Parts of a Computer

1. Central Processing Unit (CPU)
   1. 32-bit (Nios II processor)
   2. Has a bunch of registers r0, r1, r2, r3 …
   3. r0 always equal to 0
   4. Mostly use r8-r15
2. Main memory
   1. Broken into 8 bit chunks
   2. Has unit addresses
3. Input/Output unit
   1. Screen
   2. Keyboard
   3. LED
   4. button

Example: storing and adding registers

• Consider this C code:

  int A,B,C,D; //no ASM equivalent bc A = r8, B = r9, C = r10, D = r11
  A = 1;
  B = 8;
  C = A+B;
  D = B-A;
  

• Translate into Nios II Assembly:

  movi r8,1
  movi r9, 8
  add r10, r8, r9
  sub r11, r9, r8
  iloop: br iloop
  

Immediate vs Non-Immediate Instructions

• Immediate Instructions (using i):
  • andi r1, r2, 0xF - Perform bitwise AND between the contents of r2 and the immediate value 0xF, store the result in r1.
  • ori r1, r2, 0xFF - Perform bitwise OR between the contents of r2 and the immediate value 0xFF, store the result in r1.
• Non-Immediate Instructions:
  • and r1, r2, r3 - Perform bitwise AND between the contents of r2 and r3, store the result in r1.
  • or r1, r2, r3 - Perform bitwise OR between the contents of r2 and r3, store the result in r1.

32-bit Nios II Processor

• 32-bit processor
• Registers are 32-bits
• The size of the addresses are 32 bits
  • Largest address is $2^{32}-1$
• Processor can access 32 bits at one time from memory
• Byte: basic unit of storage in computer memory and represents 8 bits
  • Can represent values from 0 to 255
• Memory Address: unique identifier for a byte location in memory
  • When a program runs, it stores and retrieves data from specific memory addresses
  • CPU uses these addresses to access the memory locations to read or write data
  • Recall that memory is byte-addressable
    • Every byte has an address
    • So one word uses 4 bytes = 4 addresses
• Word: group of bytes that a processor naturally handles as a unit
  • Size of a word is 32 bits (4 bytes)
  • Word size determines how much data a processor can handle at one time
  • 32 bits is the word size of the processor
    • Although, we can access less than 32 bits at one time:
      • Half word = 16 bits
      • Byte = 8 bits
  • Nios II architecture can handle 4 bytes of data in a single operation

First few words of memory

Each byte has its own address

• Word addresses must be divisible by 4