ECE291 Computer Engineering II Lecture 13

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ECE291Computer Engineering IILecture 13

• Josh Potts
• University of Illinois at Urbana- Champaign

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Outline

• Lookup tables
• Jump tables
• Interrupt vectors
• Interrupt functions examples

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Lookup Tables

• Lookup tables are often used to convert from one
  data form to another
• A lookup table is formed in the memory as a list
  of data that is referenced by a procedure to
  perform conversions
• XLAT instruction can often be used to lookup data
  in a table
• place the offset of the array in the BX and
• the desired index value in AL
• XLAT returns the byte element in AL
• AL cannot contain an index value greater than 255
• you can use XLAT with arrays containing more than
  256 elements, simply treat each 256-byte block of
  the array as a smaller sub-array, e.g., to
  retrieve the 260th element add 256 to BX and set
  AL 3 (I.e. the 260th element is at offset 259
  which equals 256 3)

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Table-Lookup FunctionsExample 1

• MBegin CMP DIR,UP
• JE MoveUP
• CMP DIR,RT
• JE MoveRT
• CMP DIR,DN
• JE MoveDOWN
• CMP DIR,LT
• JE MoveDown
• MoveUP SUB Pos,COLSIZE
• JMP MDone
• MoveRT ADD Pos,1
• JMP MDone
• MoveDN ADD Pos,COLSIZE
• JMP MDone
• MoveLT SUB Pos,1
• JMP MDone
• MDone Finished!

• Consider movement in an array by one element
• Let Array
• Matrix of ROWSIZE x COLSIZE elements
• Let Pos Position in array
• Row COLSIZE Column
• Let Dir
• Direction (UP0, RIGHT1, DOWN2, LEFT3)
• Slow and Tedious Original Code
• Compares each possible direction value

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Table-Lookup FunctionsExample 1 (cont.)

• Fast and Compact Table-lookup Code
• Eliminate conditional jumps by defining a table
  of movements indexed by the Direction.
• Movement dw -COLSIZE Movement0
  Go UP
• dw 1 Movement1
  Go RIGHT
• dw COLSIZE Movement2
  Go DOWN
• dw -1 Movement3
  Go LEFT
• MBegin MOV BX, DIR Direction
  0..3
• SHL BX, 1
  Index words, not bytes
• MOV AX, MovementBX
• ADD Pos, AX
  Position Movement
• MDone Finished!

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Table LookupExample 2

• DAYS PROC FAR
• PUSH DX
• PUSH SI
• MOV SI, OFFSET DTAB
• XOR AH, AH clear AH
• ADD AX, AX double AX
• ADD SI, AX modify table address
• MOV DX, CSSI get string address
• MOV AX, CS change data segment
• PUSH DS
• MOV DS, AX
• MOV AH, 09h

• INT 21h
• POP DS
• POP SI
• POP DX
• RET
• DTAB DW SUN, MON, TUE, WED, THU, FRI, SAT
• SUN DB Sunday
• MON DB Monday
• TUE DB Tuesday
• WED DB Wednesday
• THU DB Thursday
• FRI DB Friday
• SAT DB Saturday
• DAYS ENDP

Converts numbers 0 to 6 to days of the week The
number is passed in AL register
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Jump Tables

• Suppose table entries are pointers to functions
• Suppose BX holds a command in the range of
  0..N-1.
• Let cmd index a function
• JFUNCTION PROC NEAR/FAR Input BX
  Command
• SHL BX,1
  Words, not bytes
• JMP JtableBX
  Jump to function
• Jtable dw offset
  Funct0
• dw offset
  Funct1
• ..
• dw offset
  FunctN-1
• Funct0 do something ..
• RET
• Funct1 do something else ..
• RET
• FunctN-1 do something else ..
• RET
• JFUNCTION ENDP

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Interrupt Vectors

• Jump Tables are the basis for interrupt vectors
• On 80x86, low 1024 bytes of memory hold vector
  table
• There are 256 pointers in the table
• Each entry is a far (4 byte segmentoffset)
  pointer
• Memory address (hex) Interrupt
  function pointer
• 003FC INT 255
• 4 x INT x
• 00008 INT 2
• 00004 INT 1
• 00000 INT 0

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Interrupt Vectors

• The term interrupt is used in three different
  contexts
• Software interrupts (e.g., 21h)
• provide a mechanism whereby the programmer can
  use the int instruction to access blocks of code
  that already exist and are resident in machine
  memory
• Hardware interrupts
• triggered by hardware events external to the
  microprocessor, rather than resulting from an
  int or any other instruction
• e.g., a request for service by external devices
  such as disk drives
• Exceptions
• hardware origin within the microprocessor itself
• e.g., anomalous event within a program, such as
  an attempt to divide by zero

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Interrupt Functions Examples DOS Functions

• To execute one of many DOS functions, you can
  specify a sub-function by loading a value into AH
  just before calling INT 21
• INT 21h, Subfunction
• AH3Dh Open File
• AH3Fh Read File
• AH3Eh Close File
• AH13h Delete File (!)
• AH2Ah Get system date
• AH2Ch Get system time
• AH2Ch Read DOS Version
• AH47h Get Current Directory
  
• AH48h Allocate Memory block
  (specified in paragraphs16 bytes)
• AH49h Free Memory block
• AH4Ch Terminate program
  (and free resources)

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Interrupt Functions Examples BIOS Functions

• SYSTEM BIOS Functions
• All PCs come with a BIOS ROM (or EPROM).
• The BIOS contains standard 80x86 opcodes to
  provide basic functions such as bootstraping and
  primitive I/O.
• INT 19h Reboot system
• INT 11h Get equipment configuration
• INT 16h Keyboard I/O

• VIDEO BIOS Functions
• Video cards come with 80x86 commands burned in a
  ROM. Video BIOS is located at C0000-C7FFF and
  holds routines for handling basic functions.
• To execute a function in video BIOS ROM, call INT
  10 with AXsub-function.
• INT 10h, Subfunction
• AH0,AL2h 80x25 Text
• AH0,AL13h 320x200, 256-color graphic

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Interrupt Functions ExamplesMOUSE Hardware
Functions

• MOUSE Functions INT 33h
• Routines to read position and status of mouse
• AX0000h Reset
• AX0001h Show Mouse
• AX0003h Get Position Status
• BX Buttons Pressed bit0left, bit2right
• CX Column Position
• DX Row Position

• HARDWARE Functions
• IRQ 0-7 FUNCTIONS INT 8-Fh
• System timer (IRQ 0 INT 8h)
• Keyboard (IRQ 1 INT 9h)
• Bus/DMA (IRQ 2 INT Ah)
• Serial Port (IRQ 3 INT Bh)
• SoundBlaster (IRQ 5 INT Dh)
• INT 8 IRQ
• IRQ 8-15 FUNCTIONS INT 70h-77h
• SCSI Request (IRQ 10d INT 72h)
• INT 68h IRQ