HW #1 Problems & Industrial Strength Assembly

1. HW #1 Problems &Industrial Strength Assembly CSCI 136

2. Reading the Question “If there are no b’s in the string, then bfind should return a pointer to the null character at the end of the string” lbu $t2, 0($t3) beq $t2, $0, term … term: la $a0, ndone li $v0 ,4 syscall Its great that you tell me whats happening.. How about putting $t3 in to $v0?

3. Reading the Question “The bfind procedure should locate the first b character in the string and return its address in register $v0” bfind: lbu $t1,0($a0) beqz $t1, Ret beq $t1, ‘b’, Ret addi $a0, $a0, 1 j bfind Ret: sb $v0, ($t1)

4. Using the bfind procedure in bcount “You must use your bfind procedure in Exercise 3.23 in your implementation of bcount.” What does bfind return? So how do we check if we’re at the end of the string? jal bfind lb $t0,0($v0) beqz $t0, endOfString

5. RTFQ – Reading the Question Tough parts of Software: • Specifying Requirements • Understanding the Specified Requirements Why it matters at GWU: Grading Scripts Besides.. It should be really frustrating to spend hours answering the wrong question…

6. RTFQ- Reading the Question Typical Errors: Didn’t write bcount as a procedure: This requires saving $ra before the call to bfind, and restoring $ra after the call to bfind main: jal bcount bcount: jal bfind What happens to $ra when jal is called?

7. Answering the Question add $t0, $0, $0 bfind: add $t3, $t0, $a0 lbu $t2, 0 ($t3) beq $t2, $0, done addi $t2, $t2, -98 beq $t2, $0, done addi $t0, $t0, 1 j bfind done: addi $t0,1 add $v0,$t0,$0 jr $ra

8. Answering the Question li $t0,0 bfind: add $t3,$a0,$t0 lbu $t2, 0 ($t3) beq $t2, $0, done beq $t2, ‘b’, done addi $t0, $t0, 4 j bfind done: move $s2,$t0 jr $ra

9. Why do we program in Assembly?

10. Why do we program in Assembly? What matters in real world: SPEED, SPEED & SPEED Other acceptable answer: Low Level Interface with hardware (mouse, ethernet card, serial port, video card, etc.) Unfortunately we can’t simulate these hardware interfaces with SPIM.

11. Naïve Approach to Style “Good Style” == lots of comments Even poor code well documented is still poor code.

12. Style • Accomplish task in straight forward manner with minimum effort • Unfortunately, “Good Style” comes from experience and experience comes from “Bad Style” We don’t expect you to have wonderful style in the first couple assignments… By the end of semester you should be writing simple straight forward code.

13. Minimum Effort la $t0, $a0 bfind: lbu $t1, 0 ($t0) beq $t1, Done beq $t1,’b’, Done addi $t0, 1 addi $a0, 1 j bfind Done: move $v0,$a0

14. Minimum Effort Registers are a Commodity. Do not waste them! Why is minimizing the use of registers so important? What is the slowest type of instruction on our processor? If we have registers available... We can use them. This will allow your programs to run faster!

15. Minimum Effort Registers are a Commodity. Do not waste them! Why is minimizing the use of registers so important? Modern processors have multiple execution units… Processors can execute several different portions of your code at the same time... Lack of registers prevents multiple execution… Help the processor out by using minimal number of registers and having minimal register “overlap”. This will allow your programs to run faster!

16. Do we need all these registers? li $s1,0x00 bfind: lbu $t2,0($a0) beq $t2,$s1,exit beq $t2,’b’,exit addi $a0,$a0,1 j bfind exit:

17. Do we need all this code? bfind: lbu $t2,0($a0) addi $a0,$a0,1 beq $t2,$zero,exit beq $t2,’b’,exit j bfind exit: subu $a0,$a0,1

18. Minimum Effort Know your architecture (load - store) and what your pseudo instructions do: Is there a difference difference between: beq $t0, ‘b’, Done and addi $at,$zero,’b’ beq $t0,$at, Done

19. Do I need everything in my loop? bfind: lbu $t0, 0 ($a0) beqz $t0, done addi $t1,$zero,’b’ beq $t0, $t1, done add $a0, $a0, 1 j bfind done: mov $v0, $a0 jr $ra

20. Extra Branches & Border Case Failure bfind: li $t1, ‘b’ lb $t0,0($a0) bne $t0,$t1,loop move $v0,$a0 quit: jr $ra loop: addi $a0,$a0,1 lbu $t0,0($a0) beq $t0,$zero,quit bne $t0,$t1,loop move $v0,$a0 jr $ra

21. Extra Branches and Loops Why are extra branches bad? -Optimizing Branches Why are extra loops bad?

22. Public and Private Variables In SPIM- When you call a function… and the function returns… the only values from that function your procedure should access are: $v0, $v1 Values saved by the function on the stack Locations in Memory DO NOT access the internal variables (registers) of another function! This “breaks” the whole “function” paradigm. It isn’t a “black box” if you go digging around…

23. Public and Private Variables GWU FUNCTION

24. Breaking the Function Paradigm addi $sp, $sp, -4 sw $ra,0($sp) jal bfind lw $ra,0($sp) addi $sp,$sp,4 quit: bne $t0, $zero, adder Who set $t0? bfind did. But the only return value was supposed to be $v0? What are the problems with this?

25. Breaking the Function Paradigm I should be able to plug in ANY bfind in to bcount and make it work. Don’t fight the framework!

26. Real World Disclaimer… Register assignments aren’t as structured… (no $t0-$t9, $s0-$s7..just registers $1-$31) Saving values on stack or in memory is slow. More registers can be used to return values. Function writers and function callers typically have specific agreements about what registers to use for passing and returning values.

27. Proper Instructions jr$1 is jump to a value given by the register $1 (Where do you typically see jr$ra ?) j is jump to a label Use addi to add a constant to a register Use add to add registers together Watch the ordering of registers in instruction… sw $sp, $ra What does this do?

28. Good Implementation of Bfind addi $t1,$zero,’b’ bfind: lbu $t0, 0 ($a0) beqz $t0, done beq $t0, $t1, done add $a0, $a0, 1 j bfind done: move $v0, $a0 jr $ra 8 lines of assembly. No extra branches, loops, variables. Common case is fast.

29. Lab Assignment: Endian Fixer • TCP/IP message headers are in Big Endian • Your PC is a Little Endian machine • Write function that takes word value in $a0 and returns a Endian reversed value in $v0 • Goals: • Learn how to use masks li $t0, 0x000000ff andi $t1, $a0, $t0 ori $v0, $t0, $t1

30. endianFixer: #1234 rol $a0,$a0,8 #2341 lui $t1, 0x00ff ori $t1, $t1, 0x00ff and $v0,$t1,$a0 #0301 sll $t1,$t1,8 #$t1=ff00ff00 rol $a0,$a0,16 #4123 and $a0,$t1,$a0 #4020 or $v0,$v0,$a0 #4321 jr $ra Why don’t I ever zeroize $v0? Is li $t1,0x00ff00ffok?

31. Adder Review • How many delays does a 4 bit ripple carry adder take to add two 4 bit numbers? Why?

32. Adder Review What are the equations for a 4 bit carry look ahead adder? Do you understand this?

33. From when a and b’s show up… how long does it take until sum appears on the output lines? How long for carry generation + how long for sum?

34. 8 bit adder We need an 8 bit adder…how can we do this?

35. 8 bit adder We could have a pure ripple carry… How long would this take to calculate the sums?

36. 8 bit adder We could use ripple carry to connect two 4 bit carry look ahead groups… How long would this take to calculate the carries? The sums?

37. 2nd layer of Carry Lookahead Lets add another layer of Carry Lookahead for our 8 bit adder… What are the equations for big Ps? What are the equations for big Gs? What are the equations for the big Cs?

38. 2nd layer of Carry Lookahead How long would it take to calculate the carries? How long would it take to calculate the sums?