COMS 361 Computer Organization

1. COMS 361Computer Organization Title: Instructions Date: 9/28/2004 Lecture Number: 10

2. Announcements • Homework 4 • Due 10/05/04

3. Review • Instructions • unconditional branch • J-Type instruction format • MIPS history • SPIM • MIPS simulator

4. Outline • SPIM • MIPS simulator

5. SPIM Program • Show me the code! • MIPS/SPIM program files usually end in .s • File is loaded into SPIM • Assembled into MIPS machine code • Executed and debugged in SPIM

6. SPIM Program ## exit.s - program simply makes a system call to exit ## ## text segment ## .text main: addi $v0, $zero, 10 # exit call code syscall # terminate program execution ## ## data segment ## ## ## end exit.s

7. SPIM Program ## exit.s - program simply makes a system call to exit ## ## text segment ## .text main: addi $v0, $zero, 10 # exit call code syscall # terminate program execution ## ## data segment ## ## ## end exit.s Comments in MIPS assembly code are denoted by the sharp (#) sign Similar to the C++ (//) comment. Starts at instance and goes to the end of the line Assembly code in NOT self-documenting. Use comments liberally, like associated with each instruction

8. SPIM Program ## exit.s - program simply makes a system call to exit ## ## text segment ## .text main: addi $v0, $zero, 10 # exit call code syscall # terminate program execution ## ## data segment ## ## ## end exit.s Comments indicating the start of the code After text segment is converted into its binary representation, it is stored in the text memory segment . indicates an assembler directive .text directs the assembler to treat what follows as instructions

9. Text Segment • The source code format is relatively standard • Proper indentation is fundamentally important in assembly language programming • [] indicate optional fields • Not all fields appear on a line of code [label:] operation [operand], [operand], [operand] [# comment]

10. SPIM Program ## exit.s - program simply makes a system call to exit ## ## text segment ## .text main: addi $v0, $zero, 10 # exit call code syscall # terminate program execution ## ## data segment ## ## ## end exit.s main: is a label Just as C++ programs need a main function, MIPS programs need a main label main: labels the first instruction of the program

11. Labels • : • Tells the assembler that the proceeding alphanumerics including (_) and (.) constitutes the label • Opcodes are reserved words and are not permitted to be used as labels • Use appropriate names for labels • Labels associate a symbol with • The address of an instruction • The address of a variable

12. SPIM Program ## exit.s - program simply makes a system call to exit ## ## text segment ## .text main: addi $v0, $zero, 10 # exit call code syscall # terminate program execution ## ## data segment ## ## ## end exit.s Program is to simple call the OS with the exit call code In the real world the OS cleans up after the program The call code for exit is the decimal number 10, which needs to be put into the $v0 register (convention) addi with the proper operands can achieve this goal

13. SPIM Program ## exit.s - program simply makes a system call to exit ## ## text segment ## .text main: addi $v0, $zero, 10 # exit call code syscall # terminate program execution ## ## data segment ## ## ## end exit.s Call the OS with the proper call code in the $v0 register

14. SPIM Program • Show me the program execute! • And some things about SPIM City!! • exit.s • hello.s

15. Datapath Diagram • Main functional units • Control unit

16. Datapath Diagram • Main functional units • Register file

17. Datapath Diagram • Main functional units • Arithmetic and logic unit (ALU)

18. Datapath Diagram • Main functional units • Program counter (PC)

19. Datapath Diagram • Main functional units • Memory

20. Datapath Diagram • Main functional units • Instruction register (IR)

21. Datapath Diagram • Other operational units • bus

22. Datapath Diagram • Other operational units • Multiplexor (data selector)

23. Fetch and Execute R-type • Instruction Fetch Phase • Fetch the word in memory at the address specified by the Program Counter (PC) • Load instruction into the IR • Increment the PC (add 4) • Operand Fetch Phase • Decode the Rs and Rt fields within the instruction Decode the Op Code

24. Fetch and Execute Cycle • Execute Phase • Perform ALU operation defined by the function code on the source operands • Write Back Phase • Result if the ALU is written into the decoded Rd register

25. MIPS Instructions • MIPS instruction set architecture • Assembly instructions that convert into machine (binary) instructions • Assembly instructions can be converted directly into machine instructions • One-to-one mapping • Assembly instructions can be converted into more than one machine instruction • One-to-many mapping • Native machine instructions • Pseudo-, Macro-, Synthetic-machine instructions • Consists of more than one native machine instruction

26. MIPS Instructions • Assembler converts pseudo-instructions with the corresponding set of actual MIPS instructions • Pseudo-instructions simplify the task of writing assembly code pseudo instruction la $a0 label load address of label into register $a0 actual MIPS instructions lui $at, upper 16 bits of label ori $Rd, $at, Lower 16 bits of label

27. Allocate space in the data segment of the SPIM memory model Allocates 4096 bytes in the data segment Assembler Directives • Used to create data structures that are available at run-time • To allocate a one-dimensional array in C++ int ARRAY[1024]; • Corresponds to the MIPS assemble directive .data ARRAY: .space 4096

28. Assembler Directives • Allocate and initialize a one-dimensional array int ARRAY[] = { 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 }; • Corresponds to the MIPS assemble directive .data ARRAY: .word 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 la $a0 ARRAY # $a0 = &ARRAY[0] lw $s0 8($a0) # $s0 = MEM[$ao + 8] $a0 contains a pointer to the array

29. Assembler Directives • String literal definition .data helloStr: .ascii “Hello, World!\n” • helloStr is the memory location of the array of characters • The string is not null terminated • Can cause problems (hell01.s) .data helloStr: .asciiz “Hello, World!\n”