When is carry bit set

What you're doing is subtracting 1 from zero. This causes a "borrow" into the high-order bit. Borrow and carry are the same thing only different.

Carry flag is set when the result of the operation is too big to fit into the registers you are using. I don't see why it is too big in this case. For subtraction, the carry flag acts as the borrow flag. Since 1 - 2 is negative, it results in a borrow - thus, the carry flag is set. Show 4 more comments. Active Oldest Votes. Improve this answer. Oliver Charlesworth Oliver Charlesworth k 29 29 gold badges silver badges bronze badges. Why does that make sense? Are you familiar with when it gets set in a subtraction?

I don't see why '' 8 numbers would not fit inside a 8 bit register. Is the carry flag inverted by definition. How about arm and mips? Remember that if you had done long subtraction i. So in that sense, yes, it's inverted. Show 2 more comments. Community Bot 1 1 1 silver badge. Tony Tony 5, 1 1 gold badge 34 34 silver badges 49 49 bronze badges.

Note that x86 sub flag-setting is not exactly equivalent to add with -src. Add a comment. It is unclear till the moment one reads how CF flag is defined. Intel CPU docs manual says: If the result of an arithmetic operation is treated as an unsigned integer, the CF flag indicates an out-of-range condition Negating anything which is treated as unsigned is obviously invalid since no unsigned value can have its sign flipped and still remain unsigned unless it is zero.

If value which you negate is treated as signed - you should look at Overflow Flag. This is the same for example in Atmel's AVRs. Artur Artur 6, 2 2 gold badges 24 24 silver badges 35 35 bronze badges. On ARM, it's inverted: it's set on no-borrow, cleared on borrow propagation out of the top bit. So ARM is most definitely not the same as x86 in the way they set their carry flag for subtraction.

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Email Required, but never shown. Learn more about Collectives on Stack Overflow. The Overflow Blog. Does ES6 make JavaScript frameworks obsolete? Podcast Do polyglots have an edge when it comes to mastering programming Featured on Meta. Now live: A fully responsive profile. Similar to the usage of the carry bit when adding there are mechanisms for subtracting that allow to integrate the result of subtraction of the lower bits into the subtraction of the next higher block of bits, where necessary.

Often CPUs use what is the carry bit for addition and interpret it as borrow bit for subtraction. It is assumed, that the CPU-word is bits long, so calculations are performed modulo. Further it is assumed, that the range is preferred, so all sign issues are excluded for the moment. When subtracting two numbers , from each other like and , the provided result is.

The next round of subtraction takes the borrow bit into account and calculates , where the condition becomes and the result is. This is how some of the older readers used to do it in school on paper, but of course with.

Combining this with the previous results in calculating. At this point some CPU-designers found it more natural, to use the carry bit instead of the borrow bit. When subtracting two numbers , from each other like and we have , the provided result is. These two ways are really equivalent and easily transformed into each other.

Neither of them provides big advantages, apart from the fact that we have the unnecessary confusion, because it depends on the CPU design, which of the two variants is used. The borrow bit is calculated and used during subtractions that are done at assembly language level, but higher languages like C or Java do provide access to this information. It is relatively easy to recover the carry bit in the case of addition based on , and.