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(nasm.info)Section B.4.285

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B.4.285. `SBB': Subtract with Borrow
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     SBB r/m8,reg8                 ; 18 /r                [8086]
     SBB r/m16,reg16               ; o16 19 /r            [8086]
     SBB r/m32,reg32               ; o32 19 /r            [386]

     SBB reg8,r/m8                 ; 1A /r                [8086]
     SBB reg16,r/m16               ; o16 1B /r            [8086]
     SBB reg32,r/m32               ; o32 1B /r            [386]

     SBB r/m8,imm8                 ; 80 /3 ib             [8086]
     SBB r/m16,imm16               ; o16 81 /3 iw         [8086]
     SBB r/m32,imm32               ; o32 81 /3 id         [386]

     SBB r/m16,imm8                ; o16 83 /3 ib         [8086]
     SBB r/m32,imm8                ; o32 83 /3 ib         [386]

     SBB AL,imm8                   ; 1C ib                [8086]
     SBB AX,imm16                  ; o16 1D iw            [8086]
     SBB EAX,imm32                 ; o32 1D id            [386]

   `SBB' performs integer subtraction: it subtracts its second operand,
plus the value of the carry flag, from its first, and leaves the result
in its destination (first) operand. The flags are set according to the
result of the operation: in particular, the carry flag is affected and
can be used by a subsequent `SBB' instruction.

   In the forms with an 8-bit immediate second operand and a longer
first operand, the second operand is considered to be signed, and is
sign- extended to the length of the first operand. In these cases, the
`BYTE' qualifier is necessary to force NASM to generate this form of
the instruction.

   To subtract one number from another without also subtracting the
contents of the carry flag, use `SUB' (*Note Section B.4.305::).