06 Assembler Programming

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Assembler ProgrammingChapter 6


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EEL-4746 Best

Practices


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EEL-4746 Best Practices

1. All programs must begin with the followingcomment

***************************

* EEL-4746 Spring 2004 Semester * Homework #NDue (Due Date)

* Problem #M

* Name of Partner A

* Name of Partner B

****************************

* Description of Program


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2. All subroutines must begin with thefollowing comment

***************************

* Subroutine Name: Mysub * Input parameter list:

* Output parameter list:

* Registers changed list: ****************************

* Description of Subroutine


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3. All lines must contain a comment

4. All labels must end with a semicolon

5. Must use a symbol for all constants


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5. 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 hereORG ResetFDB Top


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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 ALLUPPERCASELabels 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, theinput values it expects, and the registers that it modifies.


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Pseudo-code

A fictitious programming language

which allows a general algorithm for acomputer program to be developed.

Design process

1. Develop program using pseudo-code

2. Convert pseudo-code to 68HC11 assembly

language


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Structured Assembly LanguageProgramming

IfThenElseEnd IF

Pseudo-Code Syntax

If Variable_A Condition Variable_B ThenPseudo Code for Then condition

Else (Optional)Pseudo Code B for Else condition

End If


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LDAA Variable_A

CMPA Variable_B

B?? Else ; Opposite of condition

Code for Thenpart.

BRA END_IF ; need to skip else part

Else : Code for Elsepart

..END_IF: Rest of Code


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If-Then-Else-End IFCondition : Branch to Use (Opposite of condition) : BEQ (Branch if equal) = : BNE (Branch if not equal)

Signed= : BLT (Branch if less than)> : BLE (Branch if less than or equal)

Unsigned

= : BLO (Branch if lower)> : BLS (Branch if lower or same)


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If-Then-Else Example

Pseudo-code

IF temp > max_temp then

Then code

Else

Else code

End If


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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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Loops

For Loops

While-Do Loop

Do-While or (RepeatUntil) Loops


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For Loops

Pseudo-code Syntax

For loop_index = start_index to end_index Begin

Code to Execute

..

..

End For Loop


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Assembly Language Use one of the registers for the loop index.

A,B=8 bit (255 max)X,Y=16 bit (65535 max)Must use memory if index value is greater than 65535

Example Code FragmentLDAA #start_index

Loop: Start of Code to Execute (must not change A).

INCACMPA #end_index

BLS Loop


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What if you need the loop index register (e.g. A)in your code?

Example Code Fragment

LDAA #start_indexLoop: Start of Code to Execute

PSHA ; Save A on stack

Use A

.

PULA ; Restore A from stack

INCACMPA #end_index

BLS Loop


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For Loops (Count down)

Pseudo-code SyntaxFor loop_index = end_index downto start_index

Begin

Code to Execute

..

..

End For Loop


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Example Code FragmentLDAA #end_index

Loop: Start of Code to Execute

.

DECA

CMPA #start_index

BHS Loop


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For Loops (Count down from N to 1)

This counts N items

Pseudo-code SyntaxFor loop_index = end_index downto 1

Begin

Code to Execute

....

End For Loop


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Example Code FragmentLDAA #N

Loop: Start of Code to Execute

.

DECA

BNE Loop ; Save one statement


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TPS Quiz


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While-Do Loops

Pseudo-code SyntaxWhile Variable Condition Constant_VALUE

Do Code Here Begin

Code to Execute

..

.. End While


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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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While-DoCondition : Branch to Use (Opposite of condition) : BEQ (Branch if equal) = : BNE (Branch if not equal)

Signed= : BLT (Branch if less than)> : BLE (Branch if less than or equal)

Unsigned

= : BLO (Branch if lower)> : BLS (Branch if lower or same)


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Example: Temp > #$4F (unsigned)

Pseudo-code:

While Temp > $4FDo

(Execute your code here)



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While Temp > #$4F (unsigned)

Assembly CodeLoop: LDAA Temp

CMPA #$4F

BLS End_While ; (opposite of >)

Do code here

..

BRA Loop



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Do-While or (Repeat-Until) Loops

Pseudo-code SyntaxDo (or Repeat)

BeginCode to Execute

..

..

While Condition (or Until Condition)


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Do-While or (Repeat-Until) Loops

Assembly Language SyntaxLoop:

Do Code Here

..

..

LDAA Variable

CMPA #Constant B?? Loop ; (same as condition)


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Do-While or (RepeatUntil)Condition : Branch to Use (Same as condition) = : BEQ (Branch if equal) : BNE (Branch if not equal)

Signed> : BGT (Branch if greater than)>= : BGE (Branch if greater than or equal)< : BLT (Branch if less than) : BHI (Branch if higher)>= : BHS (Branch if higher or same)< : BLO (Branch if lower)


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Example

Write a 68HC11 assembly languageprogram that converts X into anequivalent ASCII signed decimal value.

Store your result in a memory locationlabeled: Result.


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Pseudo-code

X= Number_to_convert

sign= +

ifX< 0 then

sign= -

X= -X

end If

result[0] = sign

call X2ASC(X)

end program

X2ASC is a subroutine that converts the X register(unsigned) to ASCII

Well put this code into

a 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 regXJSR SX2ASC ;Signed convert

End: BRA End ;Infinite loop


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Next line

At the start of the SX2ASC subroutine, we have thepseudocode 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:ifX< 0 then end if

This converts to the assembly:CPX #0 ; Compare X to 0.BHS endIf ; If X>=0, 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 (Xgets negativeX)

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 < 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 result+0 ; Store sign char. in result[0].JSR X2ASC ; Conv. unsigned X to result[1-5].

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/ convert[B])

; where convert[]= 10000,1000,100,10

D= DA*convert[B] ; Let Dbe the remainder

A=Aor #$30 ; ConvertAfrom 0-9 to ASCII

result[B+1] =A

end for

A= D ; Dshould have the ones

A=Aor #$30 ; Convert to ASCII

result[4] =A

end functionNote: When converting this to working assembly

code, we must be careful about overlapping

registers.


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for loop outline

The skeleton of our forloop: for B = 0 to 3

(body of forloop)end for

Translates to:LDAB #0forBody: ;body code goes here

INCB ; B = B + 1

CMPB #3 ; Compare B to 3

BLS forBody ; If B


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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 convert[B]?CLRA ; Effectively sets D=B

ASLD ; Double D, its 2B.XGDY ; Transfer this to Y

LDX convert,Y ; X = *(convert+2B)

But, this only works if convert array is on page 0! More generally, wed have to do this after the ASLD:

ADDD #convert ; D = convert+2B

XGDY ; Transfer D to Y

LDX 0,Y ; X = *Y = convert[B]

U i d X2ASC i bl


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Unsigned X2ASC in assemblyX2ASC: STX varD ; Initialize variable D = X.

LDAB #0 ; For B = 0 to 3,ForBody:STAB varB ; Save variable B.

CLRA ; Clear MSB of accumulator D.ASLD ; Double D to get 2*varB.XGDY ; Move it into Y.LDX convert,Y ; Let X = *(convert+Y) = convert[varB]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 varB+1.LDAA quot+1 ; A = LSB of quotient.ORAA #$30 ; Convert A to ASCII.STAA result,Y ; result[varB+1] = A.

LDAB varB ; Load old value of variable B.INCB ; Increment B to next value.CMPB #3 ; Compare B with 3.BLS ForBody ; End For. (Continue while B


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Pseudo-code: X2ASC (w/o divide)Function X2ASC(X,Result)

D = XFor B = 0 to 3

A = 0; Count the number of times Convert can be subtracted from DWhile D>0 Do

A = A + 1

D = DConvert[B] ; Convert= 10000,1000,100,10End While

D =D + Convert[B]A = A1A = A OR #$30 ; Convert A from 0-9 to ASCIIResult[B] = A

END ForA = D ; D should have the onesA = A OR #$30 ; Convert to ASCIIResult[4] = A

End Function Note: When converting this to HC11 assemblycode, we must be careful because the HC11s

This version avoids doing division

and would work on a processorwithout a divide instruction.

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