L7

1
L7 A First Program
ECE 2560

• Department of Electrical and
• Computer Engineering
• The Ohio State University

2
A First Program

• The first program
• The algorithm
• HLL structures to assembler
• The coding of bubble sort
• Will be working through slides and code composer
  in class together.

3
The program specification

• Anytime you write software you need a
  specification for that software.
• Joanne DeGroat (Include your name)
• This is comment
• What is the first program specification.
• Write a MSP430 assembler language program that
  implements the bubble sort algorithm to sort 8
  values in memory at location labeled by xxx. The
  values are sorted using memory and registers and
  then stored back to the locations.

4
The algorithm

• The bubble sort algorithm (4 locations shown)
  On 1st pass have n locations to sort
• On 2nd pass have n-1 locations to sort.

5
Now code it

• Start with a HLL pseudocode
• nval number of items in list
• doneFALSE
• While NOT done repeat
• doneTRUE
• FOR i 1 to n-1 Loop
• IF list(i)gtlist(i1) THEN exchange
  items
• temp list(i)
• list(i) list(i1)
• list(i1) temp
• done FALSE
• END IF
• END Loop
• nn-1
• IF n1 THEN doneTRUE
• END While
• Now translate to assembler
• Note code repeats loop if any items are
  exchanged. If no exchanges exit.

6
Translating

• How do you translate HLL structures to assembler
• STRAIGHT LINE CODE
• temp list(i)
• list(i) list(i1)
• list(i1) temp
• doneFALSE
• Could be done using data memory or registers and
  the mov instruction.

7
IF THEN ELSE

• Decision structures
• Have to decide where the test variables are
• Example IF (A lt B) THEN xxx ELSE yy
• Decide where A and B are
• Also look at the possible jumps
• Code order
• TEST set the CCR bits
• Branch to else cond when F(or after,
  if no ELSE)
• Code of THEN condition
• Branch to after if ELSE
• ELSE Code of ELSE condition
• after continuation of code

8
FOR Loop

• Need location for loop control variables
• FOR i IN 1 to n LOOP
• STATEMENTS in Loop
• END FOR
• Need location for i probably memory
• Need location for n probably memory

9
FOR Loop assembler

• In .data area
• i .word 0x0001 the loop counter
• n .word 0x0004 the loop limit
• The coding - will run the code in the loop at
  least once
• tol CODE WITHIN loop
• inc i
• cmp i,n are we at the end?
• will
  compute n-i
• JGE tol BUT WHICH JUMP?
• If it was FOR i 1 to 4
• First time i1, at end before compare i2 and
  4-22
• Then i2, at end i3 and 4-31
• Then i3, at end i4 and 4-40
• Then i4, at end i5 and 4-5-1

10
The Jumps

• JGE is Jump if greater or equal
• Finding the correct jump requires some thought.
• Note There is no jump if less than or equal.
  There is JL, jump is less, and JEQ, jump if
  equal.
• How to accomplish jump if less than or equal?
• Discussion

11
While loop

• Has the form
• WHILE condition REPEAT
• Some statement in here modifies condition
• END WHILE
• Translating to assembler
• Have to set up condition to produce T/F result
• Say condition is simply NOT done
• done is a 0 (FALSE) or 1 (True)

12
In assembler

• For the example
• .data
• done .word 0
• Code
• rpt tst done
• jne cont
• BODY OF CODE
• jmp rpt
• cont

13
Now code it

• Straight code exchange items (not set up in
  loop)
• bls is the list in memory
• have ielement of list you wish to
  access
• will exchange with item i1
• mov i,R7
• dec R7
• clrc
• rlc R7 mult by 2 for word data
• add bls,R7 R7 address of item(i)
• mov _at_R7,temp list(i) to temp
• mov 2(R7),0(R7) list(i)list(i1)
• mov temp,2(R7) templist(i1)
• clr done

14
Now code the if

• What is the test? List(i) gt list(i1)
• Remember that cmp a,b computes b-a
• and it does so in twos complement arithmetic
• The means that 0xFFFF (-1) is lt 0x0001 (1)
• i is value of loop element you wish to
  access
• in memory and current element
• ifstmt mov i,R7
• dec R7
• clrc clear the carry bit
• rlc R7 rotate left with carry (x2)
• add bls,R7 address of element in R7
• cmp _at_R7,2(R7)
• jge noexch
• 4 statements to exchange the
  elements and set done
• Noexch
• Note that some the changes to the straight code
  come from having to set up the compare.

15
Put code in loop

• Loop code
• i in memory current loop count
• n in memory limit
• done is also in memory
• mov 1,i
• tol STRAIGHT LINE CODE
• inc i
• cmp i,n
• jge tol

16
Code the While Loop

• Now code the while loop
• done is a boolean in memory
• mov 7,n set number of items in list -1
• trpt tst done
• jne cont
• mov 1,done set doneTRUE
• Loop code of previous slide
• dec n
• cmp i,n
• jne trpt
• mov 1,done set doneTRUE
• jmp trpt
• cont nop program should be done
• Note change here was the you start with n-1
  rather then n

17
Try it out

• Code this into assembler in code composer.
• This information on the slides in not 100 but it
  is very close. It will be tested on the 430.
• See webpage for the assignment.
• You can load the values into R8 to R15 to see
  that they are sorted or use the memory browser to
  show the values and see that they are sorted.
• In fact you can use the memory browser to watch
  the bubble sort in action.

18
Notes from after class

• This emphasizes the point that I have been making
  in class. Look at the documentation as the
  microcontrollers all have subtle differences.
• No place does the documentation point out that
  the cmp instruction treats data as 2s complement
  values, i.e., - and values. The documentation
  does indicate this but does not state it
  explicitly. It says the function of the cmp is
  to evaluate dst .NOT.src 1 to set the
  CCR bits NVCZ
• In class the list was FF00h which represents
  0100h, i.e., 256. This has 1 added to it,
  result257. So Z not set, V not set, C not set,
  and N not set.
• Many microcontrollers treat values in compares as
  unsigned integer values.

19
Further notes

• In 16 bits you can have values from -215
  to 215-1 or -32768 to 32767
• In 8 bits the 2s complement range is -27 to
  27-1 or -128 to 127
• There will be a 1 in the msb of all 2s
  complement binary number that represent a
  negative number.