Simple RISC Computer (ezRISC)

George Trieu, Matt Garvin

Datapath Architecture

The datapath architecture is based off of a 1-BUS design.

Components

• PC: Program Counter
  • 32-bit Register
  • Stores the memory address of the current instruction
• IR: Instruction Register
  • 32-bit Register
  • Stores the instruction fetched from memory
• General Purpose Register File
  • Collection of 16 32-bit Registers Numbered from R0 to R15
• MAR: Memory Address Register
  • Stores the address that will be queried from memory
• MD_MUX/MDR: Memory Data Multiplexer/Memory Data Register
  • MD_MUX
    • 32-bit 2-1 Multiplexer
    • If Reading from memory, pass memory data to MDR
    • If Writing to memory, pass bus data to MDR
  • MDR
    • 32-bit Register
    • Stores data to be written to (Read = 0) or read from (Read = 1) memory
• HI: High Register
  • 32-bit Register
  • Stores the upper 32 bits of the 64-bit Z Register
    • Remainder in Division Operations
• LO: Low Register
  • 32-bit Register
  • Stores the lower 32 bits of the 64-bit Z Register
    • Quotient in Division Operations
• Y_MUX/Y: Y Multiplexer/Y Register
  • Y_MUX
    • 32-bit 2-1 Multiplexer
    • If used to increment PC, select constant 0x1
  • Y Register
    • 32-bit Register
    • Stores one of the operands used in ALU calculations
• ALU: Arithmetic Logic Unit
  • Performs arithmetic and logic operations
• Z: Z Register
  • 64-bit Register
  • Stores the result of the arithmetic operation
  • Can output upper or lower 32-bits onto the bus

ALU Architecture

The ALU is made up of different modular operations, that is selected using a 16-1 MUX.

Select and Encode Logic

• Special Note: When the purple logic expression on the left is true:
  • jal OP Code is 10100
  • This means this is a 'jal' (jump and link) instruction
  • It also means in this cycle, the current PC is pushed into the link register (R15)
  • Forces the decoder to activate the link register (R15) lines.