ECE 353 Introduction to Microprocessor Systems

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ECE 353Introduction to Microprocessor Systems

Discussion 4
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Topics

• Look-up Table (LUT)
• Using loops
• QA

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Look-Up Table (LUT)

• What? table of related values stored in
  memory for use by the program. (Examples
  temperature conversions, calibration info)
• Why?
• - memory access may be much faster than doing
  some set of calculations
• - natural solution for database type of
  information.

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Look-Up Table (LUT)

• Example A.) Write a code fragment that will use
  a look-up table to convert the value in the least
  significant four bits of R0 to its ASCII
  hexadecimal equivalent number and store result in
  R1.

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Look-Up Table (LUT)
Filename main.s Author ECE 353
student Description main program file for
Discussion 4 Look-up Table (LUT)
examples ARM use ARM instruction
set EXPORT __main AREA FLASH, CODE,
READONLY __main part A assumes value in
R0, so preload a sample value MOV R0,
11 AND R0, R0, 0xf only want least sig.
four bits ADR R2, HexLUT load start address
of LUT LDRB R1, R2, R0 do look-up B
__main HexLUT DCB "0123456789ABCDEF" END
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Look-Up Table (LUT)

• Example B.) Write a code fragment that will use
  a look-up table to return in R0 the 16-bit
  square of the unsigned byte in R0. Only compute
  the first and last three entries in the table.

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Look-Up Table (LUT)
Filename main.s Author ECE 353
student Description main program file for
Discussion 4 Look-up Table (LUT)
examples ARM use ARM instruction
set EXPORT __main AREA FLASH, CODE,
READONLY __main part B assumes value in R0,
so preload a sample value MOV R0, 1 AND R0,
R0, 0xff only want lsb ADR R2,
SquareLUT load start address of
LUT MOV R0, R0, LSL 1 multiply by 2 since
half-word lookup LDRH R0, R2, R0 do
look-up B __main SquareLUT DCW 0, 1,
4 first three entries more values we dont
show here DCW 64009, 64516, 65025 last three
entries END
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Looping

• Write a code fragment that will get up to 100
  characters from a byte I/O port at address
  0x00011000 and store them in an array. If a zero
  value is read from the port, store the zero and
  then quit. Otherwise, when 100 characters have
  been read, store a zero terminator in the array
  and quit.
• Outline
• Setup storage area
• Setup parameters
• Code
• initialization
• loop
• read port values
• check for zero value (branch to done if zero)
• store value
• update count
• loop if lt100 values
• done -gt store zero

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Looping

• Write a code fragment that will get up to 100
  characters from a byte I/O port at address
  0x00011000 and store them in an array. If a zero
  value is read from the port, store the zero and
  then quit. Otherwise, when 100 characters have
  been read, store a zero terminator in the array
  and quit.
• Filename main.s
• Author ECE 353 staff
• Description main program file for discussion
  problem Looping
• ARM use ARM instruction set
• EXPORT __main
• AREA FLASH, CODE, READONLY
• __main
• loop

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Looping

• Write a code fragment that will get up to 100
  characters from a byte I/O port at address
  0x00011000 and store them in an array. If a zero
  value is read from the port, store the zero and
  then quit. Otherwise, when 100 characters have
  been read, store a zero terminator in the array
  and quit.
• Filename main.s
• Author ECE 353 staff
• Description main program file for discussion
  problem Looping
• ARM use ARM instruction set
• EXPORT __main
• MAXITER EQU 100
• PORT EQU 0x00011000
• AREA SRAM, DATA, READWRITE
• array SPACE MAXITER 1 need byte for terminator
• AREA FLASH, CODE, READONLY
• __main
• MOV R0, MAXITER max iteration count
• MOV R1, PORT port address
• LDR R2, array array address
• loop

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Questions?