What does overflow flag bit mean, noun explanation overflow flag bit

The overflow flag bit is of which bit is extra in the calculation, for example, ax=ffff, bx=0001, add ax, bx is equal to 10000, and 1 of overflow will be added

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of flags. It's called the overflow flag. If the result is beyond the range that the machine can represent, it is called overflow.

Note: The overflow here is not the same as our buffer overflow.

What we are talking about here is an overflow resulting from the operation of a signed number of evidence. And it's just an overflow of the results of the operation beyond what the machine can express.

So how exactly do unsigned numbers and signed numbers convert? Since most of our compilations are represented in hexadecimal form, I will also use hexadecimal conversion here.

Because we generally have negative integers stored in computer memory in the form of complements. To put it simply, the complement of our integer is the opposite. Because only binary values can be stored in the computer memory, the negation is 1->0 0->1

Let's start with a simple binary value 1. Suppose the binary value 0000 0001 at this time, then its complement is 11111111, then it is also the binary number -1. In the same way, finding the complement of 1111 1111 is 00000001

Complement calculations in hexadecimal. Calculating the hexadecimal complement can negate all data bits and then +decimal negation, and one of the easiest ways is to subtract the corresponding digit by 15.

Here's an example. Decimal 100 = hexadecimal 64 -- negation 9b+1 = 9c ; 9c is the complement of hexadecimal 64, which is hexadecimal-64.

If you don't believe me, you can try it, because 9c is a byte. Then you send the data through the lower 8 bits of the EAX register.

The overflow flag bit (OF) is the state information generated by the CPU according to its own method when it operates according to the binary operation law. The method of giving the overflow flag is usually the carry bit XOR method, that is, when two binary numbers are operated, the carry produced by the operation of the most significant bit is different from the carry produced by the operation of the next most significant bit, and the result is the overflow flag bit of. When of=1, it indicates that the next significant bit operation produces a carry and the most significant bit operation does not carry, or the second significant bit operation does not carry and the most significant bit operation produces a carry.

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The overflow flag bit (OF) is the state information generated by the CPU according to its own method when it operates according to the binary operation law. The method of giving the overflow flag is usually the carry bit XOR method, that is, when two binary numbers are operated, the carry produced by the operation of the most significant bit is different from the carry produced by the operation of the next most significant bit, and the result is the overflow flag bit of. When of=1, it indicates that the next significant bit operation produces a carry and the most significant bit operation does not carry, or the second significant bit operation does not carry and the most significant bit operation produces a carry.

The symbol of a signed number is represented by ** in a computer, that is, the most significant bit of the data is the sign bit, and the rest of the bits are numeric bits. And still operate according to the binary rules of operation, so for signed number operations, Si indicates that the numerical bit operation does not produce a carry and the sign bit operation produces a carry, or the numerical bit operation produces a carry and the sign bit operation does not produce a carry. The former must be the addition of two negative signs, the result is that the positive sign is the latter, and the result is that the two positive signs are the addition, and the result is that the negative sign is .

Either way, from a symbolic point of view, the result is wrong.

The highest carry flag cf indicates whether the highest carry is upward to form a carry or borrow;

The overflow flag of of of indicates whether the result is outside the range of the number represented.

cf is used to represent the overflow of unsigned numbers, of of which is the overflow of signed numbers, and in general it is like this....The overflow of unsigned numbers is relatively simple to understand, and the complement representation of signed numbers should be noted....

The overflow detection method is used with double sign bits: during the operation, two sign bits participate in the operation at the same time, and if the two sign bits are different in the result, it means that an overflow has occurred. If the symbol is 01, it means that the operation result is greater than the maximum positive number of the allowable value range, which is called positive overflow. If the sign bit is 10, it means that the result of the operation is negative, and its value is less than the minimum negative number of the allowable value range, which is called negative overflow.

The highest bit of the two symbol bits is still the correct symbol.

mov al,80h (al)=80h= 10000000b

add al,80h (al)=a0h= 100000000b 1 al=0<127, so of is 1

mov al,0fch (al)=fch= 11111100b 1 1

add al,05h (al)=101h= 100000001b 1 al=1<128, so of 1, you are the oral arithmetic, use the computer to calculate. Please refer to it.

Because bai is int mid = (low+high) 2 may be inexhaustible, the value of zhi mid like du is inaccurate, and it is changed to dao

int mid = low+(high-low) 2 is not an inexhaustible question.

The value of MID is accurate, and if the MID value is too large, there will be a out-of-bounds problem, which is called overflow.

Data overflow.

In a computer, a data overflow occurs when the data to be represented is outside the representation range of the data used by the computer.

Spill Reason. Data overflow occurs if the data type exceeds the limit of the computer's word length. There are many reasons for memory overflow issues, such as:

1) Use non-type-safe languages such as C++, etc.

2) Access or copy memory buffers in an unreliable manner.

3) The memory buffer set by the compiler is too close to the critical data structure.

Factor analysis. 1.The out-of-memory problem is an inherent flaw in C or C++, which neither checks array boundaries nor type-safety. It is well known that programs developed in C C++ have direct access to memory and registers due to their close proximity to the machine core, which greatly improves the performance of C C++.

As long as it is properly coded, C C++ applications are bound to outperform other high-level languages in terms of execution efficiency. However, C C++ is also much more likely to cause memory overflow issues. Other languages also have an out-of-memory problem, but it's often not a programmer's mistake, but an error in the application's runtime environment.

2.When an application reads user (and possibly a malicious attacker) data and tries to copy it to a memory buffer created by the application, but is unable to guarantee that there is enough space in the buffer (in other words, suppose ** requests an N-byte memory buffer and then copies more than n bytes of data to it). Memory buffers may overflow.

Think about it, if you pour 12 ounces of water into a 16-ounce glass, what would you do with that extra 4 ounces of water? Of course, it will fill up to the outside of the glass!

3.On top of that, the C C++ compiler opens up memory buffers that are often close to important data structures. Suppose that the stack of a function is immediately behind the memory buffer, the function return address stored in it will be adjacent to the memory buffer.

At this point, a malicious attacker can copy a large amount of data to the memory buffer, causing the memory buffer to overflow and overwrite the function return address that was originally stored in the stack. In this way, the return address of the function is replaced by the attacker with the value specified by him; Once the function has been called, the ** at the "function return address" will continue to be executed. Not only that, but some other data structures in C++, such as v-tables, exception event handlers, function pointers, etc., can be similarly attacked.

Prohibition of terminal flags: This directive should cause RT to logically 0 the terminal flag position in the prescribed state word until otherwise directed. The RT should send the prescribed status word.

Terminal flag bit: The nineteenth digit of the status word should be reserved for the terminal flag function. The use of the standard is optional.

If used, the base is a flag of the RT failure state. Logic 1 indicates that there is a fault, while logic 0 indicates that there is no failure. If you don't use this function, the base should be set to logical 0.

0v - overflow flag, for a single byte of signed numbers, if the highest bit (b7) is used to represent positive and negative signs, there are only 7 significant digits (b6 b0), which can represent the number between -128 + 127.

If the result of the operation is outside this value range, an overflow will occur, in which case 0v=1, otherwise 0v=0. In addition, when executing a multiplication instruction, 0v=1 means that the product is more than 255; When executing a division instruction, 0v=1 means that the divisor is 0.

Explicit: The overflow is for signed numbers, and there is no overflow problem for unsigned numbers.

Note: Signed numbers are represented internally by complements, with an integer range of 127 128 for 8-bit representation and 32767 for 16-bit representation

Definition: An overflow occurs when the result of an operation is outside the range of data that can be expressed.

Conclusion: There is an overflow, which means that the result of the operation of the signed number is incorrect.

Flag: of overflow flag. If there is an overflow in the result of the arithmetic operation, then of 1;Otherwise, of 0.

Example: 3ah 7ch b6h, that is, 58 124 182, the result of this 8-bit operation has exceeded the range of 128 127, resulting in an overflow, so of 1. On the other hand, the complement b6h expresses the true value of 74, which is obviously incorrect.

Differences: The overflow flag of the spill flag and the carry flag of cf are two signs with different meanings. The carry flag indicates whether the result of the unsigned number operation is out of range, but the result is still correct; The overflow flag indicates whether the result of the operation of the signed number is out of range, and the result of the operation is incorrect.

The range of 8 binary bits for an unsigned integer is: 0 255;The range of 16-bit expression is: 0 65535.

Contrast: 3ah 7ch b6h, unsigned number operation: 58 124 182, range, no carry; Signed number operations: 58 124 182, out of range, with overflow.

aah 7ch (1)26h, unsigned number operation: 170 124 294, out of range, with carry; Signed number arithmetic: 86 124 38, range, no overflow.

Processing: When the processor calculates the two operands, the result is obtained according to the unsigned number, and the carry flag cf is set accordingly; At the same time, the overflow flag OF is set according to whether it exceeds the range of the number of signs. It is up to the programmer to decide which flag to use.

That is, if the operands participating in the operation are considered to be unsigned, one should be concerned with carrying; If you think it is a signed number, you should pay attention to whether it overflows.

Judgment: The computer uses the highest and second highest positions to carry to judge that there is an overflow. We can take advantage of a simple rule:

Only when two identical symbols are added (including the subtraction of two different symbols) and the sign of the result is opposite to the original data sign, an overflow occurs (because the result of the operation is obviously incorrect); Otherwise, there will be no overflow.

Overflow is the value of the timer reaches the maximum range, for example, the maximum value of the 16-bit timer is 65536, when the timer runs from 0 to 65536, it overflows, and can no longer be installed, at this time, it makes no sense to add it, generally overflow, the single-chip microcomputer will be the overflow flag of the corresponding timer to 1, and then trigger the interrupt, telling you that the time you need to time needs to be processed.

The status register in the microcontroller has such a flag bit, because in addition, subtraction, multiplication and division, the operation result will be beyond the range that the machine can represent, so it is used to indicate whether it is overflowing. It is also used in the counter timer to calculate whether the count value has reached a preset value.

51 MCU, in the special function register PSW, has an "overflow flag" called OV.

Its content automatically changes with the result of the operation.