Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 75)

Conditional and Arithmetic Instructions 61

The Overflow Bit

What if we were adding signed numbers? A carry out of the high-order

bit is not necessarily indicative of a result that is too large. Adding two

negative numbers will result in the carry bit being set. For example,

11111110 -2

+ l i m i i i -l

111111101 -3

A result that is too large can be detected when the sign bit (most signif

icant bit) changes when we don’t want it to. This condition is known as

overflow. The overflow bit (V) is set or reset depending on the occur

rence of the overflow condition. V is affected by all arithmetic instruc

tions regardless of whether operands are treated as signed or unsigned

numbers. The 68000 instructions don’t know the difference. Either V or

C must be used, for signed or unsigned arithmetic, respectively. The BVS

(branch on overflow set) and BVC (branch on overflow clear) instruc

tions are provided for signed arithmetic.

Subtraction also results in similar effects on C and V. If an unsigned

subtraction results in a borrow into the most significant bit position, the

carry bit is set. Since an unsigned subtraction cannot result in a negative

value, this is an error. The overflow bit is set for subtractions as well. If

we subtract a positive number from a negative number, and the result

is a negative number that is too large, V will be set. Overflow can also

result from subtracting a negative number from a positive number. If the

positive result is too large, V will be set.

The Zero and negative Bits

In addition to determining if the result of an arithmetic operation has

exceeded the size capacity of the destination, a programmer may also

desire to know if the result is positive, negative, or zero. The zero bit

(Z) and negative bit (N) are provided for these purposes. The zero bit

is set if the numerical result is zero. This means that all bits of the result

are zero. The BEQ (branch on equal) and BNE (branch on not equal)

instructions will conditionally branch depending on the value of Z.

A zero value can be used as a signal to exit an indefinite loop. The

following program outputs the integers from 10 to 1 in reverse order.

1 2 ... 70 71 72 73 74 75 76 77 78 79 80 ... 255 256

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Motorola M68000 Betriebsanweisung Seite 75