Logisim CPU Project
Erty Seidel
Spring Term 2011
Logisim
• Logic Simulation Program
– http://ozark.hendrix.edu/~burch/logisim/
– Cross-platform ( > Java 5)
•
•
•
•
Recent Update
Plugins and mods
FOSS (GPL)
[Demonstration]
CPU Project
•
•
•
•
•
•
About one month
60-100 Hours
5 Versions (1 rebuild)
Still in development
SVN repository
Probably should have been doing homework
Running a Program
•
•
•
•
•
•
Consult MIPS instruction set
Write out binary code
Use windows calculator to convert to hex
Write to a text document
Load into I-mem and D-mem
Run!
What Works?
• [See Excel spreadsheet]
Big Problems?
• No support for I-format arithmetic
• Registers cannot write and read at the same
time
• Multiplication / Division are weird when
working with highest-order bit
• No branching
• No exceptions
• No forwarding / hazard detection
The CPU
• [Demonstration]
Lessons Learned
• CPUs are serious business
• Simple pipelining is easy, anything more is
very difficult
• MIPS instructions are a bit arbitrary
• The carry-in for signed subtraction must be 1
Thanks
• Prof. Evans
– Helped talk through problems
• Jorge Seidel
– Solved subtraction, mult/div issues