Spim part II

1. Spimpart II Chun-Cheng Lin ( 林春成 ) & Jiunn-Jye Lee ( 李俊頡 ) CS, EE, NTU

2. Outline • Useful tables • SPIM program examples • Array, control, and expression • Debug • Homework bonus • Mid-term Exam

3. Table of MIPS registers

4. Set $v0 Table of system services

5. MIPS Assembler Directives • Data Types .word, .half - 32/16 bit integer .byte - 8 bit integer (similar to ‘char’ type in C) .ascii, .asciiz - string (asciiz is null terminated) • Strings are enclosed in double-quotas(”) • Special characters in strings follow the C convention • newline(\n), tab(\t), quote(\”) .double, .float - floating point .text - Indicates that following items are stored in the user text segment .data - Indicates that following data items are stored in the data segment .globl sym- Declare that symbol sym is global and can be referenced from other files

6. MIPS instruction set

7. Arrays /* Address calculation in assembler: */ address of A [x] = address of A [0] + (x * sizeof (element of A)); # $t0 = address of start of A. # $t1 = n. mul $t2, $t1, 4 # compute offset from the start of the array # assuming sizeof(element)=4 add $t2, $t0, $t2 # add the offset to the address of A [0]. # now $t2 = &A [n]. sw $t3, ($t2) # A [n] = whatever is in $t3. lw $t3, ($t2) # $t3 = A [n].

8. SPIM Program Example I • A Simple Program #sample example 'add two numbers’ .text # text section .globl main # call main by SPIM main: la $t0, value # load address ‘value’ into $t0 lw $t1, 0($t0) # load word 0(value) into $t1 lw $t2, 4($t0) # load word 4(value) into $t2 add $t3, $t1, $t2 # add two numbers into $t3 sw $t3, 8($t0) # store word $t3 into 8($t0) .data # data section value: .word 10, 20, 0 # data for addition

9. SPIM Program Example II • A Program with System Call #sample example 'system call' .text .globl main main: la $t0, value li $v0, 5 # read_integer syscall sw $v0, 0($t0) li $v0, 5 # read_integer syscall sw $v0, 4($t0) lw $t1, 0($t0) lw $t2, 4($t0) add $t3, $t1, $t2 sw $t3, 8($t0) li $v0, 4 # print_string la $a0, msg1 syscall li $v0, 1 # print_integer move $a0, $t3 #pseudoinstructions syscall .data value: .word 0, 0, 0 msg1: .asciiz "Result = "

10. SPIM Program Example III • A Program with Procedure Call # sample example ‘swap two numbers’ .text .globl main main: la $a0, array addi $a1, $0, 0 addi $sp, $sp, -4 sw $ra, 0($sp) jal swap lw $ra, 0($sp) addi $sp, $sp, 4 jr $ra .data array: .word 5, 4, 3, 2, 1 .text # swap(int v[], int k) # { # int temp; # temp = v[k]; # v[k] = v[k+1]; # v[k+1] = temp; # } swap: add $t1, $a1, $a1 add $t1, $t1, $t1 add $t1, $a0, $t1 lw $t0, 0($t1) lw $t2, 4($t1) sw $t2, 0($t1) sw $t0, 4($t1) jr $ra

11. if ( condition ) { statements } # MIPS code for the # condition expression. beqz $t0, if_end_label # MIPS code for the statements. if_end_label: if ( condition ) { if-statements } else { else-statements } # MIPS code for the # condition expression. beqz $t0, if_else_label # MIPS code for the if-statements. j if_end_label if_else_label: # MIPS code for the else-statements. if_end_label: Control statements (if, if-else)

12. Example • A Program with “if” if ( i == j ) go to L1; f = g + h; L: f = f – i; # $s0 = f; $s1 = g; $s2 = h # $s3 = i; $s4 = j; beq $s3, $s4, L1 # go to L1 if i equals j add $s0, $s1, $s2 # f = g + h ( skipped if i equals j ) L: sub $s0, $s0, $s3 # f = f – i ( always executed )

13. Example • A Program with “if-else” if ( i == j ) f = g + h; else f = g - h; # $s0 = f; $s1 = g; $s2 = h # $s3 = i; $s4 = j; bne $s3, $s4, Else # go to Else if i  j add $s0, $s1, $s2 # f = g + h ( skipped if i j ) j Exit # go to Exit Else: sub $s0, $s1, $s2 # f = g – h ( skipped if i = j ) Exit:

14. Control statements (while) while ( condition ) { statements } while_start_label: # MIPS code for the condition expression beqz $t0, while_end_label # MIPS code for the statements. j while_start_label while_end_label: Break ----> j while_end_label Continue ----> j while_start_label

15. Control statements (do-while) do { statements } while ( condition ); do_start_label: # MIPS code for the statements. do_cond_label: # MIPS code for the condition expr: beqz $t0, do_end_label j do_start_label do_end_label: Break ---> j do_end_label Continue ---> j do_cond_label

16. Example • A Program with “while” while ( save[i] == k ) i = i + j; # $s3 = i; $s4 = j; $s5 = k # $s6 = address of save[0] Loop: add $t1, $s3, $s3 # Temp reg $t1 = 2 * i add $t1, $t1, $t1 # Temp reg $t1 = 4 * i add $t1, $t1, $s6 # $t1 = address of save[i] lw $t0, 0($t1) # Temp reg $t0 = save[i] bne $t0, $s5, Exit # go to Exit if save[i]  k add $s3, $s3, $s4 # i = i + j j Loop Exit:

17. Control statements (for) for ( init ; condition ; incr ) { statements } # MIPS code for the init expression. for_start_label: # MIPS code for the condition expression beqz $t0, for_end_label # MIPS code for the statements. for_incr_label: # MIPS code for the incr expression. j for_start_label for_end_label: Break ---> j for_end_label Continue ---> j for_incr_label

18. switch ( expr ) { case const1: statement1 case const2: statement2 ... case constN: statementN default: default-statement } # MIPS code to compute expr. # Assume that this leaves the value in $t0. beq $t0, const1, switch_label_1 beq $t0, const2, switch_label_2 ... beq $t0, constN, switch_label_N # If there is a default, then add b switch_default # Otherwise, add the following line instead: b switch_end_label switch_label_1: # MIPS code to compute statement1. switch_label_2: # MIPS code to compute statement2. ... switch_label_N: # MIPS code to compute statementN. # If there's a default: switch_default: # MIPS code to compute default-statement. switch_end_label: Break ---> b switch_end_label Control statements (switch)

19. Example • switch ( k ) { • case 0: f = i + j; break; /* k = 0 */ • case 1: f = g + h; break; /* k = 1 */ • case 2: f = g - h; break; /* k = 2 */ • case 3: f = i - j; break; /* k = 3 */ • } • A Program with “switch” # $s0 = f; $s1 = g; $s2 = h; # $s3 = i; $s4 = j; $s5 = k; # $t2 = 4; slt $t3, $s5, $zero # Test if k < 0 bne $t3, $zero, Exit # if k < 0, go to Exit slt $t3, $s5, $t2 # Test if k < 4 beq $t3, $zero, Exit # if k >= 4, go to Exit add $t1, $s5, $s5 # Temp reg $t1 = 2 * k add $t1, $t1, $t1 # Temp reg $t1 = 4 * k add $t1, $t1, $t4 # $t1 = address of JumpTable[k] lw $t0, 0($t1) # Temp reg $t0 = JumpTable[k] jr $t0 # jump based on register $t0 L0: add $s0, $s3, $s4 # k = 0 so f gets i + j j Exit # end of this case so go to Exit L1: add $s0, $s1, $s2 # k = 1 so f gets g + h j Exit # end of this case so go to Exit L2: sub $s0, $s1, $s2 # k = 2 so f gets g - h j Exit # end of this case so go to Exit L3: sub $s0, $s3, $s4 # k = 3 so f gets i - j Exit: # end of switch statement

20. Expression evaluation (and) cond1 && cond2 # MIPS code to compute cond1. # Assume that this leaves the value in $t0. # If $t0 is zero, we're finished (and the result is FALSE). beqz $t0, and_end # MIPS code to compute cond2. # Assume that this leaves the value in $t0. and_end:

21. Expression evaluation (or) cond1 || cond2 # MIPS code to compute cond1. # Assume that this leaves the value in $t0. # If $t0 is not zero, we‘re finished (and the result is TRUE). bnez $t0, or_end # MIPS code to compute cond2. # Assume that this leaves the value in $t0. or_end:

22. Debug • Three kinds of error • Syntax error • Your program will not compile • Run error • Your program will compile but will crash or hang • Logical error • Your program will run successfully but not do what you intend

23. When loading … .globl Debug syntax error # add.s .text .global main main: la $t0, value lw $t1, 0($t0) lw $t2, 4($t0) add $t3, $t1, $t2 sw $t3, 8($t0) .data value: .word 10, 20, 0

24. Debug run error • Set break points

25. Debug run error • Set break points

26. Debug run error • Set break points

27. Debug logical error • Review the program • Run the program step by step, and watch the dynamics of registers

28. Homework bonus • Practice MIPS procedures • Chap3: 3.26-27 ( tail recursion ) • Due: 5:00 PM, 11/25/2003 • Save as ID_1.s and ID_2.s, and send to sanlin@cobra.ee.ntu.edu.tw • The sample code of answers will be online after two weeks

29. Mid-term Exam • 日期：11/11 9:10 AM ~ 11:00 • 分數分配 • 座位 • EE R143: 單數座號 • EE R105: 雙數座號