memory operands

• memory is byte addressed
  • identifies an 8-bit byte
• words are aligned in memory
  • address must be multiple of 4
• MIPS is Big Endian
  • most-significant byte at least address
  • eg:
    • a word: A0 B1 C2 23
    • store in memory increasing:
    • A0 B1 C2 23

data transfer operations

• C code:

  g = h + A[8]
  

• MIPS code:

  lw    $t0, 32($s3)
  add    $s1,$s2, $t0
  

• C code:

  A[12] = h + A[8]
  

• MIPS code:

  lw $t0, 32($s3)
  add $t0,$s2, $t0
  sw $t0, 48($s3)
  

  • define address:
    • g -> $s1
    • h -> $s2
    • A -> $s3

register vs memory

• register are faster to access than memory
  • 所以要安排好，常用的盡量長時間放在register

immediate operands

• addi
  • 可直接用const value
  • 用法:

    addi $s3, $s3 , 4
    

the constant zero

• MIPS register 0 ($zero) is the constant 0
  • 不能寫入
  • 已寫死是0
• useful command
  • 當作move

    add $t2, $s1, $zero
    

representing instruction

• machine code
  • instruction are encoded in binary

MIPS R-format instruction

• instruction fields
  1. op:
     • 6bits
     • operation code (opcode)
  2. rs:
     • 5bits
     • first source register number
  3. rt:
     • 5bits
  4. rd:
     • 5bits
  5. shamt:
     • 5bits
  6. funct:
     • 6bits

• eg:

  add $t0, $s1, $s2
  

  • 數學運算 的opcode => 0
  • rs => $s1
  • rt => $s2
  • rd => $t0
  • shamt => 0
  • funct => 32 (add)
  • 數學運算 的opcode

MIPS I-format instruction

• instruction fields
  1. op:
     • 6bits
  2. rs:
     • 5bits
  3. rt:
     • 5bits
  4. constant or address:
     • 16bits
• 會有兩個format的原因
  • principle 3

• eg:

  lw $t0 ,1200($t1)
  

  • op => 35
  • rs => $t1
  • rt => $t0
  • address

MIPS logical operator

• shift operator
  • shift left (sll)
    • shift left, fill 0 bits
    • 意義：×2
  • shift right (srl)
    • shift right, fill 0 bits
    • 意義：/2
  • 有另一種指令sla，sra
    • 會根據+-號補上1||0

• and operator

  • operator：

    and $t0 $t1 $t2
    

    • t1,t2 做and， 存入t0

  • useful to mask bits in a word

    • select some bits, clear other to 0

• or operator

  • operator:

    or $t0 $t1 $t2
    

  • 跟and的方式一樣
  • useful to include bits in a word
    • select some bits to 1, leave other unchanged
• not operator
  • MIPS does not have not operator
  • operator:

    nor $t0 $t1 $zero
    

  • 利用nor operator 和 $zero 來做not
  • useful to invert bit in a word

flow control operation

• beq
  • 例子

    beq $s0 $s1 L1
    

  • 意思：

    if (s0==s1)
    jump L1
    

• bne
  • 例子：

    bne $s0 $s1 L2
    

  • 意思：

    if(s0!=s1) jump L2
    

• j
  • 就是jump

compiling loop statements

• c/c++:

  while(save[i]==k)i+=1;
  

• MIPS code:

  Loop:
     sll $t1, $s3, 2
     add $t1, $t1, $s6
     lw $t0, 0($t1)
     bne $t0, $s5, Exit
     addi $s3, $s3, 1
     j Loop
  Exit:
     ...
  

more conditional operation

• set result to 1 if a condition is true; otherwise,set to 0
• operator:

  slt rd, rs, rt
  slti rt, rs, const
  sltu
  sltui
  

• meaning

  if(rs<rt)rd = 1;else rd= 0
  if(rs<const) rt = 1, else rt = 0
  Unsigned operators
  

• 不做blt的原因：
  • 硬體比較難做，而且成品可能會拖垮其他的指令的速度
• blt 實做

procedure call instruction

• jal
  • 直接跳
  • 會記下下一行的address在$ra
• jr
  • 跳回$ra

Byte/Halfword operations

• could use bitwise operations
• code:

  lb rt, offset(rs)
  lh rt, offset(rs)
  lbu
  lhu
  

  • load 一個byte
  • load 兩個byte
  • unsigned operator

jump addressing

• j format
  • op code
    • 6bits
  • address
    • word
    • 26 bits
    • 所儲存的address是原始address/4
    • byte => word

branching far away

• beq
  • 可以跳+-128k的位置
• j
  • 可跳256MB

assembler pseudo instruction

• 可以提供假的指令
  • 如 move

fallacies

• powerful instruction => higher performance
  • 不一定
  • 可能跑比較久
  • 可能會拖慢其他的instruction
• use assembly code for high performance
  • 除非寫assembly 寫得比較好
  • 現在的compiler都很好
• backward compatible => instruction set doesn't change
  • 會增加新的instruction

chapter 3 arithmetic for computers

problem of ripple carry adder

• 下一個bit要等上一個bit運算完才能執行

IEEE floating point format

• s
  • sign bit
  • 0: non-negative
  • 1: negative
• exponent
  • single: 8bits
  • double: 11 bits
• fraction