CSE 378 Spring 2007

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CSE 378 Spring 2007

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Arithmetic Components The center of the computer is the Arithmetic and Logic Unit, but we also provide a simple adder component for use in computing branches and offsets.  Module: Adder     Simple unsigned adder of arbitrary size. No overflow checking  Parameters:   Name Default Description WIDTH 32 bit width of inputs and outputs Port Description:   Name Dir Width Description inA In WIDTH one input for the addition inB In WIDTH second input for the addition out Out WIDTH out = inA + inB Symbol:   This is a variable width module. When connecting buses to the ports of this module, you must specify the width of the bus, or it will default to an 8-bit bus. Module: MIPSALU This is an ALU that supports the full complement of arithmetic and logical operations of both signed and unsigned varieties. In general, the only difference between signed and unsigned operations is the overflow checking. Signed operations perform overflow checking, while unsigned operations do not.  The lone exception is the Set on Less Than (SLT) operation.  Consider the following case: SLTI SLTIU $t0, $0, 0xFFFF $t1, $0, 0xFFFF # $t0 = 0. SignExt(0xFFFF) = -1 (2's complement) # $t1 = 1. SignExt(0xFFFF) = 2^32-1 (unsigned)   This happens because the unsigned version compares the numbers as unsigned numbers, while the signed version treats them as 2's complements numbers.  It's worth noting that SLTIU still performs sign extension on its immediate.     Note: Does not implement any shift operations. Parameters:   Name Default Description AND 0 control code for logical AND OR 1 control code for logical OR ADD 2 control code for Addition ADDU 3 control code for unsigned addition SUBU 4 control code for unsigned subtraction SUB 6 control code for subtraction SLT 7 compare A and B as 2's complement numbers SLTU 8 compare A and B as unsigned numbers XOR 9 control code for XOR NOR 12 control code for logical NOR   Port Description:   Name Dir Width Description A In 32 left hand operand B In 32 right hand operand ALUctl In 4 determine which operation to perform ALUOut Out 32 result of the operation Zero Out 1 (ALUOut == 0)? 1 : 0 Overflow Out 1 Overflow status of signed operations Symbols:

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