Assembler Programming

1
Assembler Programming

• Chapter 6

2
EEL-4746 Best Practices
3
EEL-4746 Best Practices

• All programs must begin with the following
  comment
• EEL-4746 Spring 2004 Semester
• Homework N Due (Due Date)
• Problem M
• Name of Partner A
• Name of Partner B
• Description of Program

4
EEL-4746 Best Practices

• All subroutines must begin with the following
  comment
• Subroutine Name Mysub
• Input parameter list
• Output parameter list
• Registers changed list
• Description of Subroutine

5
EEL-4746 Best Practices

• All lines must contain a comment
• All labels must end with a semicolon
• Must use a symbol for all constants

6
EEL-4746 Best Practices

• Program must have the following format
• Header Comment
• Standard Symbols
• Data EQU 0000
• Program EQU E000
• Stack EQU 00FF
• Reset EQU FFFE
• Place your symbols here
• Program
• Top ORG Program
• Label Program line comment
• Data
• ORG Data
• Variable Directive Data goes here
• ORG Reset

7
Additional Comments

• To Indent or Not Indent
• Not really used in assembly language programming
• Upper,Lower, and Mixed Cases
• Be consistent
• Symbols for constants in ALLUPPERCASE
• Labels for in-memory variables in lowercase
• Code labels in MixedCase
• Instructions all uppercase or all lowercase
• Etc
• Subroutine Headers
• Develop the habit of placing comments before
  subroutines. The comment should give the
  function of the subroutine, the input values it
  expects, and the registers that it modifies.

8
Pseudo-code

• A fictitious programming language which allows
  a general algorithm for a computer program to be
  developed.
• Design process
• Develop program using pseudo-code
• Convert pseudo-code to 68HC11 assembly language

9
Structured Assembly Language Programming

• If Then Else End IF
• Pseudo-Code Syntax
• If Variable_A Condition Variable_B Then
• Pseudo Code for Then condition
• Else (Optional)
• Pseudo Code B for Else condition
• End If

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Structured Assembly Language Programming

• LDAA Variable_A
• CMPA Variable_B
• B?? Else
  Opposite of condition
• Code for Then part
• .
• BRA END_IF need to skip else
  part
• Else Code for Else part
• ..
• END_IF Rest of Code

11
Structured Assembly Language Programming

• If-Then-Else-End IF
• Condition Branch to Use (Opposite of
  condition)
• ltgt BEQ (Branch if equal)
• BNE (Branch if not equal)
• Signed
• lt BGT (Branch if greater than)
• lt BGE (Branch if greater than or equal)
• gt BLT (Branch if less than)
• gt BLE (Branch if less than or equal)
• Unsigned
• lt BHI (Branch if higher)
• lt BHS (Branch if higher or same)
• gt BLO (Branch if lower)
• gt BLS (Branch if lower or same)

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If-Then-Else Example

• Pseudo-code
• IF temp gt max_temp then
• Then code
• Else
• Else code
• End If

13
If-Then-Else Example

• Assembly Language
• LDAA Temp
• CMPA MAX_TEMP
• BLS Else
• Then code here
• BRA ENDIF
• ELSE
• ELSE code here
• EndIf
• Rest of program here

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Structured Assembly Language Programming

• Loops
• For Loops
• While-Do Loop
• Do-While or (Repeat Until) Loops

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Structured Assembly Language Programming

• For Loops
• Pseudo-code Syntax
• For loop_index start_index to end_index
• Begin
• Code to Execute
• ..
• ..
• End For Loop

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Structured Assembly Language Programming

• Assembly Language
• Use one of the registers for the loop index.
• A,B8 bit (255 max)
• X,Y16 bit (65535 max)
• Must use memory if index value is greater than
  65535
• Example Code Fragment
• LDAA start_index
• Loop Start of Code to Execute (must not change
  A)
• .
• INCA
• CMPA end_index
• BLS Loop

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Structured Assembly Language Programming

• What if you need the loop index register (e.g. A)
  in your code?
• Example Code Fragment
• LDAA start_index
• Loop Start of Code to Execute
• PSHA Save A on stack
• Use A
• .
• PULA Restore A from stack
• INCA
• CMPA end_index
• BLS Loop

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Structured Assembly Language Programming

• For Loops (Count down)
• Pseudo-code Syntax
• For loop_index end_index downto start_index
• Begin
• Code to Execute
• ..
• ..
• End For Loop

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Structured Assembly Language Programming

• Example Code Fragment
• LDAA end_index
• Loop Start of Code to Execute
• .
• DECA
• CMPA start_index
• BHS Loop

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Structured Assembly Language Programming

• For Loops (Count down from N to 1)
• This counts N items
• Pseudo-code Syntax
• For loop_index end_index downto 1
• Begin
• Code to Execute
• ..
• ..
• End For Loop

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Structured Assembly Language Programming

• Example Code Fragment
• LDAA N
• Loop Start of Code to Execute
• .
• DECA
• BNE Loop Save one statement

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TPS Quiz
23
Structured Assembly Language Programming

• While-Do Loops
• Pseudo-code Syntax
• While Variable Condition Constant_VALUE
• Do Code Here
• Begin
• Code to Execute
• ..
• ..
• End While

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Structured Assembly Language Programming

• While-Do Loops
• Loop LDAA Variable
• CMPA Constant_Value
• B?? End_While (opposite of
  condition)
• Do code here
• ..
• BRA Loop
• End_While End of While loop

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Structured Assembly Language Programming

• While-Do
• Condition Branch to Use (Opposite of
  condition)
• ltgt BEQ (Branch if equal)
• BNE (Branch if not equal)
• Signed
• lt BGT (Branch if greater than)
• lt BGE (Branch if greater than or equal)
• gt BLT (Branch if less than)
• gt BLE (Branch if less than or equal)
• Unsigned
• lt BHI (Branch if higher)
• lt BHS (Branch if higher or same)
• gt BLO (Branch if lower)
• gt BLS (Branch if lower or same)

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Structured Assembly Language Programming

• Example Temp gt 4F (unsigned)
• Pseudo-code
• While Temp gt 4F
• Do
• (Execute your code here)
• End_While End of While loop

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Structured Assembly Language Programming

• While Temp gt 4F (unsigned)
• Assembly Code
• Loop LDAA Temp
• CMPA 4F
• BLS End_While (opposite of
  gt)
• Do code here
• ..
• BRA Loop
• End_While End of While loop

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Structured Assembly Language Programming

• Do-While or (Repeat-Until) Loops
• Pseudo-code Syntax
• Do (or Repeat)
• Begin
• Code to Execute
• ..
• ..
• While Condition (or Until Condition)

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Structured Assembly Language Programming

• Do-While or (Repeat-Until) Loops
• Assembly Language Syntax
• Loop
• Do Code Here
• ..
• ..
• LDAA Variable
• CMPA Constant
• B?? Loop (same as condition)

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Structured Assembly Language Programming

• Do-While or (Repeat Until)
• Condition Branch to Use (Same as condition)
• BEQ (Branch if equal)
• ltgt BNE (Branch if not equal)
• Signed
• gt BGT (Branch if greater than)
• gt BGE (Branch if greater than or equal)
• lt BLT (Branch if less than)
• lt BLE (Branch if less than or equal)
• Unsigned
• gt BHI (Branch if higher)
• gt BHS (Branch if higher or same)
• lt BLO (Branch if lower)
• lt BLS (Branch if lower or same)

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Example

• Write a 68HC11 assembly language program that
  converts X into an equivalent ASCII signed
  decimal value. Store your result in a memory
  location labeled Result.

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Pseudo-code

• X Number_to_convert
• sign
• if X lt 0 then
• sign -
• X -X
• end If
• result0 sign
• call X2ASC(X)
• end program
• X2ASC is a subroutine that converts the X
  register (unsigned) to ASCII

Well put this code intoa subroutine
calledSX2ASC that convertsthe X register
(signed)to ASCII.
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Lets convert the code

• For this line of pseudo-code
• X Number_to_convert
• Well create the following code
• NUMBER EQU 8001 -32767
• ORG CODE_AREA
• Main LDX NUMBER Put it in regX
• JSR SX2ASC Signed convert
• End BRA End Infinite loop

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Next line

• At the start of the SX2ASC subroutine, we have
  the pseudocode line
• sign
• Well convert this to
• ORG DATA_AREA
• sign RMB 1 1 byte for sign char
• ORG CODE_AREA
• SX2ASC LDAA Take an ASCII sign.
• STAA sign Store it in sign var.

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Next line

• Next, we have
• if X lt 0 then end if
• This converts to the assembly
• CPX 0 Compare X to 0.
• BHS endIf If Xgt0, skip if body
• (IF body goes here)
• endIf (code after IF goes here)

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A more difficult case

• Consider the pseudocode line
• X -X (X gets negative X)
• Problem Theres no NEGX instruction!
• How can we work around this?
• One solution
• varX RMB 2 Reserve space for X in mem.
• STX varX Save X in memory at varX
• LDD 0 Load accum. D with a 0
• SUBD varX Let D 0 varX -X
• XGDX Copy D back to X.

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Full SX2ASC in assembly

• SX2ASC LDAA ' Take an ASCII "" sign.
• STAA sign Store it in "sign"
  variable.
• CPX 0 Compare X to 0.
• BHS endIf If X lt 0, then...
• LDAA '- Take an ASCII "-"
  sign.
• STAA sign Store it in "sign"
  variable.
• STX varX Save X in "varX"
  variable.
• LDD 0 D 0.
• SUBD varX D 0 - varX -varX
  -X.
• XGDX Copy D back to X.
• endIf LDAA sign A sign character.
• STAA result0 Store sign char. in
  result0.
• JSR X2ASC Conv. unsigned X to
  result1-5.
• RTS End of subroutine
  return.

X -X
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Pseudo-code X2ASC (w. divide)

• function X2ASC(X,result)
• D X
• for B 0 to 3
• A floor(D / convertB)
• where convert 10000,1000,100,10
• D D AconvertB Let D be the
  remainder
• A A or 30 Convert A from 0-9 to
  ASCII
• resultB1 A
• end for
• A D D should have the ones
• A A or 30 Convert to ASCII
• result4 A
• end function

Note When converting this to working
assemblycode, we must be careful about
overlapping registers.
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for loop outline

• The skeleton of our for loop
• for B 0 to 3 (body of for loop)end for
• Translates to
• LDAB 0
• forBody body code goes here
• INCB B B 1
• CMPB 3 Compare B to 3
• BLS forBody If Blt3, continue
• But, we must be careful to preserve the value of
  B within the body code!

40
Accessing an array of words

• We can set up the convert array as follows
• convert FDB 10000,1000,100,10
• But, how do we access convertB?
• CLRA Effectively sets DB
• ASLD Double D, its 2B.
• XGDY Transfer this to Y
• LDX convert,Y X (convert2B)
• But, this only works if convert array is on page
  0!
• More generally, wed have to do this after the
  ASLD
• ADDD convert D convert2B
• XGDY Transfer D to Y
• LDX 0,Y X Y convertB

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Unsigned X2ASC in assembly

• X2ASC STX varD Initialize variable D
  X.
• LDAB 0 For B 0 to 3,
• ForBodySTAB varB Save variable B.
• CLRA Clear MSB of
  accumulator D.
• ASLD Double D to get
  2varB.
• XGDY Move it into Y.
• LDX convert,Y Let X (convertY)
  convertvarB
• LDD varD Load old variable D.
• IDIV (X,D)
  (quotient,remainder) of D/X.
• STX quot Save quotient
  temporarily.
• STD varD Save remainder as new
  variable D.
• CLRA Clear MSB of
  accumulator D.
• LDAB varB Set LSB of D
  variable B.
• XGDY Move value of B from D
  into Y.
• INY Increment Y to be
  varB1.
• LDAA quot1 A LSB of quotient.
• ORAA 30 Convert A to ASCII.
• STAA result,Y resultvarB1 A.
• LDAB varB Load old value of
  variable B.

XconvertB
resultB1A30
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Pseudo-code X2ASC (w/o divide)
This version avoids doing divisionand would work
on a processorwithout a divide instruction.

• Function X2ASC(X,Result)
• D X
• For B 0 to 3
• A 0
• Count the number of times Convert can be
  subtracted from D
• While Dgt0 Do
• A A 1
• D D ConvertB Convert
  10000,1000,100,10
• End While
• D D ConvertB
• A A 1
• A A OR 30 Convert A from 0-9 to ASCII
• ResultB A
• END For
• A D D should have the ones
• A A OR 30 Convert to ASCII
• Result4 A
• End Function

Note When converting this to HC11 assemblycode,
we must be careful because the HC11sregister D
overlaps registers A and B.