L11-HLL to Assembler

1. ECE 2560 L11-HLL to Assembler Department of Electrical and Computer Engineering The Ohio State University ECE 3561 - Lecture 1

2. HLL to Assembler • Pseudo HLL • HLL structure • Their flow chart • HHL code • Corresponding Assembler ECE 3561 - Lecture 1

3. 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 ECE 3561 - Lecture 1

4. 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 ECE 3561 - Lecture 1

5. Pseudo HLL • Sequence of Actions • Do action A • Do action B • …. • Do action Q ECE 3561 - Lecture 1

6. 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 ECE 3561 - Lecture 1

7. Examples and demo • Example and demo of decision structures • IF THEN ELSE • Set up • Test condition • Branch to after the code • The action code ECE 3561 - Lecture 1

8. Example • Is a > b ? • a and b are memory location labels • .data • a .word 0x00FF • b .word 0x00FF • The code if a>b THEN actions • cmpa,b ;computes b-a • but what jump? • after ECE 3561 - Lecture 1

9. 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 ECE 3561 - Lecture 1

10. 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 ECE 3561 - Lecture 1

11. 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 ECE 3561 - Lecture 1

12. 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. ECE 3561 - Lecture 1

13. 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. • cmpa,b • jgeelsecond • ;THEN condition actions • jmpifendpt • elsecond ;ELSE condition actions • Ifendpt ;following code ECE 3561 - Lecture 1

14. 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; ECE 3561 - Lecture 1

15. 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 ECE 3561 - Lecture 1

16. The structure • ;start of loop – setup • mov &stval,R8 ;i to R8 • tolcmp R8,&endval • jeq done • ;actions of loop • inc R8 • jmptol • done ;code after loop ECE 3561 - Lecture 1

17. 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 ECE 3561 - Lecture 1

18. New material - TA • New material starts here ECE 3561 - Lecture 1

19. 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 ECE 3561 - Lecture 1

20. 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; ECE 3561 - Lecture 1

21. 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; ECE 3561 - Lecture 1

22. 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. ECE 3561 - Lecture 1

23. Straight line code - Sequence • Straight line Pseudo HLL – little modification is needed. • A = m*x + 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 ECE 3561 - Lecture 1

24. Decision • Decision structure • IF condition THEN action if true • ELSE action if false • END IF; • Example: • IF (A<B) THEN temp=A; A=B; B=temp • END IF; ECE 3561 - Lecture 1

25. Assembler for example • A and B are in memory • Will use a register for temp. • cmp A,B ;B-A is positive if A<B • jl noexch ;jump if B<A • mov A,R6 • mov B,A • mov R6,B • noexch ECE 3561 - Lecture 1

26. A more efficient coding • Could be made a little more efficient • Previous code also exchanges when A=B • cmp B,A ;A-B is negative if A<B • jgenoexch ;jump if B<=A • mov A,R6 • mov B,A • mov R6,B • noexch ECE 3561 - Lecture 1

27. When there is an else • Else is similar to the previous code • IF (A<B) THEN temp=A; A=B; B=temp; • ELSE A = 0; • END IF; • Much like previous • cmp B,A ;A-B is negative if A<B • jge else ;jump if B<=A • mov A,R6 • mov B,A • mov R6,B • jmp after • else clr A • after ECE 3561 - Lecture 1

28. 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? ECE 3561 - Lecture 1

29. 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 • tolcmp endval,R9 ;R9-endval • jgelpexit • mov R9,R8 • dec R8 • clrc • rolc R8 • add #element,R8 • add @R8,sum • inc R9 • jmptol • lpexit ECE 3561 - Lecture 1

30. 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!! ECE 3561 - Lecture 1

31. The while loop example • i=0; flag=TRUE; • WHILE flag LOOP • increment i; • Actions when value i; • IF i = 5 THEN flag = False; END IF; • END LOOP; • Now translate this to assembler ECE 3561 - Lecture 1

32. Assembler for while example • mov #0,&i • mov #1,&flag ;1=TRUE • rptcmp &flag,#0 • jeq done • inc &i • nop ;code to do actions • cmp &i,#5 • jnerpt • mov #0,&flag • jmprpt • done nop ;code after loop ECE 3561 - Lecture 1

33. Demo • Demo of loop code. ECE 3561 - Lecture 1