L11-HLL to Assembler

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L11-HLL to Assembler
ECE 2560

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

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HLL to Assembler

• Pseudo HLL
• HLL structure
• Their flow chart
• HHL code
• Corresponding Assembler

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What is Pseudo HLL

• Pseudo HLL is a way of expressing an algorithm or
  procedure for performing a task.
• Very similar to modern High Level programming
  Languages
• Best illustrated with an example

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Pseudo HLL example

• Sum a list of 10 integers
• 3 7 8 2 4 6 5 5 9 1
• There are several methods
• The algorithm here
• Create sum and initialize to 0
• Add first number
• Repeat adding number until done with list

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Pseudo HLL

• Sequence of Actions
• Do action A
• Do action B
• .
• Do action Q

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Decisions

• Decision structures
• IF condition THEN action if true
• ELSE action if false
• END IF
• Example
• Remove front bike wheel
• TURN Bike over
• IF wheel-has-quick-release-skewer
• THEN flip level and remove wheel
• ELSE use wrench to remove nuts from axle
• remove wheel
• END IF

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Examples and demo

• Example and demo of decision structures
• IF THEN ELSE
• Set up
• Test condition
• Branch to after the code
• The action code

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Example

• Is a gt b ?
• a and b are memory location labels
• .data
• a .word 0x00FF
• b .word 0x00FF
• The code if agtb THEN actions
• cmp a,b computes b-a
• but what jump?
• after

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The jumps

• Jump Instructions
• JC,Jump if carry set JHS,Jump if high or same
• The C bit is tested
• If set (i.e. 1) then the branch is taken
• If clear, the next instruction is executed
• JEQ, JZ Jump if equal, Jump if zero
• The Z bit is tested
• If set (i.e. 1) then the branch is taken
• If clear, the next instruction is executed

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The Jumps (2)

• JGE Jump if greater or equal
• The N and V bits are used
• If N and V are set or reset (both 0 or both 1)
  the branch is taken
• JL Jump if less
• The N and V bits are used
• If only one of N or V is set then the branch is
  taken
• JMP Jump unconditionally

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Jumps (3)

• JN Jump is negative
• The N bit is tested
• If N is set (i.e. 1) the branch is taken
• JNC Jump if carry not set
  JLO Jump is lower
• The carry bit C is tested.
• If C is 0 the branch is taken
• JNE Jump not equal JNZ Jump if not zero
• The Z bit is tested
• If Z 0 the branch is taken

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A test program

• Write and compile a test program that sets up two
  values, uses cmp to compare them, and then runs
  through the branches.
• Have values that are both positive, both
  negative, and one positive-one negative.

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Adding the ELSE condition

• If you have an else condition, set up the branch
  such that when the condition is not met, you have
  a label to branch to this code section.
• cmp a,b
• jge elsecond
• THEN condition actions
• jmp ifendpt
• elsecond ELSE condition actions
• Ifendpt following code

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Repeat structures - Do Loops

• Finite number of times DO LOOP
• Set up any values for loop
• FOR counter start TO end STEP x LOOP
• actions of loop
• END FOR
• Example
• Sum 0
• FOR i 1 to 10 LOOP
• Sum Sum element(i)
• END FOR

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A fix number of times

• For I in 1 to 10 loop
• action in loop
• End loop
• Assembler
• Use a register for the current value of the loop
  counter
• In data area have
• stval the starting value, endval the ending
  value
• .data
• stval .word 0x0001
• endval .word 0x000F loop 15 times

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The structure

• start of loop setup
• mov stval,R8 i to R8
• tol cmp R8,endval
• jeq done
• actions of loop
• inc R8
• jmp tol
• done code after loop

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Demo

• Want to do loop 4 times
• Setup stval of 1, endval of 4
• But only execute the loop 3 times
• Could you solve this with the type of branch?
  Difficult
• Easy endval is number of times through the loop
  1 OR
• i is one less than the iteration through the loop
  at compare, i.e. initial value is 0 move inc
  instruction to just after the jump

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New material - TA

• New material starts here

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Repeat structures

• Indeterminate number of times Repeat loop or
  decision loop action inside the loop causes
  condition to occur that ends the loop.
• 2 structures
• One has code that executes at least once
• One has code that may or may not execute at all

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While condition

• While loop may or may not execute code
• WHILE condition LOOP
• SEQUENCE_of_ACTIONS
• END loop
• Example
• WHILE not EOF LOOP
• Read line from file
• Process line
• END LOOP

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Repeat

• Repeat Until structure
• REPEAT
• Sequence_of_statements
• UNITL condition
• Example - attempt to read will be done at least
  once
• REPEAT
• Read line from file
• UNTIL EOF

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Now what is the assembler?

• For each of these Pseudo HLL structures what is
  the corresponding assembler.
• Each will have assumption as to were data to be
  tested is.

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Straight line code - Sequence

• Straight line Pseudo HLL little modification is
  needed.
• A mx b in Pseudo HLL
• Where are values say in memory locations
  labeled by the same name
• Code becomes
• mov m,R8
• push R8
• push x
• call smult
• pop R8
• pop R8
• add b,R8
• mov R8,A

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Decision

• Decision structure
• IF condition THEN action if true
• ELSE action if false
• END IF
• Example
• IF (AltB) THEN tempA AB Btemp
• END IF

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Assembler for example

• A and B are in memory
• Will use a register for temp.
• cmp A,B B-A is positive if AltB
• jl noexch jump if BltA
• mov A,R6
• mov B,A
• mov R6,B
• noexch

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A more efficient coding

• Could be made a little more efficient
• Previous code also exchanges when AB
• cmp B,A A-B is negative if AltB
• jge noexch jump if BltA
• mov A,R6
• mov B,A
• mov R6,B
• noexch

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When there is an else

• Else is similar to the previous code
• IF (AltB) THEN tempA AB Btemp
• ELSE A 0
• END IF
• Much like previous
• cmp B,A A-B is negative if AltB
• jge else jump if BltA
• mov A,R6
• mov B,A
• mov R6,B
• jmp after
• else clr A
• after

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Repeat structures

• Finite number of times DO LOOP
• Set up any values for loop
• FOR counter start TO end STEP x LOOP
• actions of loop
• END FOR
• Example
• Sum 0
• FOR i 1 to 10 LOOP
• Sum Sum element(i)
• END FOR
• How would this example be coded?

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Coding for Do Loop

• Where are control values?
• i the current index value use R9
• startval the starting value for i memory loc
• endval the ending value for i memory
• sum in memory
• element label of list of values in memory
• mov i,R9
• tol cmp endval,R9 R9-endval
• jge lpexit
• mov R9,R8
• dec R8
• clrc
• rolc R8
• add element,R8
• add _at_R8,sum
• inc R9
• jmp tol
• lpexit

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The repeat and while loops

• The coding of the repeat and while loops is very
  similar.
• Documents where variables are
• Need loop control and testing of loop control
• Which branch instruction to use takes some
  thought
• Need to remember what the cmp instruction does!!

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The while loop example

• i0 flagTRUE
• WHILE flag LOOP
• increment i
• Actions when value i
• IF i 5 THEN flag False END IF
• END LOOP
• Now translate this to assembler

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Assembler for while example

• mov 0,i
• mov 1,flag 1TRUE
• rpt cmp flag,0
• jeq done
• inc i
• nop code to do actions
• cmp i,5
• jne rpt
• mov 0,flag
• jmp rpt
• done nop code after loop

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Demo

• Demo of loop code.