Common Questions About Project 2

Last updated 04/13/2005

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Contents

Notes:

1. Do not use StorageObject's backDoor function

Questions:

1. How do the set operations REALLY work?
2. How does that short data work in store instructions?
3. Doing comparisons of value in the datapath?

Notes

 
1. StorageObject void backDoor (long x): The StorageObject class does contain a function void backDoor (long x) that allows you to load a value into any storage object. You are not allowed to use this function, as it is a hack that exists for testing. Any use of this function will result in point loss on the project.
   

Questions

 

1. > I discovered a question during testing of my project 2.  It has to do with
   > the Set instructions. Since we're comparing the full width of the
   > register, we can't just fill the top half of a word with zeros like we
   > can with the immediate forms of this instruction.
   >
   

   You have to do the subtraction, and look at all the outputs from the ALU:

   	OUT()	the difference
   	CARRY()	carry/borrow flag: 0 = no, 1 = yes
   	OVERFLOW()	arithmetic overflow flag: 0 = no, 1 = yes

   Signed comparisons have these results:

   	equal	OUT() == 0
   	less	!equal && sign_of_OUT() != OVERFLOW()
   	greater	!equal && !less

   where sign_of_OUT() is the most significant bit of the result.

   Note that equal and greater than are included for completeness.  

2. >        I have a doubt, while executing inst. SH do we have to shift the
   > rd, 16 bits or do we mask the upper order bits? before stoing in MAR.
   >
   

   Inside the CPU's registers short data should always be kept in the low order bits (so that we can use it correctly in arithmetic operations). In memory short data sits at the address specified. So if you wanted to SH R1 at mem location A with:

   	R1 =    0000f3f2
   	mem[A]= 12345678
   

   in the end memory should look like:

     
   	mem[A]= f3f25678
   

   i.e. the half word of reg 1 was saved at the memory location A (without destroying any other memory but the 16 bit at mem location A.

   So to do this you need to:

   	temp = R1<<16
   	MDR  = mem[A]
   	MDR  = MDR & 0000ffff
   	MDR  = MDR | temp
   	mem[A] = MDR
   

    

3. > You were talking about the project tonight and said something about
   > doing instructions in 'if' statements.  If I did something like:
   > if( ir( 20, 25) - lmm(20,25) < 0 )...
   >
   > Thos are just fake values, but it gets my point across.  Is this illegal?
   

   You are allowed to use C++ if's to do comparisons with set patterns (ie comparing the opcode part of the IR with the constant 0x04 to see if is a BEQ instruction) but you are not allowed to do arbitrary comparisons between bit ranges (ie comparing A.value() < B.value()) as doing this requires a complex circuit. In addition, you are not allowed to use C++ code to manipulate the data (you can't do ir(20,25) - lmm(20,25)). All manipulation of the data inside the simulated program should be done through the arch package.

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