CS400 Lab: 8 Bit CPU

Objective of this lab:


	To study the operation of a CPU and control unit. 

Preparation


	Read chapters 7 and 8 of your text book. Read lab lecture notes.

Apparatus


	PC computer and LogicWorks simulation software.

Procedure

  1. Obtain an 8-bit CPU working circuit file
    
    	~ftp/pub/class/400f/8bitcpu.cct
    
    
  2. The circuit just obtained is a 8 bit version of the 1 bit CPU we built in last lab with a few additions:
    
    	To get a feel for the circuit before more details are examined, 
    	do the following: 
        
    	1.	load the number 8 into memory location 2 
    
    	2.	move (the contents in) memory location 2 to the PC 
    
    	3.	increment the PC 
          
           
    	Write down the signals you used:
    
            ________________________________
    
            ________________________________
    
            ________________________________
    
            ________________________________
    
    
  3. Machine Instructions
  4. Execute the program
    
    Given the following program stored in memory write out the equivalent 
    assembly language mnemonics: 
      
    
    	
    
          Location 	Contents 	Assembly Language Mnemonics 
          =============================================================
          0 	10000001  
          -------------------------------------------------------------
          1 	00010110  
          -------------------------------------------------------------
          2 	00000101  
          -------------------------------------------------------------
          3 	00001000  
          -------------------------------------------------------------
    
    
    Pre-load the value 1 in the B register, preload 0 in the memery location 8, and then load the preceding program (in machine code) to the memory. Make sure the PC points to the start of the program. Write out the microinstructions necessary to execute the program. Execute the program.

Questions

 
	1) Describe the purpose of the 3->8 decoder. 

	2) Describe how the RAM chip control signals are used to read 
	   and write to memory. 

	3) Write out the microinstructions to perform the operation XOR #5,(C) 

	4) Describe how unconditional branches could be added to the CPU and 
	   what additional things this would require. 
	   Remember that a branch instruction uses an offset from the current 
	   location. 

	5) Add a new instruction (OR operation) to the CPU and 
	   include a print out of your circuit in your report.

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