## sbb - subtract with borrow

    dest = dest - (source + CF)

The sbb instruction subtracts the source value (second operand) plus the carry frag from the destination (either a register or a memory location). The destination can be a 64, 32, 16 or 8 bit register or memory location. The source can be either a register, memory location or an immediate value. You can not use 2 memory addresses. You can subtract either 8, 16 or 32 bit immediate values to larger registers and the immediate value will be sign extended.

sbb is designed to simplify subtracting large integers. In the code below a 256 bit difference is produced.

            mov     rax, [b]    ; get the low quadword of b, b[0]
            sub     [a], rax    ; subtract b[0] from a[0]
            mov     rax, [b+8]  ; get the next quadword, b[1]
            sbb     [a+8], rax  ; subtract the 2 quadwords + CF
            mov     rax, [b+16] ; get the next quadword, b[2]
            sbb     [a+16], rax ; subtract the 2 quadwords + CF
            mov     rax, [b+24] ; get the next quadword, b[3]
            sbb     [a+24], rax ; subtract the 2 quadwords + CF

### flags: OF SF ZF AF CF PF