Computer Architecture Overview

Control Unit

• Coordinates all actions of the computer

Input Unit

• Receives coded information from various sources
• Keyboard translates key presses into binary code
• Touchpad, mouse, joystick, trackball, and other graphic input devices are used with displays
• Microphones capture audio
• Cameras capture video
• Digital communication facilities like the Internet provide input from other computers and servers

Output Unit

• Sends processed results to the user
• Examples include printers
• Graphic displays can provide both output (showing text and graphics) and input

Types of Output

• Hard copy: Printed on paper or other permanent media
• Soft copy: Displayed on a screen or using other non-permanent methods

Multimedia

• Combines text, graphics, audio, and video

Branch Instructions

• beq (branch if equal):
  • Format: beq $s1, $s2, label
  • Branches to the address specified by the label if the value in register $s1 equals the value in register $s2
• j (jump unconditional):
  • Format: j label
  • Unconditionally jumps to the address specified by the label

Conditional Branch Instruction

• Allows altering the program flow based on a condition

Unconditional Branch Instruction

• Always executes the branch

Branch/Jump Instruction

• A key instruction for program control

Operands

• Values used in arithmetic and logical operations

Types of Operands

• Register operands: Located in special hardware components called registers
• Memory operands: Access data stored in memory
• Constant or immediate operands: Fixed values directly included in instructions

Register Operands

• Restricted to a limited number of registers
• Instructions involving arithmetic operations act only on registers

Memory Operands

• Data transfer instructions move data between memory and registers
• ADD A: Adds the contents of the memory cell at address A to the accumulator

Indirect Addressing

• The effective address of the operand is the contents of the memory location whose address is in the instruction
• ADD (A): Looks in the memory location A to find the address of the operand and then adds that operand to the accumulator
• ADD (A), R0: Same as above, but the result is stored in register R0

Register Addressing

• The operand is the content of the register specified in the instruction
• Example: Add R1, R2: Adds the content of register R1 to register R2
• Advantages:
  • Shorter instructions
  • Faster instruction fetch
  • Very fast execution due to no memory access
• Disadvantages:
  • Very limited address space

MIPS Assembly Language

• Uses specific registers for different purposes
• $s0 to $s7 map to registers 16 to 23
• $t0 to $t7 map to registers 8 to 15

Logical Operations

• Work on individual bits in the operands
• Include AND, OR, NOT, and XOR

Shift Operations

• Used to move bits within a register
• shamt (shift amount) specifies the number of positions to shift
• Shift left logical (sll): Shifts bits to the left, filling with 0s; multiplying by 2 to the power of the shift amount
• Shift right logical (srl): Shifts bits to the right, filling with 0s; dividing by 2 to the power of the shift amount

AND Operations

• Returns a 1 if both input bits are 1, and a 0 otherwise

OR Operations

• Returns a 1 if at least one input bit is 1, and a 0 otherwise

NOT Operations

• Inverts the bits of an operand; changes 0s to 1s and 1s to 0s

Instructions for Making Decisions

• Used to control program flow based on conditions
• beq register1, register2, L1: Branch to label L1 if the values in register1 and register2 are equal
• bne register1, register2, L1: Branch to label L1 if the values in register1 and register2 are not equal

Pseudodirect Addressing

• Used in jump instructions, where the jump address is formed by concatenating the upper bits of the program counter (PC) with the 26 bits of the instruction

Addressing Modes

• Different ways the CPU calculates the address of an operand
• Immediate Addressing Mode: The operand is directly specified as part of the instruction
• Register Addressing Mode: The address of the operand is stored in a register
• Base or Displacement Addressing Mode: The address of the operand is calculated by adding a displacement value to the contents of a register
• PC Relative Addressing Mode: The address of the operand is calculated relative to the program counter (PC)
• Pseudodirect Addressing Mode: The jump address is formed by concatenating the upper bits of the program counter (PC) with the 26 bits of the instruction

RISC (Reduced Set Instruction Set Architecture)

• Emphasizes simplicity of hardware and uses a limited set of basic instructions
• Aims to reduce the cycles per instruction, even if it means more instructions per program.

CISC (Complex Instruction Set Architecture)

• Allows more complex and powerful instructions, but can lead to increased cycles per instruction

Performance

• Both RISC and CISC aim to improve CPU performance
• RISC focuses on reducing cycles per instruction, sacrificing instructions per program
• CISC focuses on reducing instructions per program, sacrificing cycles per instruction
• The rise of high-level programming languages has favored RISC architecture, as it simplifies instruction set design