Digital Level Design of Processors

Low level overview of simple processor.

• Processor fetches machine encoded instructions from memory and executes these instructions
• The memory also holds data for reading/loading and writing/storing
• We access I/O through special memory locations that cause things to happen, or receive information in the outside world

Simple Processor: Instructions and Encoding

• Has 4 by 8-bit registers
  • Nios II is 32 by 32 bit
• Memory is only 256 bytes (vs 1G byte)
  • All addresses are 8 bits ($2^8 = 256$)
• Separate Program Counter (PC) 8 bits wide
  • Holds the address of the next instruction to be executed
• Only 10 instructions → encoded into 8 bits

Condition Code Flag Registers (Z and N bits)

1 bit each, as follows, used for the conditional branch instructions

1. Z bit: If the operation causes the result to be exactly zero, this bit is set
   1. 1 when the result of the last instruction was zero
   2. 0 when
2. N bit: set if the result of the last operation was nagetive
   1. 1 when the result is negative
   2. 0 when the result is positive

Key Parts

• Program Counter
  • Address of the next instruction to be executed
• Memory
  • Where both encoded instructions and data are stored
• Register file
  • 4 by 8 bit register
• Arithmetic Logic Unit
  • Where the functions happen
  • Conditional bits (N and Z)