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Chap.2: Instructions: Language of the computer

Chap.2: Instructions: Language of the computer. Jen-Chang Liu, Spring 200 6 Adapted from http://www-inst.eecs.berkeley.edu/~cs61c/. Example for Lab#1: 印出 sum=5. .globl main main: .data str: .asciiz "sum=" .text li $v0, 4 # $v0=4 la $a0, str # $a0=str

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Chap.2: Instructions: Language of the computer

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  1. Chap.2:Instructions: Language of the computer Jen-Chang Liu, Spring 2006 Adapted from http://www-inst.eecs.berkeley.edu/~cs61c/

  2. Example for Lab#1: 印出 sum=5 .globl main main: .data str: .asciiz "sum=" .text li $v0, 4 # $v0=4 la $a0, str# $a0=str syscall # print the string li $s0, 2 li $s1, 3 add $a0, $s0, $s1 # 2+3=5 li $v0, 1 # print integer syscall

  3. 不要亂寫 assembly code • add $s0, 1

  4. Pseudo-instruction • SPIM can recognize a large instruction set than the original MIPS • Example: li (load immediate) SPIM Pseudo-instruction MIPS Bare machine real inst.

  5. Shut down pseudo-instruction option

  6. Outline • C operators, operands • Variables in Assembly: Registers • Comments in Assembly • Addition and Subtraction in Assembly • Memory Access in Assembly 指令

  7. Assembly Operands: Memory • C variables map onto registers; what about large data structures like arrays? • 1 of 5 components of a computer: memory contains such data structures • But MIPS arithmetic instructions only operate on registers, never directly on memory. • Data transfer instructions transfer data between registers and memory: • Memory to register: load • Register to memory: store

  8. MIPS CPU store load

  9. MIPS memory allocation (Textbook Fig2.17) • 32 bits addressing • one byte in an address 20000000 1 byte It is software convention

  10. Data Transfer: Memory to Reg (2/4) • To specify a memory address to copy from, specify two things: • A register which contains a pointer to memory • A numerical offset (Max. offset???) • Example: 8($t0) • specifies the memory address pointed to by the value in $t0, plus 8 bytes • The desired memory address is the sum of these two values, 8+$t0.

  11. Memory access: 8($t0) Pointer $t0 (register) Offset 8 (constant) Start of an array 8 (constant) Offset $t0 (register)

  12. Data Transfer: Memory to Reg (3/4) • Load Instruction Syntax: 1 2,3(4) • where 1) operation name 2) register that will receive value 3) numerical offset in bytes 4) register containing pointer to memory • Instruction Name: • lw (meaning Load Word, so 32 bits(4 bytes) or one word are loaded at a time) Example: lw $t0,12($s0)

  13. Example: load word

  14. Data Transfer: Reg to Memory (1/2) • Also want to store value from a register into memory • Store instruction syntax is identical to Load instruction syntax • Instruction Name: sw (meaning Store Word, so 32 bits or one word are loaded at a time)

  15. Data Transfer: Reg to Memory (2/2) • Example: sw $t0,12($s0) This instruction will take the pointer in $s0, add 12 bytes to it, and then store the value from register $t0 into the memory address pointed to by the calculated sum

  16. Pointers v.s. Values • Key Concept: A register can hold any 32-bit value.(typeless) • That value can be a (signed) int, an unsigned int, a pointer (memory address), etc. • If you write add $t2,$t1,$t0 then $t0 and $t1 better contain values • If you write lw $t2,0($t0) then $t0 better contain a pointer • Don’t mix these up!

  17. C: word address => assembly: byte address • What offset in lw to select A[8] in C? • A is a 4-byte type (ex. long int) • Compile by hand using registers:g = h + A[8]; • g: $s1,h: $s2,$s3:base address of A • 4x8=32 bytes offset to select A[8] • add $s1, 32($s3) lw $t0,32($s3) # $t0 gets A[8] add $s1,$s2,$t0 # $s1 = h+A[8]

  18. 0 1 2 3 Aligned Not Aligned More Notes about Memory: Alignment • MIPS requires that all words start at addresses that are multiples of 4 bytes • Called Alignment: objects must fall on address that is multiple of their size.

  19. Role of Registers vs. Memory • What if more variables than registers? • Compiler tries to keep most frequently used variable in registers • Why not keep all variables in memory? • Smaller is faster:registers are faster than memory • Registers more versatile: • MIPS arithmetic instructions can read 2, operate on them, and write 1 per instruction • MIPS data transfer only read or write 1 operand per instruction, and no operation

  20. Brief summary (1/2) • In MIPS Assembly Language: • Registers replace C variables • One Instruction (simple operation) per line • Simpler is Better • Smaller is Faster • Memory is byte-addressable, but lw and sw access one word at a time. • A pointer (used by lw and sw) is just a memory address, so we can add to it or subtract from it (using offset).

  21. Brief summary (2/2) • New Instructions: add, addi, sub lw, sw • New Registers: C Variables: $s0 - $s7 Temporary Variables: $t0 - $t9 Zero: $zero

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