Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Digital systems, linear algebra, introduction to formal proof / 1 / d

A subtract with carry instruction consumes the no-borrow carry flag. Its arithmetic is $Z=X-Y-(1-C_{\mathrm{in}})(\mathrm{mod}{2}^{32})$. Hence a low-limb subs followed immediately by a high-limb sbcs propagates exactly the low subtraction's borrow.

For example, with the same limb assignment as in (c):

subs r0, r0, r2
sbcs r1, r1, r3

It uses an ordinary 32-bit adder by complementing every bit of the subtrahend and adding the incoming arithmetic carry. For $M=2^{32}$, the unreduced adder sum isIf this is $Z+M{C}_{\mathrm{out}}$, rearrangement gives $X-Y-(1-C_{\mathrm{in}})=Z-M(1-C_{\mathrm{out}})$. Thus both the result and outgoing no-borrow flag have the required meanings. With incoming arithmetic carry one this becomes ordinary subtraction.^[ref]