN Accumulator, Port (0-0FFH)
• The accumulator can be AX or AL depending on
  whether a 8 or 16 bits port is used.

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E.g. Programming a Parallel Printer (1)

• The Ports used in a standard parallel printer
• Data Port ? 3BCH
• Output Control Port ? 3BEH
• Printer Status ? 3BDH
• To print one character
• Initializing the Printer
• Put the character in the data port
• Check for printer ready in the status port
• Send the character using the control port

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E.g. Programming a Parallel Printer (2)

• Initializing the Printer
• Bit 2 of the control output port should be set
  to 0 for at least 50 microseconds. Then set it to
  1.
• MOV DX, 3BEH control port address
• MOV AL, 08H 0000 1000
• OUT DX, AL
• MOV CX, 50D
• WAIT NOP
• LOOP WAIT
• MOV AL, 0CH 0000 1100
• OUT DX, AL

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E.g. Programming a Parallel Printer (3)

• Setting the Data port to the character to be
  printed
• MOV AL, 42H ASCII of B
• MOV DX, 3BCH data port number
• OUT DX, AL Setting the port
• Checking the Printer if it is ready (Polling)
• MOV DX, 3BDH status port address
• Test_If_Busy IN AL,DX
• TEST AL, 80H
• JZ Test_If_Busy

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E.g. Programming a Parallel Printer (4)

• Sending the Character
• To start printing bit 0 should be 1
• To Stop printing bit 0 should be 0
• Bits 2 and 3 must always be 0
• Code to start printing the char B
• MOV DX, 3BEH control port address
• MOV AL, 0DH 0000 1101
• OUT DX, AL start printing
• MOV AL, 0CH
• OUT DX, AL stop printing

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Asynchronous Serial Communication

• What is asynchronous serial communication serial
  communication in which serial characters
  transmitted are separated by special bit
  patterns.
• Difference between serial and parallel
  transmission (cost vs. speed)
• Serial transmission causes extra problems
• Recognize beginning of a character e.g. Figure
  20.5, p 527 add a start bit(a 0 bit before each
  char Figure 20.6 p 527)
• Corruption of data
• E.g 01000011 (c) instead of 01000010 (b)
• Add one more bit for checking (odd or even parity
  systems)
• Synchronization between transmitter and receiver
• Stop bits (E.g Figure 20.7 and 20.8, p 529)

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Speed of Transmission

• The unit is the baud (Baudot an early pioneer in
  data communication) bits/second
• Early US based teletype computers used
• 1 start bit 7 data bits 2 stop bits 11 bits
• 10 characters/ s
• 110 baud slowest transmission rate used in the
  microcomputer industry
• General Electric produced 300 baud terminals 10
  bits (1 start bit1 stop bit 7 data bits even
  parity), 30 chars/s
• Speeds of 1200, 2400, and 9600 are common now

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Programming an asynchronous interface

• The most popular support chip for asynchronous
  serial communications 8250 asynchronous
  communication element
• 7 I/O ports (Table 20.1, p 531)
• 2 data ports
• 2 status port
• 6 control ports
• Initializing the interface requires five of these
  ports
• Baud rate divisor (LSB)
• Baud rate divisor (LSB)
• Line Control register
• Modem Control register
• Interrupt enable register

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Programming an asynchronous interface (2)

• To print on character via a serial interface
• Set the baud rate
• Set the character format
• Set the modem control register
• Disable interrupts
• Check the communication status
• Send the character

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Example

• Specifications
• Print a character via a serial interface
• Baud Rate 1200 baud
• Data format 7 data bits 1 stop bit odd
  parity
• Steps
• Set the baud rate divisor registers
• MSB 00H and LSB 60H
• Initialize the line control register to determine
  the data format
• Set bits 5, 6, and 7 to 0
• Initialize the modem control register
• Set bit 0 and 1 to 1
• Clear the interrupt enable register to disable
  all interrupts
• Check the line status register before sending any
  character.

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Setting the Baud Rate

• Select the baud rate divisor registers
• MOV DX, 3FBH
• MOV AL, 80H
• OUT DX, AL
• Set the baud rate divisor registers To 1200 baud
• MOV DX, 3F8H Setting LSB
• MOV AL, 60H
• OUT DX, AL
• MOV DX, 3F9H Setting MSB
• MOV AL, 00H
• OUT DX, AL

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More initializations

• Initialize the line control register Format of
  7 bits 1 stop bit odd parity
• MOV DX, 3FBH Line control register
• MOV AL, 0000 1010 B
• OUT DX, AL
• Initialize the modem control register setting
  the RTS and DTR signals
• MOV DX, 3FCH
• MOV AL, 3H bits 0 and 1 initialized to 1
• OUT DX, AL

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More initializations (2)

• Disabling Interrupts
• MOV DX, 3F9H Giving access to the interrupt
  identification register
• MOV AL, 0
• OUT DX, AL
• MOV DX, 3FAH accessing the II register
• OUT DX, AL
• Checking the Status Control Register
• READY_CHECK
• MOV DX, 3FDH
• IN AL, DX
• TEST AL, 20H
• JZ READY_CHECK
• Send the character
• MOV DX, 3F8H
• OUT DX, AL

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Direct Memory Access I/O and the Display Screen
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What is DMA?

• Direct Memory Access A technique for
  transferring data from memory to a device without
  passing through the CPU.
• Why DMA?
• DMA is useful when the neither the programmed I/O
  nor the interrupt I/O techniques can provide the
  required data transfer rates.
• The computers display requires high data rates
  gt DMA is used
• Communicating to devices that use DMA is done
  simply by accessing a specific group of locations
  in memory through DMA channels
• Example Display of a text character on the
  screen in a specific location

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Text Display (1)

• To work with text in black and white (see Table
  21.4)
• BIOS INT 10H
• Selecting the mode of 07H monochrome text
• The screen has 80252000 characters to display
  at the same time in this mode
• Display has 16K bytes addressable memory
• The DMA address starts at 0B8000H
• To display a character in the position (x,y) in
  the 8025 screen
• Store the character in the even address
• 0B8000H (160 x) (2 y)
• Store the attribute of the character in the
  address
• 0B8000H (160 x) (2 y) 1

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8025 screen mode
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Examples (1)

• There are 8 different Display attributes for
  color display in black and white for the 8025
  text mode (see figure 22.1)
• Displaying character A in the position (1,5) in
  reverse video mode
• MOV AX, 0B800H
• MOV ES, AX
• MOV DI, (1160D)(52)
• MOV AL, A
• MOV ESDI1, AH
• MOV ESDI, AL

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Examples (2)

• Clearing the screen
• MOV AX, 0B800H DMA segment
• MOV ES, AX
• CLD Move forward
• MOV DI, 0 (0,0) position
• MOV AH, 07H Normal attribute
• MOV AL, space character
• MOV CX, 2000D number of characters in the
  screen
• REP STOSW

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The 6845 CRT controller

• The 6845 Cathode Ray Tube (CRT) controller is a
  chip that controls what appears on the screen
• Main functions
• Select the character code to be displayed
• Drawing the character on the screen using
  miniature dots
• Generating cursor images
• Positioning the cursor
• Synchronizing the different operations of the
  graphical adapter circuitry
• 16 registers accessible through 2 ports
• 3D4H gt Address register
• 3D4H gt 16 registers (see Table 22.1, p 568)

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The 6845 CRT controller registers

• Horizontal registers determine the of
  characters per line and the width of each
  character
• Vertical registers determine the the of
  characters per column and how many miniature dots
  the character requires
• Cursor start and end registers determine where
  the cursor is displayed according to the text,
  its position and its height.
• Start address registers determine where in the
  the 16 k memory The first character will be
  displayed

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Using the 6845 CRT controller

• To display a character in the position (x,y) the
  corresponding offset from the DMA start address
  is (x 80D) y a 0 bit is all the time
  appended to the character code address and a 1
  bit to the attribute address
• Example Routine that positions the cursor in
  position (ROW, COLUMN).
• Assume we have the following declaration
• COLUMN DW ?
• ROW DB ?

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Position Cursor Routine

• POSITION_CURSOR
• Calculating the offset address
• MOV AX, 80D
• MUL ROW
• ADD AX, COLUMN
• MOV BX, AX
• Setting the 6845 address register to access
  register 14D
• MOV DX, 3D4H
• MOV AL, 14D
• OUT DX, AL
• Set Cursor address (high)
• MOV DX, 3D5H
• MOV AL, BH
• OUT DX, AL
• Set the 6845 address register to access register
  15
• MOV DX, 3D4H
• MOV AL, 15D
• OUT DX, AL
• Set cursor address low

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Displaying Text in Color

• The color/graphics adapter has to be installed in
  the microcomputer
• Select the color mode using BIOS INT 10H
• Several color modes can be selected (See the BIOS
  Interrupts sheet or Table 21.1)
• Each color mode has its resolution
• Some modes have complete displays, or pages of
  text stored in memory. Each page has 2K bytes
• Example 4 pages of the 8025 display can be kept
  sequentially in DMA memory. 8 pages can be kept
  for the 4025 display.
• Select the background and foreground color using
  the attribute bye and the 6845 chip (port 3D9H)
• 16 possible foreground colors
• 8 possible background colors (see Table 22.2)

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Displaying Text in Color (2)

• There are 2 sets of 8 colors for the background
  colors but they cannot be used simultaneously.
• Port 3D9H gives access to the color select
  register, which determines the screen border
  color which set is to be used for the background
  color (Background I bit) see Figure 22.4 and
  22.5, p 575
• Hardware Scrolling
• The current page number is determined by the
  value in the start-address registers of the 6845
  chip
• The display is scrolled faster
• Example Scrolling with one line for the 8025
  display
• MOV DX, 3D4H address register to select
• OUT DX, 13 Start address low
• MOV DX, 3D5H register value
• OUT DX, 160D to move to the second row

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Displaying Text in Color (3)

• Switching to other pages or scrolling can also be
  achieved using the BIOS INT 10 (see the
  interrupts sheet)
• AH 05h, to switch to the page whose value in AL
• AH 06H, to scroll up the window
• AH 07h, to scroll down the window
• To Draw, you need to
• Select the graphic mode
• Set a target column, row, and page number
• Find the corresponding offset and DMA address
• Choose the color through the attribute byte
• Store the character and attribute in the proper
  DMA location
• Cycle the pages with a pause for each

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Instruction Encoding
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Introduction

• The machine code generation phase of a compiler
  is tightly dependent on the different Addressing
  modes
• How instructions are encoded

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Instruction Encoding

• How is a particular instruction encoded from
  assembly to machine code?
• 2 Basic variations single operand and 2
  operands.
• Why it is important to know Instruction encoding
• A better understanding of the 8086 architecture
• Implementation of 8086 assembler and debugger
• Help explain the decoding process

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Single Operand Instructions (1)

• Example The INC Instruction
• 2 encoding patterns
• Short form When INC is used with register
  addressing (16 bit registers)
• A general pattern register addressing (8-bit
  registers), Direct and indirect addressing mode

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Single Operand Instructions (1)

• Short form
• Occupies 1 Byte gt The most efficient
  instructions
• 5 bits- opcode(25 32 different ops possible)
• 3 bits- operand (8 different registers)
• 01000 011gt 43H gt INC BX
• 01000 is the opcode for the INC mnemonic
• See Table 13.3 (P 331) for register values

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Single Operand Instructions (12)

• Most general form
• Increment 8 bit or 16 bit registers
• Contents of directly and indirectly addressed
  memory locations
• 2 Bytes encoding
• FE C3 gt INC BL
• 1111111 0 11 000 011

r/m field
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Single Operand Instructions (3)

• Opcode (10 bits) split into 2 parts (7 bits in
  the 1st byte and 3 bits in the 2nd byte)
• w field (1bit ) size of the operand 0 8 bits 1
  16 bits
• Mod field (2 bits) 11 register, other memory
• R/m (3 bits) register or memory locations
• Both the mod and the r/m field give the
  addressing mode
• Another example
• INC BBX-4
• 1111111 0 01 000 111 1111 1100

-4 for the displacement
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Single Operand Instructions (4)

• FE 06 01 A gt INC my_location (01A3)
• 1111111 0 00 000 110 0000 0001 1010 0011
• Op 111111
• W 0, 8 bit
• Mod 00, 16 bit displacement
• OP 000
• R/m no index register or base
• Offset 0000 0001 1010 0011

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Operands Instructions (1)

• One operand must be a register
• Max length restricted to 6 bytes
• One more additional bit the d field to specify
  the destination
• 0 register is the source
• 1 register is the destination
• ADD BL, DH
• 000000 10 11 011 110
• Opcode dw mod Reg r/m
• ADD my_location, DX
• 000000 01 00 010 110 10100010
  10111100
• Opcode dw mod Reg r/m Offset address