4.3 Addition and Substraction

• Overflow occurs when adding two positive numbers and the sum is negative, or vice versa
• Adding or subtracting two 32-bit numbers can yield a result that needs 33 bits to be fully expressed
• The lack of 33 bits means that overflow occurs when the carry out occurred into the sign bit 
• Unsigned integers are commonly used for memory addresses where overflows are ignored
• Two kinds of arithmetic instruction to ignore overflow or not:
• add. addi, sub cause exceptions on overflow
• addu, addiu, subu do not cause exceptions on overflow
• Because C ignores overflows, the MIPS C compilers will always generate unsigned version of arithmetic instructions (addu, addiu etc)
• MIPS detects overflow with an exception/interrupt
• An exception/interrupt is essentially an unscheduled procedure call
• MIPS includes a register called the exception program counter (EPC) to contain the address of the instruction that caused the exception
• The instruction move from system control (mfc0) is used to copy EPC into a general-purpose register so that MIPS software has the option of returning to the offending instruction via a jump register instruction