twos-complement

问题 In one of the books that I am reading, following function is used to determine 2's complement integer multiplication overflow. int tmult_ok(int x, int y) { int p = x*y; return !x || p/x == y; } While this works, how do I prove its correctness in all the cases? How do I ensure that p != x*y when there is an overflow? Here is what I understand: When you multiply 2 integers of size "w- bits", the result can be 2w bits. The computation truncates higher order w bits. So we are left with lower

问题 In one of the books that I am reading, following function is used to determine 2's complement integer multiplication overflow. int tmult_ok(int x, int y) { int p = x*y; return !x || p/x == y; } While this works, how do I prove its correctness in all the cases? How do I ensure that p != x*y when there is an overflow? Here is what I understand: When you multiply 2 integers of size "w- bits", the result can be 2w bits. The computation truncates higher order w bits. So we are left with lower

问题 In one of the books that I am reading, following function is used to determine 2's complement integer multiplication overflow. int tmult_ok(int x, int y) { int p = x*y; return !x || p/x == y; } While this works, how do I prove its correctness in all the cases? How do I ensure that p != x*y when there is an overflow? Here is what I understand: When you multiply 2 integers of size "w- bits", the result can be 2w bits. The computation truncates higher order w bits. So we are left with lower

问题 As we know that the range of char is -128 to 127 . The 2's compliment of -128 and the binary of 128 is same, which is 10000000 So why the range of char is -128 to 127 but not -127 to 128 . Where as in case of int , 128 and -128 both are different. 回答1: In twos-complement notation, whenever the high-order bit is 1, the number is negative. So the biggest positive number is 01111111 = 127 and the smallest negative number is 10000000 = -128 The same thing happens for int , but its range is much

问题 I know that -4 is copied into the EAX register. My doubts: -4 will be converted into two's complement binary notation before copying to EAX or not? If it is converted into two's complement binary notation , who does the job? Is there any special opcode for denoting negative numbers? What is the possible maximum negative number we can store in EAX register? Is there any special opcode or instructions for signed arithmetic? what happens when we multiply a negative and positive number in CPU?

问题 I am trying to figure out how the imul and idiv instructions of the 8086 microprocessor work. I know this: 1. mul and div are multiplications and division for unsigned numbers 2. imul and idiv, are also multiplications and divisions but for signed numbers I searched all the web, and what I just wrote above, that's the only info that I've found, but written in different ways. I have this: mov AX, 0FFCEh idiv AH Because ah it's a byte, AL=AX/AH (the result) and AH=remainder After the

问题 thx for help my question is about ax value received from code below? mov al,22h mov cl,0fdh imul cl Actual machine result: ff9a What I expected: 00:9a (by multiplying in binary) The first number is 22h so its 34 decimal its already unsigned the second number is fd in binary its goes like 11111101 so its signed that mean its like -3 so 22* -3 its 66; and -66 on signed 9a so why there is ff at the beginning 回答1: imul cl does AX = AL * CL , producing a full 16-bit signed product from 8-bit

问题 The ISO C standard allows three encoding methods for signed integers: two's complement, one's complement and sign/magnitude. What's an efficient or good way to detect the encoding at runtime (or some other time if there's a better solution)? I want to know this so I can optimise a bignum library for the different possibilities. I plan on calculating this and storing it in a variable each time the program runs so it doesn't have to be blindingly fast - I'm assuming the encoding won't change

问题 The ISO C standard allows three encoding methods for signed integers: two's complement, one's complement and sign/magnitude. What's an efficient or good way to detect the encoding at runtime (or some other time if there's a better solution)? I want to know this so I can optimise a bignum library for the different possibilities. I plan on calculating this and storing it in a variable each time the program runs so it doesn't have to be blindingly fast - I'm assuming the encoding won't change

问题 The ISO C standard allows three encoding methods for signed integers: two's complement, one's complement and sign/magnitude. What's an efficient or good way to detect the encoding at runtime (or some other time if there's a better solution)? I want to know this so I can optimise a bignum library for the different possibilities. I plan on calculating this and storing it in a variable each time the program runs so it doesn't have to be blindingly fast - I'm assuming the encoding won't change