Comp Org Week 7

How does MIPS handle subtraction?

It uses addition, which means that instead of doing 7 - 6, it does 7 + -6 (this uses no additional opcode)

What is overflow?

A carry past the highest bit. This doesn’t happen adding operands of different signs or subtracting operands of the same sign.

How can you tell there’s been an overflow?

The sign bit is set with the value of the result instead of the correct sign.

Which instructions do you use if overflow detection is not needed?

addu, addui, and subu

Which instructions do you use if there’s a chance of an overflow?

add, addi, sub

What are examples that signal there was an overflow?

1. Adding two positive numbers give a negative number

2. Adding two negative numbers give a positive number

3. Subtracting a positive number from a negative number gives a positive number

4. Subtracting a negative number from a positive number gives a negative number

How many bits are needed for binary integer multiplication?

64 bits

How many bits are needed for binary integer division?

31 bits

What registers are used for integer multiplication and division in MIPS?

mult, div, hi, lo, mfhi, mflo

What does ‘hi’ used for in MIPS?

With multiplication, it stores the resultant (because it’s able to store a 64 bit integer). With division, it stores the remainder

What is ‘lo’ used for in MIPS?

It’s used with ‘hi’ in multiplication to store 64 bits integers. In division, it stores the quotient.

What does ‘mfhi’ and ‘mflo’ mean in MIPS?

Move the contents from hi/lo into rd