COMP541 Completing the MIPS Datapath

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COMP541Completing the MIPS Datapath

• Montek Singh
• Mar 27, 2007

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Datapath Lab

• Last Fridays lab Included this
• Need more also memory/control

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Next few labs

• March 30 lw and sw and branching
• April 6 Complete CPU (Holiday?)
• Add VGA, keyboard/joystick

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Next Lab

• Implement the lw and sw instructions.
• The address for lw/sw generated by ALU
• Format
• sw r1, 100(r2)
• Store contents of register 1 to
• address r2 100
• Implement branch instructions

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Store Word

• Easier
• Just disable writing to register
• Set an address and write to memory
• Could use second memory port and write on same
  cycle as next instruction
• Lets look at block diagram

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Block Diagram (just lw/sw)
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R-Type and lw/sw
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Load Word

• Need to determine memory address
• Using ALU
• Then need to read memory and write to register

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Careful

• If you use one memory port, your instruction will
  disappear
• You could use the other memory port
• Could add an instruction register
• More complex control
• Harvard Architecture another possibility

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Sign Extension

• Dont get sidetracked into complexity
• Just extend value of bit 15 to upper 16 bits
• How?

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Branching

• Add beq (branch on equal)
• That should give enough breadth to actually write
  full programs

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Resulting MIPS Datapath
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With Control
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Memory

• Addressing
• Byte address or word address?

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The MIPS Datapath
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Expanding Memory Block

• First lets look in detail at data memory block
• See how to add block memories
• Add I/O

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Memory Block Signals
Mem Write 1 bit
Address 32 bits
Write Data 32 bits
Read Data 32 bits
Mem Read 1 bit
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Using 2 Block RAMs

• Illustration from book
• Lets work it out for Block RAMs
• Specs next

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Block RAM Specs

• Try 2 256x16

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Adding More Memory

• Illustration from text
• Could add 2 more block RAMs to increase mem to
  512 words

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Memory Map

• Refers to how physical memory is populated
• In our example, memory from 0 to 511
• Its in two blocks, but thats invisible to
  programmer

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Another Portion of Memory

• Lets see how to add a block of x8 character
  memory for the VGA
• Need to make 24 bits be zero

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Memory Map
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Instruction and Data Memory
Empty
01 0000 0000 -256
Character Memory
10 0000 0000 512
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Adding I/O

• Look at memory-mapped I/O
• Conceptually like this

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In Reality

• Isolated from CPU
• On standard buses
• PCI
• Ours will be simpler
• Just memory addresses

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Later

• Youll add joystick or keyboard (or both) into
  memory locations that you can read

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How Does Processor Begin?

• One way is to initialize PC to 0
• Make sure to have instructions there
• Real computers have flash memory to boot CPU or
  go to configuration utility
• On (very) old computers had to enter boot program
  on front panel